EDWARD GROTH III is a staff officer, Environmental Studies Board, Commission on Natural Resources, National Research Council, Washington, D.C

ENVIRONMENTAL CONTAMINATION by fluorides exposes many organisms to potentially toxic effects and may exert some stress on the ecological interrelationships among plant and animal populations in natural biological communities. Research to date has focused on human beings and species important to humans; (1) relatively little is known of the potential ecological consequences of fluoride pollution. This article presents a literature review of what is known about the ecological effects.

In brief, the available data fall short of providing conclusive proof that any major, significant, or irreversible ecological changes have occurred, or are likeIy to occur, as a result of existing levels of fluoride pollution. (In this context, ecological effects means changes in the balance of natural ecosystems, not the very severe damage to commercial timber crops and livestock that has occurred because of fluoride pollution. See, for example, “Fluorides in the Air,” Environment, April 1973.) Nevertheless, the available evidence does support the view that fluorides are pollutants with considerable potential for producing ecological damage. The compounds are potentially serious contaminants not only when present in highly localized, massive concentrations, but also when distributed in low-level amounts over a long period of time. As future research begins to bring potential ecological impacts of fluoride into better focus, it seems very likely that proof will develop that the ecosystem does suffer damage when fluoride levels of the magnitude discussed here are present.

The evidence which supports concern over potential ecological impacts of low-level fluoride pollution can be summarized as follows:

* Levels of fluoride air pollution capable of leading to significant accumulation in vegetation and consequent injury to some sensitive plants have occurred several miles or more from sources of fluoride emissions, despite air pollution controls.

* Significant fluoride accumulation has been demonstrated in insects and in birds and mammals that feed on plants in the vicinity of pollution sources. The accumulated levels have been high enough, in some cases, to be potentially toxic, and such buildup represents a major increase of fluoride in food chains.

* Water pollution from both industrial sources and municipal sewage seems capable of producing downstream concentrations of 0.5 to 3 parts per million (ppm). Concentrations are highest during summer months, when biological activity is also at its peak. Some reports of toxic effects in algae and freshwater vertebrates at 1 to 2 ppm fluoride have been published. Most invertebrate species studied can accumulate significant bodily burdens of fluoride at this level of pollution, and there are indications that aquatic vegetation may also concentrate the element. It seems very likely that fluoride is accumulating, and probably being magnified, along aquatic food chains.

* Substantial amounts of fluoride are transferred to the soil each year. The degree to which this fluoride is available for uptake by soil organisms, and the extent to which soil life may be affected by fluoride in the environment, remain unknown.

* Possible conversion of fluoride into fluoroacetate (more toxic than fluoride itself and related organic forms), and the likelihood that fluoride may enter into synergistic actions with other contaminants, greatly expand the potential for ecological damage by low-level fluoride contamination.

Fluoride Air Pollution

Estimates by the National Research Council and the Environmental Protection Agency (3) suggest that between 120,000 and 155,000 tons of fluoride (calculated as hydrogen fluoride) are emitted into the atmosphere each year in the U.S. Fluoride is released from a variety of sources including aluminum smelting and phosphate processing operations; the combustion of coal; and the manufacture of steel, brick, tile, clay, and glass products. Reductions in fluoride emissions with increasing application of control regulations may be offset by the rapid growth of some fluoride sources, particularly phosphate fertilizer and aluminum production.

Most major fluoride sources use wet scrubbers to remove the pollutant from exhaust streams. Such controls are essential because concentrations as low as one part per billion (ppb) in ambient air are capable of causing serious damage to vegetation and may threaten livestock.(2) Concentrations of 10 ppb or higher have been measured in the immediate vicinity of a source, (4) and fluoride levels in the 1 ppb range may occur for several miles downwind of an emission point. In general, however, except downwind of a source, or in urban areas where many sources are present, the air rarely contains measurable fluorides.

Environmental Effects

According to a review by the US Department of Agriculture, fluorides have done more damage to livestock, worldwide, than any other air pollutant. (6) Some plants, including several important timber varieties of coniferous trees, are sensitive to fluoride damage. (2) Concentrations of 1.0 ppb or less can lead to long-term environmental damage because of biological magnification (the significant increases in pollutant concentrations which occur at each successively higher level in a food chain).(7) Some forage grasses can accumulate 200,000 times the level of fluoride present in the surrounding air. (2) Prolonged ingestion of contaminated forage by livestock can lead to excessive accumulation of fluoride in the bones which may, in turn, produce skeletal deformities and other damage to the animals’ health. (6)

Several studies in the past five years have begun to explore potential effects of fluoride on natural vegetation and wildlife species not previously investigated. For example, when samples of lichens and mosses were exposed to pollution from an aluminum smelter in Quebec in four- or twelve-month studies at distances of from about one-half to nine miles downwind of the source, the lichens showed severe fluoride injury symptoms, especially near the source, and both lichens and mosses accumulated the pollutant. Lichens, exposed for four months about one-half mile from the source had 990 ppm, mosses, 570 ppm Even samples nine miles from the source showed 190 ppm (lichens, at four months) and 78 ppm (mosses, at twelve months). (8) Similar accounts of the effects of fluorides on lichens in Pennsylvania and Scotland have been published. (9)

Several scientific groups in Montana recently investigated the effects of fluoride on a wide range of plants and animals exposed to the contaminant. The polluted areas studied were near the Anaconda Aluminum Company smelter in Columbia Falls and the Rocky Mountain Phosphate Company plant in Garrison. Despite pollution control measures employed by both companies (reported to be 99 percent efficient in controlling fluoride emissions), fluoride contaminated the environment and accumulated in a large number of organisms.

Vegetation in a 400-square-mile area downwind of the Columbia Falls aluminum smelter accumulated significantly elevated levels of fluoride (more than 10 ppm); on more than one-quarter of that area, foliage levels exceeded 30 ppm (10) Several species of pines, firs, grasses, hay, and a large number of of shrubs and herbs were sampled, and many were found to contain significant amounts of fluoride, even at distances of more than twenty miles from the source. Insects of several dozen species were captured in the polluted area, and almost all samples had high levels of fluoride. Control samples, taken from a nonpolluted area, showed fluoride levels of 3.5 to 16.5 ppm in their tissues while insects from the study area had 6.1 to 585 ppm. Insects from the pollinator group (such as bees) generally had the highest fluoride levels. Some species that are predatory throughout their life cycles had an elevated fluoride content, suggesting the transfer of the pollutant through the food chain. (11)

University of Montana investigators analyzed the thigh bones of more than 300 animals taken from different parts of the study area. They found that skeletal fluoride accumulation was 10 to 40 times higher than that in animals taken from nonpolluted areas. Many of the chipmunks, ground squirrels, and other mammals and birds in the sample had bone fluoride levels in excess of 1,000 ppm, and several individual animals had concentrations of from 5,000 to 13,333 ppm (12)

The investigation of the area around the Garrison phosphate operation revealed a similar, although geographically more limited, pattern. The fluoride levels of many samples of vegetation exceeded the 35 ppm state standard, some samples contained more than 100 ppm. Animal specimens had above normal accumulations which correlated well with the concentrations in plants at the sites where the animals were trapped. (13)

A similar study showed significantly elevated fluoride levels in grass, and in bones of sparrows and frogs near an aluminum smelter in Czechoslovakia. (14) In general, however, few other data have been gathered on the potential impact of fluoride pollution on wildlife species.

Although the data available to date are few, they fit a pattern. The ability to accumulate fluoride from very low ambient air concentrations, and to build up levels of 10 to 100 ppm or more appears to be very widespread among different kinds of vegetation. A broad range of herbivorous (plant-eating) animals in polluted regions, sometimes many miles from the source of pollution, seem to be accumulating substantial fluoride, primarily through their diet. Levels in animals are generally higher relative to control sample levels than levels in plants, reflecting the magnified effects which occur as a pollutant moves up the food chain. Since very few samples of predatory animals have been analyzed, no solid conclusions can be drawn about the potential hazards to animals higher in food chains. However, experience with other food chain pollutants (for example, DDT) indicates that predators are often hardest hit by cumulative contaminants. It seems urgent, therefore, to obtain further data on fluoride accumulation in predatory species.

Fluoride Toxicity

The potential biological and ecological significance of fluoride accumulation, as reported in these studies, is not easy to evaluate. In general, there is little information available on the toxicity of fluoride to most wildlife species. Data on domestic plants and livestock indicate wide differences in the sensitivity, of various species to fluoride injury. (5) Some conifers are among the most sensitive plant varieties. Investigators in Montana reported that pines, especially the western white pine, were dying out over hundreds of acres near the aluminum plant in their study. They concluded that loss of the pine trees was altering the normal ecological succession of the forest community at those sites and could lead to major changes in the vegetation patterns of the area. (16) It is not known whether fluoride may be having injurious effects on other important members of the plant community in the polluted areas, but should such effects occur, they would alter not only the balance of vegetative types, but of animals as well.

Extensive studies on domestic animals indicate that 30 to 40 ppm fluoride in forage can be seriously toxic to cattle when ingested on a prolonged basis, and that sheep, swine, and other species seem to be able to tolerate higher amounts of fluoride in their feed.(6) Data on herbivorous wildlife species are not available, but it should be assumed, in the absence of contrary information, that fluoride levels of 30 ppm or more found in large areas in Montana may represent a hazard to animals which habitually feed on the contaminated vegetation.

In domestic cattle, skeletal concentrations ranging from 1,450 to over 8,000 ppm have been associated with fluorosis (fluoride poisoning), and bones from a horse injured by fluoride pollution had 1,060 to 1,500 ppm (6) Although no direct relationship was established between skeletal fluoride accumulation and health effects in the animals in Montana, it seems logical that at least those animals in which skeletal fluoride exceeded 5,000 ppm could have suffered some adverse health impact.

Some information on fluoride toxicity to insects is available. Mulberry leaves containing 10 to 15 ppm fluoride were lethal to silkworm larvae, while leaves containing lower fluoride levels led to reduced growth of the insects. (17) In other studies, (18) sodium fluoride added to flour affected the survival and reproduction of the flour beetle, Tribolium confusum; some concentrations appeared to inhibit, and others to enhance, egg production. Considerable evidence is available to indicate that honeybees are highly sensitive to fluoride. Bee colonies in the vicinity of fluoride sources have frequently been heavily damaged. Two of the Montana investigators commented that the highest accumulation of fluoride among insects in their study was in members of the pollinator group. (16) They speculated that if other pollinators should prove as susceptible to fluoride injury as the honeybee, patterns of pollination in a polluted region could be substantially altered; and, as a consequence, the abundance of many insect-pollinated plants could shift, with attendant major changes in the ecology of an entire community.

It must be emphasized that research to date has not probed for such ecological effects, and we cannot say that they are occurring in the vicinity of fluoride air pollution sources. Nevertheless, the potential for such effects seems real enough, making this an area in which more research would be desirable.

Water Pollution Sources

While fluoride air pollution primarily occurs in the vicinity industrial sources, fluoride is released into the aquatic environment by a far wider range of sources, and it seems very likely that most bodies of water are contaminated by fluoride to some extent. Some fluoride is present in waters from natural sources. Many minerals contain soluble fluoride, and when ground water passes through such fluoride-bearing rock formations, the water may become contaminated. A few sources, primarily deep wells, contain 1 ppm fluoride or more. Most surface waters contain less than 0.2 ppm fluoride, and the majority are below 0.1 ppm (19) The oceans, as the result of eons of leaching of mineral salts from the land, contain from 1.2 to 1.4 ppm fluoride, about half in the form of fluoride ion and half in the relatively insoluble, magnesium fluoride complex ion. (20) Although natural, or “background,” fluoride levels in most fresh-water streams are in the 0 to 0.2 ppm range, available data indicate that concentrations above 0.5 ppm, and occasionally as high as 2 or 3 ppm, may be fairly common in watercourses contaminated by human activities.

Several human activities result in substantial fluoride input to the aquatic environment. Many of the industries which have fluoride air pollution problems are also sources of fluoride water pollution. Air pollution control equipment often produces a fluoride laden liquid waste which requires disposal. (Fluoride can be removed from wastewater by treatment with lime in settling ponds, a form of treatment which can reduce the fluoride content of an effluent stream from more than 5,000 ppm to about 5 to 50 ppm) (21) Aggregate figures for all fluoride sources are not available, but the phosphate industry may discharge from 6,000 to 30,000 tons of fluoride into waterways in the US annually.(22) The Environmental Protection Agency has proposed standards for the primary aluminum industry which, starting in 1977, would restrict fluoride in wastewater discharge to an average of two pounds per ton of aluminum produced. If all aluminum smelters were currently meeting that standard, fluoride discharges would be 4,000 to 5,000 tons per year from this industry. However, only about one-third of the plants now in operation are presently in compliance, so actual fluoride pollution from the aluminum industry is probably substantially larger. (23) Fluoride discharges from other industries are not negligible, but are probably smaller than from phosphate and aluminum operations.

Another significant source of fluoride water pollution is domestic sewage. Approximately one-half of the communities in the US which have centralized water distribution systems now add fluoride to their water supplies for the partial control of tooth decay. (24) Provision of fluoridated water for 100 million people requires the addition of approximately 20,000 tons of fluoride to domestic water supplies each year. Most of the water used in urban areas, and thus most of the fluoride added to water supplies, is returned through sewage systems to the aquatic environment.

A study of fluoride levels in sewage in 56 California cities demonstrated that domestic sewage already contains fluoride, over and above that naturally present in water or added for dental health. (26) Fluoride in human wastes, originating with fluoride in foods, was tentatively identified as the source of the excess. The investigator concluded that fluoride from toothpastes and other sources would make a negligible contribution, and that no industrial sources were contributing fluoride to the sewage samples studied. The findings suggest that the total input of fluoride into the environment from domestic sewage is probably more than the 20,000 tons estimated to be added to water supplies in communities where fluoridation of drinking water takes place. Thus, even communities not fluoridating water may release significant fluoride into receiving streams in their sewage.

The same study showed that secondary sewage treatment (biological digestion of wastes) reduced fluoride in the final effluent by an average of 57 percent, while primary treatment had no appreciable effect on fluoride levels. Even with secondary sewage treatment, however, it was concluded that significant amounts of fluoride persisted in effluents.

Fluoride is present in phosphate fertilizers, and some fluoride may be carried into surface waters in runoff from agricultural lands. It is also likey that some portion of fluorides emitted into the air is eventually carried by precipitation into surface waters. (27) While these sources may be significant, good quantitative estimates of the magnitude of fluoride input to the aquatic environment by these routes are not available.

Environmental Concentrations

Although fluoride air pollution leads to significant environmental concentrations only in the vicinity of sources, low-level fluoride water pollution appears to be more widespread.

The US Geological Survey monitors water quality at several thousand sites around the country, but fluoride data are not routinely included in chemical analyses. Fluoride readings for some streams are available, however. Many rivers have fluoride contents ranging from 0 to 0.2 ppm, but some have much higher levels. For example, 1967 data for the Santa Ana River in California showed fluoride levels of 0.9 to 3.6 ppm (average, 1.1 ppm), and single readings in the Pit River (also in California) reached 1.8 and 2.1 ppm (28) (Earlier monitoring at the same sites on the Pit River recorded levels of 0.1 to 0.2 ppm) (29)

A number of published studies relate environmental fluoride concentrations to specific sources of the contaminant. Tributaries of the East Gallatin River above the town of Bozeman, Montana, contain 0.1 ppm fluoride or less, while the river below the city’s sewage outfall (the only fluoride source in the area) has been found to have concentrations of 0.3 to 0.8 ppm (30)

Fluoride concentrations of from 0.17 to 2.06 ppm were measured in a study of the Illinois River. (31) The highest concentrations occurred during the summer months, when stream volume was lowest. Fluoride sources upstream from the monitoring site included several communities with fluoridated water supplies and several major fertilizer manufacturing plants. A study of fluoride input to Narragansett Bay, in Rhode Island, showed that 36 percent of the fluoride entering the bay was due to fluoridation of water supplies in five communities on rivers feeding into the estuary. (32) In midsummer, pollution from these sources was enough to double the fluoride content of the rivers. A similar study in Japan showed fluoride concentrations of 0.15 to 1.07 ppm in rivers feeding into Tokyo Bay. (33)

Pollution near industrial sources, especially where only limited wastewater treatment to remove fluoride is employed, can be much more serious. Concentrations of 20 ppm or more were reported for the Pamlico River (in North Carolina) near a phosphate plant. (34) In most states where industrial fluoride discharge is a problem, relevant water quality standards have been adopted. Standards for drinking water sources generally are based on the US Public Health Service Drinking Water Standards and prohibit concentrations in excess of 1.5 to 2.0 ppm Some states permit levels of 5 to 10 ppm for bodies of water which are not sources of public water supplies in order to prevent toxic effects to wildlife. (35)

Ecological Effects

The critical question for biologists is whether chronic exposure to these fluoride concentrations, which may be from two to ten or more times higher than the background level, poses any significant physiological or ecological hazard to aquatic life. It seems reasonable to conclude that fluoride at these levels poses no major risk to marine organisms. (32) Both the dilution factor, and the fact that most oceanic forms evolved in an environment that contains from 0.6 to 0.7 ppm fluoride ion, suggest that potential effects on marine life should be minimal if fluoride in rivers rarely exceeds 2 ppm However, freshwater organisms evolved in an environment that was almost fluoride-free, and thus might be expected to be less well-equipped to tolerate fluoride concentrations encountered in polluted streams.

Relatively little is known about the potential impact of fluoride on either freshwater or marine organisms. A number of investigators have measured the short-term toxicity of various fluoride compounds for a good number of species, but systematic inquiries on the more general effects of long-term, low-level pollution, analagous to the Montana air pollution studies discussed above, have rarely been published. Thus, we may know the lethal concentrations for many organisms, but we have very little knowledge of the sublethal effects of fluoride on behavior or reproductive processes, or of potential accumulation of the pollutant in aquatic food chains. Yet such effects, should they occur, would probably be more important ecologically than the mortality which might result from very high, but short-lived, pollution episodes. (36)

Several investigators have exposed a variety of bacteria and microscopic animal species that live in freshwater to a range of fluoride concentrations extending well above those likely to be encountered in streams, without any demonstrable toxic effects. (37) Not many species have yet been tested, however, and the criteria for evaluating toxicity were not sophisticated. The finding that bacterial digestion of sewage removes much of the fluoride content of the effluent (26) suggests that some bacteria may accumulate fluoride from water. The importance of bacteria as a basic element in food chains makes it important to learn more about the capacity of microorganisms to bioconcentrate this contaminant.

The single-celled green alga Chlorella showed a 37 percent reduction in growth over 48 hours when exposed to a 2 ppm fluoride solution; (38) 43 ppm was reported lethal to another alga, Scenedesmus. (39) Few other data on toxicity of fluoride to aquatic plants are available, but several studies suggest that water plants can accumulate the element. Five-day exposures to 100 ppm led to a 50-told concentration of fluoride by aquatic plants, and fourteen days at 20 ppm produced a 38 fold increase. (27) Water hyacinths absorb fluoride efficiently at concentrations above 10 ppm, and to a much lesser extent at lower Ievels. (40) Several species of marine algae (exposed to 0.5 to 0.7 ppm) contained 2 to 22 ppm fluoride. Eel grass and the alga Cladophora, however, showed no significant fluoride buildup after seventy-two days in sea water with 52 ppm fluoride. (42) One Russian study found an average fluoride content of 40.5 ppm in samples of several freshwater plants, (43) and other studies strongly suggest that aquatic vegetation accumulates fluoride. (44) However, the evidence as a whole is still too fragmentary to provide a clear or systematic picture of the capacity for fluoride buildup in aquatic plants.

Effects on Aquatic Animals

Short-term fluoride toxicity data are available for a number of invertebrate species, the majority of them marine varieties. Water fleas are killed or immobilized by concentrations of various fluoride compounds ranging from 5 to 500 ppm (45) Lobsters are not harmed by 5 ppm fluoride. (46) Mussels may be killed by 1.4 to 7.2 ppm, (42) and concentrations of 20 ppm or higher for extended periods have been shown to be toxic or lethal to oysters, two species of crabs, and a sand shrimp, but not to two types of prawns. (47) More significant than the lethal effects of high concentrations, however, is the marked ability demonstrated by almost all species studied in these investigations to accumulate substantial bodily burdens of fluoride. Even animals kept in sea water containing only 1 ppm fluoride had bodily concentrations of from 100 to 300 ppm (48) The entry of fluoride into food chains through bioconcentration in aquatic invertebrates is a subject in need of much more careful research.

Studies of the effects of fluoride on fish are far more numerous than for any other form of aquatic life . (49)

Short-term lethal effects may occur at concentrations as low as 3 ppm in sensitive species (for example, rainbow trout), while other fish are not damaged until fluoride levels reach 100 ppm Water temperature, hardness, chlorinity, and other environmental factors, as well as the age and physiological state of the fish, can influence the toxicity of a given concentration of fluoride. (50)

Sublethal concentrations may have adverse effects on fish behavior or reproduction, which could be ecologically significant. Research findings are few and not confirmed, but trout eggs seem to be delayed in development and hatching by 1.5 ppm fluoride. (51)

Fish are important food-chain organisms, and the ability of many fish, like many other vertebrates, to accumulate elevated fluoride levels in their skeletons (52) can introduce the contaminant into the diet of fish-eating predators. Levels of 550 to 6,800 ppm have been reported in bones of ocean fish, and 400 to 1,600 ppm in trout from a naturally high-fluoride stream in Yellowstone National Park. Such accumulation might pose a hazard to animals that eat whole fish.

Data on other aquatic vertebrates which may be exposed to fluoride are sparse. Frogs were killed in one week by 900 ppm fluoride, (53) and decreased red and white blood cell counts were observed in frogs kept in fluoride concentrations of 5 to 300 ppm (54) There have also been indications that sublethal fluoride concentrations may adversely affect amphibian reproductive cycles. (55) Frog eggs were retarded in development but hatched prematurely in 1 ppm fluoride in well water, higher concentrations (13 to 450 ppm) had the same effects on toad eggs, and metamorphosis in tadpoles was significantly delayed by fluoride at 0 5 and 4.5ppm. (56)

Most research on the effects of fluoride on aquatic organisms dates back to the early 1960s or before, and more definitive studies are required on the potential hazards suggested here. There is also a pressing need to examine the potential impact of chronic, low-level bioaccumulation of fluoride on predatory animals higher in aquatic-based food chains. As is the case with fluoride air pollution, the logic of ecosystem energy and nutrient flow patterns suggests that species at the highest levels of a food chain are likely to bear the greatest risk of harm, but virtually no effort has been made to look for such damage. If fluoride has had such adverse effects on aquatic wildlife, they have thus far been too subtle to attract attention. In the absence of any substantive research data, it would be unwise to assume that no risks exist.

Soil Pollution Sources

Because fluoride is a common constituent of several relatively abundant minerals, most soils contain this element. The range for most normal soils is 100 to 300 ppm, but levels of up to 8,300 ppm have been found in heavy clay soil. (2) Additional sources of fluoride input to the soil may be present in many localities. Air pollution can lead to a substantial increase in soil fluoride content, both through fallout of particulate fluorides and through the absorption of gaseous fluorides in rain and snow. (57) Phosphate fertilizers may contain 0.5 to 4.0 percent fluoride by weight as an impurity. One investigator calculated that fertilizer applications in Germany were adding from 7.0 to 17.6 pounds of fluoride each year per acre of land fertilized. (58) This compared to 1.8 pounds per acre of fluoride added to the soils in his study area by air pollution, and to values of 6.1 to 19.2 pounds for each acre input from air pollution in similar studies. In the US, 5 million tons of phosphate fertilizers were applied to soils in 1973. (59) If it is assumed that the average fluoride content of that fertilizer was 2 percent by weight, this represents an input of 100,000 tons of fluoride to US soils.

Additional fluoride input to soils may occur when fluoride-containing waters are used in irrigation. No quantitative estimates are available for the magnitude of such contributions to fluoride contamination of the soil, however.

Fate of Fluoride in Soils

More than 90 percent of the natural fluoride content of soils is insoluble, or tightly bound to soil particles. (27) Most soil samples show lower fluoride content near the surface than at depths of a few feet, indicating that the soluble fraction of fluoride may be removed from the surface by water seeping into the ground. It appears, therefore, that under normal conditions very little fluoride is available for uptake by plants, even in soils that may be relatively rich in fluoride.

Research findings differ on the degree to which fluoride added by pollution or fertilization is available for uptake in the plant roots. When soluble fluoride compounds (for example, sodium fluoride) were added to soils in concentrations of 150 ppm or more during one experiment, significant uptake by plants occurred. (60) Other experiments showed that a substantial amount of fluoride was removed from polluted soils by water. (61) On the other hand, it has been found that as much as 90 percent of fluoride from fertilizers and air pollution may remain in the soil; (58) another report showed that some soils, especially those with relatively high calcium content, were very effective in fixing fluoride, with the result that little was available for plants to incorporate. (62)

It seems very likely that a number of soil characteristics, as well as other environmental factors, can have a marked influence on the availability of fluoride to plants. For example, fluoride is more readily available in sand or acid soils than in high-clay soils. (63) Also, a relationship exists between the type of nitrogen fertilizer applied and the toxicity of fluoride to crops. (64) The use of certain boron-containing fertilizers leads to a dramatic increase in the accumulation of fluoride in the leaves of fruit trees. (65)

Biological Effects

The only research on the biological impacts of soils contaminated by fluoride has dealt with uptake of the chemical by plants. Data from one study showed that grasses grown in soils containing 1,350 ppm fluoride could contain as much as 1,330 ppm (14) In many similar reports, it has been observed that when fluoride is present as both an air pollutant and a soil pollutant, plant uptake from air (through the tiny openings in leaves where gases are exchanged) is far more significant than from soil. Several investigators have shown, however. that substantial uptake can occur from soil alone under some conditions. (66)

A number of investigators have shown that the uptake of fluoride pollution from soil can have toxic effects on some plants. For example, 1,000 to 1,500 ppm fluoride added to soil in one experiment reduced the yield of winter wheat by 40 to 65 percent and 400 ppm reduced growth of Tradescantia, a flowering plant by 28 to 34 percent; (60) a strong correlation has been demonstrated between inhibition of pea seedling growth and increased fluoride content of the soil, (67) and fluoride concentrations of 1.9 to 190 ppm in soil reduced the growth of loblolly pine and red maple trees. (68)

The most obvious ecological concerns arising from fluoride pollution of the soil center around uptake of the contaminant by plants, not only because of potential toxic effects to the plants themselves, but also because the process may introduce additional fluoride into the diets of animals.

But uptake by plants is just a small part of the possible impact soil fluoride might have on living things. The soil is anything but a sterile medium; it is, in fact, a very rich, and highly diverse, ecosystem which includes thousands of species of microbes, fungus, worms, and insects. (69) Many of these soil organisms are essential to the fertility of the land – for example, they convert nitrogen to a form useful to plants, help break down organic matter and by turning the soil, help aerate it. Disruptions by soil ecology by toxic pollutants could potentially reduce the land’s ability to support plant life, and thus, all life.

Whether the fluoride now being added to soils in fertilizers and as fallout from air pollution poses any real threat to the ecological balance of the soil community cannot be determined yet. There are virtually no published data on the toxicity of fluoride to soil organisms, or the potential for accumulation of fluoride in soil food chains. Until research has been conducted on this subject, we will have no way of knowing, but the possibility must be considered that fluoride may be potentially as dangerous to some soil organisms as it is known to be to some terrestrial and aquatic varieties.

Biosynthesis of Organofluorides

Many environmental contaminants may be altered chemically by the action of living things, and in this way be transformed into substances more toxic than the pollutants in their original form. The methylation of mercury by bacteria is one example of such biotransformation. (See “Mercury in the Environment,” and “Mercury in Man,” Environment, May 1971.) Another is the synthesis of highly toxic azo compounds from aniline-based herbicides which has been reported to occur in soil microorganisms. (See “The Soil Transforms,” Environment, May 1971.) There is convincing evidence now that some plants can synthesize organic fluoride compounds, primarily fluoroacetate and fluorocitrate, from inorganic fluorides. Although inorganic fluorides are themselves quite toxic, fluorocitrate and fluoroacetate are much more toxic. According to one expert in the field, “Fluoroacetates and their related compounds…are among the most poisonous substances known.” (70)

The biosynthesis of organofluorides was initially demonstrated in certain tropical plants noted for their extreme toxicity to livestock. It has been observed that these plants may contain several hundred ppm of fluoro-organic compounds in their leaves, but the plants usually grow in soils which are low in fluoride (11 and 216 ppm). (71) Fluoroacetate levels of up to 1,100 ppm have been measured in the leaves of one tropical plant growing in an area where soils contained 1 to 6 ppm fluoride, and the water only 0.05 ppm No other plants in the vicinity contained more than 2 ppm fluoride. Some of the plants that synthesize fluoro-organic compounds, therefore, appear to have an exceptional ability to extract fluoride from an environment in which the element is present only in extremely small traces. (72)

More than two dozen toxic plants are known to be able to synthesize fluoroacetate, (73) but much interest has been generated by recent findings, which could be of great ecological importance, that suggest that the ability to make organic fluoride toxins may be much more widespread than was previously suspected. Fluoro-organic residues have been detected in several salad and forage crops.(74) Measurements have shown 179 ppm fluoroacetate and 896 ppm fluorocitrate in forage crops grown in fields near a phosphate plant. (75) Soybean plants exposed to hydrogen fluoride in the laboratory had concentrations of 40 and 140 ppm of the same two compounds. (76) Other workers have reported that single-cell cultures of soybeans possess the ability to synthesize fluoro-organic compounds; (77) lettuce can convert fluoroacetate to fluorocitrate. (78) At least one attempt to detect organic fluoride compounds in crop plants exposed to fluoride was not successful (79) although others have repeatedly confirmed these findings. (73)

Compared to the amounts of fluoro-organic toxins found in some of the poisonous tropical plants mentioned above, the quantities detected in most of the more common plants tested are quite small and may not be a toxic threat. However, organofluorides have also been reported in tea and in oatmeal (80) and may be very widespread in both the human and natural food chains. Recent reports that fluoro-organic residues are present in the bones of cattle and horses (81) are suggestive of food-chain transfer. Sodium fluoroacetate, sold commercially under the name “Compound 1080” is a widely employed rodenticide, (92) and unintended transfer of the poison through the food chain has had adverse effects on some predators which feed on rodents. If increasing fluoride pollution of the environment should lead to a general buildup of fluoro-organic compounds in natural food webs, it is possible that the ecological damage which might occur could be severe.

A great deal of research is needed to determine whether biosynthesis of organic fluorides does in fact add a serious new dimension to the potential ecological consequences of fluoride pollution. We need to know which organisms possess the ability to synthesize these toxins, and in particular whether such abilities exist in members of aquatic and soil communities. There is evidence that some soil microorganisms may synthesize fluoroacetate; (83) the existence of this capacity in microbes, as well as higher plants, needs to be explored. Some plants have also been shown to break down organic fluoride compounds.(84) Some bacteria may also be able to defluorinate these substances. (71) A great deal of work is still needed to track the environmental fate of fluoro-organic toxins in natural biological systems, and to determine the magnitude of any threat arising from the biosynthesis of such compounds that may occur in a fluoride-contaminated environment.

Potential Synergisms

It seems likely that fluoride may interact synergistically with other environmental pollutants to produce greater effects than either pollutant could cause were it acting alone. One study shows a pronounced synergistic effect between fluoride and copper which resulted in the inhibition of cellular respiration in Chlorella. (85) The influence of boron, contained in certain fertilizers, on fluoride uptake by plants has been noted above. (65) Other evidence suggests that there are synergistic effects between hydrogen fluoride and sulfur dioxide in the air.

Other factors in the environment may also modify, and in some cases offer protection against, the toxic effects of fluoride. The presence of some mineral elements, especially calcium, in soil and water seems to reduce potential fluoride availability and, therefore, the potential damage. Many other geochemical, physical, and biological parameters may well influence the effects of a given level of fluoride on any organism or ecosystem. This complexity, combined with the lack of solid experimental data, make evaluation of existing toxicological information all the more difficult.

Conclusions

Although based on still fragmentary research data, the conclusions summarized at the start of this paper comprise a fairly compelling case for treating fluorides as pollutants with a great capacity to do ecological harm. Research to provide a more definitive assessment of the environmental risks of low-level fluoride pollution is urgently needed. Priority areas for study should include the following: the effects of fluoride in food chains, particularly on predators at the higher levels of trophic webs; the physiological impact of chronic exposure to fluoride, at concentrations now present in the environment, on many of the most important species in land, aquatic, and soil ecosystems; the sublethal toxic effects, such as interference with reproduction, alteration of behavior, or increased susceptibility to diseases, predation, or parasitic attack. Data are needed as well on the effects of fluoride on soil organisms, an area in which we are virtually completely without information, and better understanding is needed of the synthesis and transfer of organic fluoride poisons in ecosystems. In addition, much more complete monitoring to record levels of fluoride actually present in ecosystems, especially freshwater streams, would be very useful.

To date, except for instances of gross spillage of fluoride into the air or water, fluoride has received relatively little attention as a contaminant of the ecosystem. In the case of water pollution especially, there have been many other pollutants which have been present in massive amounts, and which have had a very significant impact. It is easy to understand how a pollutant like fluoride, which is usually present at fairly low levels, and which has more subtle, insidious effects, when it has effects at all, has been given relatively low priority, both in terms of research attention and regulatory control. It is possible that fluoride may have had some adverse effects on aquatic life, but that such damage has been masked by the far more severe effects of untreated sewage, industrial effluents, pesticides, and other major pollutants. As controls on these more easily recognized pollution problems are becoming more effective and widespread, attention can turn to less prominent pollutants such as fluoride, whose impacts may be more easily detected as water quality improves in respect to other parameters.

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NOTES

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1. National Research Council, Fluorides, Committee on Biological Effects of Atmospheric Pollutants, National Academy of Sciences, Washington, DC, 1971. World Health Organization, Fluorides and Human Health, WHO Monograph No. 59, Geneva, 1970.

2. National Research Council, ibid.

3. US Environmental Protection Agency, Engineering and Cost Effectiveness Study of Fluoride Emissions Control, Washington, DC, Jan. 1972.

4. Prival, M.J., and F. Fisher, “Fluorides in the Air,” Environment, 15(3):25-32, 1973. Cross, F.L., and R.W. Ross, “Fluoride Emissions from Phosphate Processing Plants,” Fluoride Quarterly Reports, 2(2):97-105, 1969.

5. Yunghans, R.S., and T.B. McMullen, “Fluoride Concentrations Found in NASN Samples of Suspended Particles,” Fluoride, 3(3):143-152, 1970.

6. Lillie, R.J., Air Pollutants Affecting the Performance of Domestic Animals, A Literature Review, Agricultural Handbook No. 380, Agricultural Research Service, USDA, Washington, DC, 1970.

7. Hill, A.C., “Air Quality Standards for Fluoride Vegetation Effects,” J. Air Poll. Cont. Assoc. , 19(5):331-336, 1969.

8. LeBlanc, F., G. Comeau, and D.N. Rao, “Fluoride Injury Symptoms in Epiphytic Lichens and Mosses,” Can. J. Bot., 49:1691-1698, 1971.

9. Gilbert, O.L., “The Effect of Airborne Fluoride on Lichens,” Lichenologist, 5:26-32, 1971. Nash, T.H. III, “Lichen Sensitivity to Hydrogen Fluoride,” Bulletin Torrey Botanical Club, 98:103-106, 1971.

10. Carlsson, C.E., and J.E. Dewey, “Environmental Pollution by Fluorides in Flathead National Forest and Glacier National Park,” USDA, U.S. Forest Service, Missoula, Montana, 1971.

11. Dewey, J.E., “Accumulation of Fluorides in Insects Near an Emission Source in Western Montana,” Environ. Entom., 2:179-182, 1973.

12. U.S. Environmental Protection Agency, Environmental Effects of Fluoride: Glacier National Park and Vicinity, Report No. EPA-908/1-74-001, Air and Water Programs Division, Region VIII, Denver, Colo., Feb 1974.

13. Kay, E., “An Inquiry into The Distribution of Fluoride in the Environment of Garrison, Montana,” Fluoride, 7(1):7-31m 1974.

14. Macuch, P., E. Hiuchan, J. Mayer, and E. Abel, “Air Pollution by Fluoride Compounds near an Aluminum Factory,” Fluoride Quarterly Reports, 2(1): 28-32, 1969.

15. Hindawi, I.L. Air Pollution Injury to Vegetation, Nat. Air Poll. Cont Admin, Pub. No. AP-71, Raleigh, N.C., 1970. Lillie, loc. cit.

16. Carlson and Dewey, loc. cit

17, Fujii, M., and S. Honda, “The Relative Oral Toxicity of Some Fluorine Compounds for Silkworm Larvae,” J. Sericul. Scl. Japan,, 41(2):104-110, 1972 (Abstract in EngIlsh).

18. Johansson, T.S.K., and M.P. Johansson, “Sublethal Doses of Sodium Fluoride Affecting Fecundity of Confused Flour Beetles.” J. Econ. Entom.. 65(2):356-357, 1972.

19. Dobbs, G., “Fluoride and the Environment,” Fluoride, 7(3).1123-134, 1974. Marler, J.R., and D. Rose, Environmental Fluoride, Pub. No. 12,226, National Research Council of Canada, Ottawa, 1971.

20. Riley, J.P., and G. Skirrow, Chemical Oceanography, vol. 2, Academic Press, New York, 1965.

21. U.S. Environmental Protection Agency, Development Document for Proposed’ Effluent Limitations Guidelines and New Source Performance Standards for the BASIC FEW TILIZER CHEMICALS Segment of the Fertilizer Manufacturing Point Source Category, Report No. EPA 440/1-73-011, Washington, D.C., Nov. 1973; Development Document for Proposed Effluent Limitations Guidelines and New Source Performance Standards for the PRIMARY ALUMINUM SMELTING Subcategory of the Aluminum Segment of the Nonferrous Metals Manufacturing Point Source Category, Report No. EPA 440/1-73-019a, Dec. 1973.

22. The estimated tonnage of fluoride discharged as water pollution by the phosphate industry was calculated as follows: Some 40 million tons of phosphate rock are mined annually in the U.S.. which may contain 2.5 to 4.5 percent fluoride by weight (USEPA, Rep. No. EPA 440/1-73-011, ibid.). From 30 to 90 percent of the fluoride may be evolved in gaseous or particulate form in the processing of the rock into various phosphate products (USEPA, ibid.; Marier and Rose, loc. cit.). If it is assumed that the average fluoride content of rock processed is 3 percent and that 50 percent of this is evolved in processing, some 600,000 tons of potential fluoride air pollutants will be generated. Air pollution control devices range up to 99 percent, plus. in efficiency; thus up to 594,000 tons of fluoride (or more) Is likely to be retained in scrubber liquors. Lime treatment and settling in gypsum ponds can remove 95 to 99 percent of the fluoride from wastewaters. Lacking exact data on the efficiency of control measures currently employed throughout the industry, I have simply assumed that between 1 percent and 5 percent of the fluoride in waste streams eventually reaches the environment In effluent discharges, that is 5,940 to 29,700 tons per year. If the actual state of controls in the industry averages less than 95 percent efficient, the figure would of course be higher.

23. U.S. EPA, Rep. No. EPA 440/1-73-011, loc. cit.

24. U.S. Public Health Service, Fluoridation Census, National Institutes of Health, Bethesda, Md., 1970.

25. The estimate for the amount of fluoride added to community water supplies for dental caries prevention is based on the following data and assumptions: The average optimal level is assumed to be 1.0 ppm, for simplicity. The average per capita water consumption from public water supplies Is about 160 gallons per day, all of which must of course be fluoridated at I ppm, even though per capita ingestion of water averages only about one quart or so. (Consumption data from Todd, D.K., The Water Encyclopedia, Water Information Center. Part Wasington, N.Y., 1970). With approximately 100,000.000 Americans. living in communities which now fluoridate their water, 23,800 metric tons per year of fluoride are put into the water. Since most water supplies contain some fluoride already (average 0.1 to 0.2 ppm), the figure was rounded off to 20,000 tons.

26. Masuda, T.T., “Persistence of Fluoride from Organic Origins In Waste Waters,” Developments in Industrial Microbiology, 5:53-70, 1964.

27. Marier and Rose, loc. cit.

28. U.S. Geological Survey, Water Quality Data, 1967, Part 11, USGS Water Supply Paper No. 2015, Dept. of the Int., Washington, D.C., 1972.

29. US. Geological Survey, Water Quality Data, 1962, Parts 9-14, USGS Water Supply Paper No. 1945, Dept. of the Interior, Washington, D.C., 1964.

30. BahiS, L.L., “Diatom Response to Primary Wastewater Effluent,” J. Water Poll. Cant. Fed. 45:134-144, 1973. Soitero, R.A., “Chemical and Physical Findings from Pollution Studies on the East Gallatin River and its Tributaries,” Water Research, 3:687-706, 1969.

31. Wang, W.C., and R.L. Evans, “Dynamics of Nutrient Concentration in the Illinois River,”. J. Water Poll. Cant. Fad.. 42: 2 117’3123, 19M.

32. Miller. G.R., Jr., K. Woolsey, and D.R. Kester, “Fluoride Chlorinity Ratios In Narragansett Bay.” Graduate School of Oceanography, University of R.I, Kingston, R.I., ref. no. 72-1, 1972.

33. Kitime, Y., and V. Furukawa, “Distribution of Fluoride in Waters of Tokyo Bay,” J. Oceanographic Sac. Japan, 28(3):121-125, 1972.

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35. McKee, J.E., and H.W. Wolf, Water Quality Criteria, California State Water Quality Control Board, Pub. No. 3a, Sacramento, Calif., 1963. Federal Water Pollution Control Administration, Water Quality Criteria, report of the National Technical Advisory Committee on Water Quality Criteria, to the Secretary of the Interior, USGPO, Washington, D.C., 1968. U.S. Environmental Protection Agency, Inorganic Fertilizer and Phosphate Mining Industries: Water Pollution and Control, Rep. No. EPA 12020 FPD 09/71, Washington, D.C., Sept. 1971.

36. Sprague, J.B., “Measurement of Pollutant Toxicity to Fish, III. Sublethal Effects and ‘Safe’ Concentrations,” Water Research. 5-245-266, 1971.

37.Wantland, W.W., “Effects of Various Concentrations of Sodium Fluoride on Parasitic and Free-living Protozoa and Rotifera,” J. Dental Res., 35:763-772, 1956. Grune, W.N., and R.Q. Sload, “Effect of Fluoride Concentration on Sludge Digestion,” Sewage & Industrial Wastes. 27:1-7, 1955. Vajdic, A.H., “The Effect of Sodium Fluoride on the Growth and Survival of Some Bacterial Species Important in Water Quality Measurement,” Ontario Water Resources Commis. sion, Division Research Paper No. 21006, 1966.

38. Smith, A.O., and B.R. Woodson, “The Effects of Fluoride an the Growth of Chlorella pyrenoidosa,” Virg. J. Sci., 16:1-8, 1965.

39. McKee and Wolf, loc. cit.

40. Rao, K.V., A.K. Khandekar, and D. Vaidyanadham. “Uptake of Fluoride by Water Hyacinth, Eichhornia crassipes,” Indian J. Exper. Bio., 11:68-69, 1973.

41. Young, G.E., and W.M. Langille. “The Occurrence of Inorganic Elements in Marine Algae of the Atlantic Provinces of Canada,” Can. J. Bot., 36:301-310, 1958.

42. Hemens, J., and R.J. Warwick, “The Effects of Fluoride on Estuarine Organisms,” Water Research. 6:1301-1308, 1972.

43. Danilova, V.V., “The Geochemistry of Dispersed Fluorine. 11. Determination of Fluorine in Plants,” Trav. Lab. Biogeochim. Acad. Sci. URSS, 7:83-85, 1944 (English Abstract. Chemical Abstracts, 1947).

44. Mun, A.I., Z.A. Bazilevich, and K.P. Budeyeva, “Geochemical Behavior of Fluorine in the Bottom Sediments of Continental Basins,” Geochem. Internat., 3:698-703, 1966. Windom, H.L., “Fluoride Concentration in Coastal and Estuarine Waters of Georgia,” Limnology and Oceanography, 16:806-810, 1971. Kilham, P., and R.E. Hecky. “Fluoride: Geochemical and Ecological Significance in East African Waters and Sediments,” Limnology and Oceanography, 18(6):932-945, 1973.

45. Sanders, H.O., and O.B. Cope. “Toxicities of Several Pesticides to Two Species of Cladocerans.” Trans. Am. Fish. Soc., 95:165-169, 1966. Anderson, B.G., “The Toxicity Thresholds of Various Sodium Salts Determined-by the Use of Daphnia Magna ” Sewage, Works J., 18:82-97, 1946. Bringmann, G., and R. Kuhn, “The Toxic Effects of Waste Water on Aquatic Bacteria, Algae, and Small Crustaceans,” Gesund-heits-ing., 80-115-123, 1959. FWPCA, loc, cit.

46. Stewart, J.E., and J.W. Cornick, “Lobster (Homarus americanus) Tolerance for TRIS Buffer. Sodium Fluoride, and Seawater Extracts of Various Woods,” J. Fish. Res. Board Can., 21:1549-1556, 1964.

47. Moore, D.J., “A Field and Laboratory Study of Fluoride Uptake by Oysters,” Report No. 20, Water Resources Research Institute, University of N.C., Raleigh, N.C.

1969. Moore, 1971, loc. cit. Hemens and Warwick, loc. cit.

48. Moore, 1971, loc. cit. Hemens and Warwick, loc. cit.

49. Neuhold, J.M., and W.F. Sigler, “Effects of Sodium Fluoride on Carp and Rainbow Trout,” Trans. Am. Fish. Soc.. 89:358-370, 1960. Angelovic, J.W., W.F. Sigler. and J.M. Neuhold, “Temperature and Fluorosis in Rainbow Trout,” J. Water Poll. Cont. Fed., 33:371-381. 1961. Neuhold, J.M. and W.F. Sigler, “Chlorides Affect the Toxicity of Fluorides in Rainbow Trout,” Science, 135:732-733, 1962. Herbert. D.W.M., and D.S. Shurben, “The Toxicity of Fluoride to Rainbow Trout,” Water & Waste Treatment. 10:141-142, 1964. Vallin, S., “The Toxicity of Fluoride to Fish,” Vatten, 24:51-57, 1968. Sigler, W.F., and J.M. Neuhold, “Fluoride Intoxication in Fish: A Review,” J. Wildlife Diseases, 8:252-254, 1972. DeRoos, C.D., “The Effects of Sodium Fluoride on the Weight Gain and Gills of the Common Goldfish,” Thesis, Utah State University, Logan, Utah, 1957. Ellis. M.M., B.A. Westfall, and M.D. Ellis, “Determination of Water Quality,” Research Report No. 9. U.S. Fish and Wildlife Service, U.S. Dept. of the Int., Washington. D.C., 1948. Wallen, I.E., N.C. Greer. and R. Lasater, “Toxicity to Gambusia affinis of Certain Pure Chemicals in Turbid Waters.” Sewage and Industrial Wastes, 29:695-711, 1957. Simonin, P., and A. Plerron, “Toxicite Brute des Derivis Fluores,” Comptes Rendus, 124:133-136, 1937. Hemens and Warwick, loc. cit.

50. Sigler and Neuhold, ibid.

51. Ellis et al., loc. cit.

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53. Simonin and Pierron, loc. cit.

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55. Cameron, J.A., “The Effect of Fluoride on the Hatching Time and Hatching Stage in Rana pipiens.” Ecology, 21:288-292, 1940. Kuusisto, A.N., and A. Telkka, “The Effect of Sodium Fluoride on the Metamorphosis of Tadpoles,” Acta Odontologica Scandanavica, 19:121-127, 1961. Kawahara, H., and K. Kawahara, “Pretiminay Report on the Influence of NaF Solution Upon the Early Development of Toad Embryos.” Tokushima J. Exper. Mod.. 1:98-104, 1954. (Extended abstract appeared in Fluoride, 4(4):167-171, Oct. 1971.)

56. Kuusisto and Telkka, ibid.

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58. Oelschlager. W., “Fluoride Uptake in Soil and its Depletion,” Fluoride, 4(2):80-84, 1971.

59. Chemical and Engineering News, June 3, 1974.

60. Garber, PC. “Fluoride Uptake in Plants,” Fluoride Quarterly Reports, 1(1):27-33, 1968.

61. Gisiger, L., “The Solubility of Various Fluorine Compounds in Soil,” Fluoride Quarterly Reports, 1(1):21-26, 1968. Macuch et al., loc. cit.

62. Macintire, W.H., “Air Versus Soil as Channels for Fluoric Contamination of Vegetation in Two Tennessee Locates,” In McCabe, L.C., ed., Proceedings, U.S. Technical Conference. on Air Pollution, Interdepartmental Committee on Air Pollution, Washington, D.C., 1950, pp. 53-58.

63. Gisiger. loc. cit.

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65. Bovay, E. “Fluoride Accumulation in Leaves due to Boron-Containing Fertilizers,” Fluoride Quarterly Reports, 2(4):222-223, 1969. Bolay, A., E. Bovay, J.P. Quinche, and R. Zuber, “Amounts of Fluorine and Boron in the Leaves and Fruits of Fruit trees and Vineyards, Fertilized by Certain Boron- and Fluorine-Containing Fertilizers,” Revue Suisse de Viticulture et Arboricultural, (Laussane), 3(3):54-61, 1971.

66. Gardner, loc. cit. Bovay, ibid.

67. Hadjuk, J., “Extension Growth in Seedlings as a Biological Test of Soils Contaminated with Fluorine Exhalates,” Biologia, 24(10):728-737, 1969. (In German; English abstract in U.S. EPA Pub No. AP-119, loc. cit.)

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69. Smith, R.L., Ecology and Field Biology, Harper and Row, N.Y., 1966.

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71. Hall, R.J., “The Distribution of Organic Fluoride in Some Toxic Tropical Plants,” New Physiology, 71:855-871, 1972.

72. Vickery, B., and M.L. Vickery, “Fluorine Metabolism in Dichapetalum toxicarium,” Phytochemistry, 11:1905-1909, 1972.

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76. Cheng, J.Y., M.H. Yu, G.W. Miller, and G.W. Welkie, “Fluoro-organic Acids in Soybean Leaves Exposed to Fluoride,” Environ. Sci. and Tech., 2:367-370, 1968.

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81. Peters, R.A., “Organic Fluorides in Plants,” Fluoride, 6(3):189-194, 1973.

82. Peters, J.A., and K.J. Baxter, “Analytical Determination of Compound 1080 (Sodium Fluoroacetate) Residues in Biological Materials,” Bull of Environ. Contamination and Toxic., 11(2):177-183, 1974.

83. Hall, R.J. and R.B. Cain, “Organic Fluorine in Tropical Soils,” New Phytology, 71:831-853, 1972.

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85. Sargent, D.F., and C.P.S. Taylor, “The Effect of Cupric and Fluorie Ions on the Respiration of Chlorella,” Canadian J. Botany 50:905-907, 1972.

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