Tucson does not add fluoride to its drinking water and remains one of six major U.S. cities without optimally fluoridated tap water.

The government lowered the recommended level to 0.7 milligrams per liter on Monday. But most Tucson Water users — 709,000 people — receive water that averages 0.4 milligrams per liter of naturally occurring fluoride.

In addition to Tucson, cities without optimal fluoridation levels are Wichita, Kansas; Fresno, California; Albuquerque; Portland, Oregon; and San Jose, California, according to research by the American Dental Association and the CDC’s Division of Oral Health.

San Jose’s water district has voted in favor of fluoridating, but the process of retrofitting its complex system of water plants to bring fluoride to consumers is expected to take several years.

According to the American Dental Association, fluoride supplementation should be considered for children who drink water with less than 0.6 milligrams per liter.

Many health experts say that drinking water with below optimal fluoridation levels puts residents at an increased risk for cavities.

While 75 percent of Americans who use community water systems get fluoridated tap water, in Arizona it is lower than the national average. Fifty-seven percent of Arizona’s population gets fluoridated water.

Arizona cities and towns that add fluoride to their water supplies include Bisbee, Mesa, Chandler, Phoenix, El Mirage, Tempe, Gilbert, Yuma and Glendale.

Three major U.S. cities don’t add fluoride to their water, but they have naturally occurring levels at or above the 0.7 milligram-per-liter optimal level. Those cities are Colorado Springs, Colorado; Jacksonville, Florida; and El Paso.

A group of dentists in 1992 persuaded the Tucson City Council to fluoridate the local water supply in conjunction with the introduction of Central Arizona Project water. But the fluoridation never happened.

Adding fluoride to Tucson’s water now would require a directive from the mayor and council, and it would also mean higher water rates, city officials have said.