Ottawa’s drinking water has been without cavity-fighting fluoride for extended periods in recent months because of a shortage of the chemical across North America.

For more than four decades, the city has added fluoride to its drinking water to prevent tooth decay in children, but lately cities have been struggling to buy the chemical as shortages have sent prices soaring.

The supply problem has been attributed to Hurricane Katrina damaging Gulf Coast manufacturing facilities two years ago, a downturn in the phosphate mining industry and the shutdowns of some American suppliers.

Ottawa’s fluoride comes from Jacksonville Beach, Florida.

In the past year, prices have doubled for the dwindling supply of fluoride, a byproduct of phosphate mining, said Dixon Weir, the city’s manager of drinking water services.

“There have been problems noted with the supply chain. We are aware that there are shortages that have been reported in other communities,” Mr. Weir said.

The water treatment plant in Britannia was forced to stop adding fluoride to its water between March 1 and May 23, he said. Another halt in fluoride supply also occurred there between June 20 and July 16. Interruptions also took place at the Lemieux Island water treatment plant for shorter periods in March and July, Mr. Weir said.

The long-term future remains uncertain for Ottawa’s fluoride supply, as cities across North America have also reported shortages or suspended adding the compound.

But in the short term, Ottawa’s fluoride supply is steady, Mr. Weir said.

“We’re not forecasting an exhaustion or any stopping of addition at this point,” Mr. Weir said, adding that the city can’t predict the long-term effects of the shortage.

The shortages have caused the price of fluoride in Ottawa to nearly double from last year, from $285 per tonne to $566 per tonne, or from the city spending $150,000 a year to $300,000.

The city reports to the medical officer of health and the Ministry of the Environment when a fluoride stoppage occurs, Mr. Weir said.

Fluoride protects tooth enamel against acids that cause cavities. Its use in water is safe, and it reduces tooth decay by about 30 to 40 per cent, according to Health Canada.

Kimberly Carriere, spokeswoman for the Alberta Dental Association, said going without fluoridated water for short periods is not likely to harm dental health.

“For long periods, you’d certainly see an increase in dental (decay) but you do have fluoride in your toothpaste, so the impact wouldn’t be that large in the short term,” Ms. Carriere said.

Ottawa residents approved fluoridation in a 1964 plebiscite, but Quebec cities such as Gatineau and Montreal rejected the idea. This spring, the Quebec government was urged to force Montreal to fluoridate its water supply and end what it described as “an epidemic of tooth decay” among the city’s preschoolers.

Some towns, including Kamloops, B.C., and Cobalt, Ont., have stopped fluoridating their water over the years.

Edmonton is the latest city facing a fluoride shortfall, possibly as soon as this fall if an expected shipment falls through.

“At this time, we have sufficient fluoride to meet Edmonton’s needs until the fall,” said Mike Gibbs, spokesman for Epcor, which supplies water to the region. “But it is a situation we’re closely monitoring and actually what’s happening is there’s a shortage in North America of fluoride.” Not everyone, however, thinks a shortage is a problem.

“It’s an excellent thing for them to be short of this chemical,” said Paul Connett, executive director of the U.S.-based Fluoride Action Network.

“If having a shortage of it makes them question the practice, that would be very, very good.” The Fluoride Action Network released a statement this week signed by more than 600 professionals — including more than 100 dentists — calling for an end to water fluoridation.

They say while using fluoride in toothpaste has proven effective in preventing tooth decay, putting it in the water supply has shown minimal benefits.

“It makes as much sense as swallowing sunblock,” said Mr. Connett.

His group says the detrimental effects of fluoride include an increased risk of bone fractures, lowered IQ, decreased thyroid function and a staining of teeth.

About 40 per cent of Canadians receive fluoridated water.