Cities and towns across North America are scrambling for supplies of fluoride - a chemical they add to their water systems to prevent tooth decay.

Ottawa has been without the additive for extended periods in recent months because of a global shortage.

The shortage has stirred debate over a decades-old practice, with critics flashing big, toothy grins at the prospect of cities stopping fluoridation, something they say does more harm than good.

"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."

Ottawa's fluoride comes from East Tampa, Fla., and its shortage is blamed on Hurricane Katrina damaging Gulf Coast manufacturing plants two years ago, a downturn in the phosphate mining industry, and shutdowns of some U.S. suppliers.

In the past year, prices have doubled for the dwindling supply of fluoride, a by-product of phosphate mining, said Dixon Weir, Ottawa's manager of drinking-water services.

Edmonton foresees a fluoride shortfall this autumn if an expected shipment falls through.

The city adds 0.8 parts of fluoride per one million parts of water. Edmonton has treated its water with fluoride since 1967.

U.S. cities as diverse as New Orleans, Portland, Me., and Greensboro, N.C., have reported shortages or had to temporarily suspend water fluoridation.

Fluoridation has long been promoted by dentists, and by the Canadian Dental Association, as a cost-effective and far-reaching way to lower tooth decay.

According to Health Canada, many studies show fluoridated water greatly reduces the number of cavities in children's teeth.

However, the practice has long had its detractors. The Fluoride Action Network released a statement this week signed by more than 600 professionals - including over 100 dentists - calling for an end to water fluoridation.

While putting fluoride in toothpaste has proved effective in preventing tooth decay, they say putting it in the water supply has shown minimal benefits. "It makes as much sense as swallowing sunblock," Connett said.

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.

Most Quebec municipalities, such as Montreal and Gatineau, have rejected the idea.

Ottawa residents approved fluoridation in a 1964 plebiscite.

This spring, a health coalition urged the Quebec government to force Montreal to fluoridate its water and end what it called "an epidemic of tooth decay" among the city's preschoolers.

Kimberly Carriere, spokesperson for the Alberta Dental Association, said going without fluoridated water for short periods isn't likely to harm dental health.

"You do have fluoride in your toothpaste, so the impact wouldn't be that large in the short term," Carriere said.