When it comes to fluoridating drinking water, Ontario and Quebec couldn't be further apart. Ontario has the country's highest rate of adding the tooth-enamel-strengthening chemical into municipal supplies, while Quebec has one of the lowest, with practically no one drinking fluoridated water. But surprisingly, the two provinces have very little difference in tooth-decay rates, a finding that is likely to intensify the ongoing controversy over the practice of adding fluoride to water as a public health measure. Quebeckers have more cavities than people in Ontario, but the difference is slight. Among children 6 to 19, considered the most decay-prone part of the population, the rate in Ontario was lower by less than half a cavity per child. In the 6-11 age group, Ontario kids have 3.5 per cent fewer cavities than those in Quebec: 1.7 cavities compared to 1.76.