Ontario will enact the toughest rules in Canada when its pesticide ban takes effect today, a move that could land the province in the legal crosshairs of an international trade dispute.

Ontario is joining Quebec in restricting the use of pesticides, but its rules go further by prohibiting the sale and cosmetic use of more than 80 ingredients and 250 products, with few exceptions, experts say.

Other provinces are considering similar restrictions to protect the environment and public health, including British Columbia, Prince Edward Island and New Brunswick, said Lisa Gue of the David Suzuki Foundation.

Science hasn't yet proven what effect, if any, pesticides have on our health, but given the risks, governments must err on the side of caution, Gue said from Ottawa.

"The lack of scientific certainty cannot be an excuse to delay action to protect health and the environment," she said.

"And that's exactly what Quebec, and now Ontario, have done."

Both provinces have even banned weed killer 2,4-D even though it has been deemed safe by Health Canada – a move that sparked a NAFTA challenge of Quebec's ban.

Dow AgroSciences LLC, which manufactures 2,4-D, has decided to sue the federal government and seek $2 million in damages, arguing that Quebec's rules violate Canada's trade obligations because it prohibits a product without any scientific basis.

The company wouldn't say whether it will go after Ontario's ban as well.

"Our legal action is focused solely on Quebec at this time, and I won't speculate on any other action that we might take in Canada," said spokeswoman Brenda Harris.

"What I do think is important is that when provinces or jurisdictions are considering these types of things, that they really look closely at science, and making sure that science is part of the process."

Ontario's environment minister said he isn't worried by the potential threat of a lawsuit.

"We think we're doing absolutely the right thing by banning pesticides from our laws and gardens, and we'll have to wait and see what the courts say about that," said John Gerretsen.

"But I'm quite confident that they will uphold the right of Ontario to take this kind of an action."

Experts, such as the Ontario College of Family Physicians, have warned that the long-term effects of exposure to pesticides can be devastating, especially to pregnant women and children.

But unlike Quebec, Ontario municipalities are forbidden from enacting tougher anti-pesticide rules.

When Premier Dalton McGuinty announced the ban a year ago, he mistakenly said towns and cities could have stronger bylaws if they wanted to – a move many applauded.

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Two weeks later, he admitted that he "screwed up," but shifted the blame to Gerretsen, who had failed to correct his boss in front of reporters.

The government's refusal to change the legislation angered some municipalities and health groups, as well as the Ontario New Democrats.

"I still think municipalities should be able to go beyond what the province put in place," said Peter Tabuns, the party's environment critic.

"The City of Toronto had rules in place that had gone beyond what's going to be enforced in Ontario."

Ontario will still allow pesticides to be used in farming, forestry or for health and safety reasons, such as controlling mosquitoes that can carry diseases like the West Nile virus.

Golf courses will also be able to use pesticides, but must meet certain conditions to minimize the effects on the environment.

CropLife Canada, a pesticide industry association, says lawns and gardens represent only about four per cent of the pesticide business across Canada.

Quebec, which announced its ban in 2003, prohibits the use and sale of 20 ingredients in lawn pesticides and has additional restrictions for their use outside public daycares and schools.

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