Ontario has filed an amicus brief in the ozone standards case now before the U.S. Court of Appeals for the District of Columbia Circuit in Washington, D.C.. Ontario has been a vocal opponent to the new U.S. ozone standards and has supported legal action against the U.S. Environmental Protection Agency (EPA).

The EPA has set new ambient air quality standards for ozone of 75 ppb reduced from the previous level of 80 ppb. The Canada-Wide standard is 65 ppb. Ontario submitted comments to the EPA in September 2007, urging the adoption of standards at least as stringent as the Canada-wide standards. The new, less stringent standards are contrary to the advice of the agency's own Clean Air Scientific Advisory Committee.

Transboundary air pollution is responsible for about half of Ontario's smog. Ontario wants stronger standards in the U.S. in order to reduce ozone levels in the province.