Canada could significantly influence U.S. policy by pushing the U.S. to clean up air pollution that blows across its northern border, American enviros said yesterday at a conference held by Ontario’s Medical Association. Fifty percent of Ontario’s air pollution originates in the U.S., said Jason Grumet of the Northeast States for Coordinated Air Use Management. Ontario recently asked for intervenor status in a U.S. dispute in which companies are fighting strict proposed regulations for nitrogen oxide emissions.