Public health officials are warning of worsening air quality across much of western North America, as fire season heats up amid a historic drought. In Canada, nearly 200 wildfires are burning across British Columbia. Among those forced to evacuate the flames Monday were 350 workers with the Trans Mountain oil pipeline expansion project. A thick pall of smoke has settled over the city of Vancouver, which had the worst air quality in the world on Monday. Meanwhile, Seattle is experiencing some of its worst air pollution in years. In Northern California, fire crews are battling the Mosquito Fire, which has burned nearly 65 square miles, threatening thousands of homes east of Sacramento. In Southern California, rescue crews pulled several trapped drivers from their vehicles Monday, after the remnants of a rare post-tropical cyclone moved into the region, triggering flash flooding and mudslides.