Large wildfires in western Canada and the western United States have burned hundreds of thousands of acres over the past week, forcing thousands of people to evacuate, sending thick plumes of smoke southeastward and compromising air quality. Many of the fires have erupted from pinpoint lightning strikes amid record-breaking heat and expanding drought.  Several massive blazes covering at least 100,000 acres are burning in Oregon and western Canada. With the smoke from the fires infiltrating the Rockies, air quality alerts were in effect Wednesday morning in most of eastern Colorado, including Denver; northern Wyoming, including Yellowstone National Park; and eastern Oregon. Much of western Canada is also under air quality alerts. Some smoke has even spread into the Midwest. Both Denver and Chicago have ranked among the top 10 most polluted large cities in the world since Tuesday, according to IQAir.com. [to access the full story a Washington Post subscription is required]