NEW YORK, June 7 (Reuters) – More than a dozen U.S. states were under air-quality alerts on Wednesday as smoke from hundreds of wildfires burning in eastern Canada wafted south, casting a dull gray pallor over the skyline of New York and other big cities. Health authorities in 15 states from Vermont to South Carolina as well as Ohio and Kansas in the Midwest warned that fine matter in the atmosphere could exceed unhealthy levels and make breathing difficult for millions of residents. Washington, D.C., was also under an air-quality warning, according to the National Weather Service. People were instructed to limit time outdoors, while those with respiratory issues were advised to consider wearing a mask. The smoke is crossing the U.S. northern border from Canada, where wildfire season got off to an unusually early and intense start due to persistent warm and dry conditions. Canada is on track for its worst-ever wildfire season. The skies above New York and many other North American cities were a uniform gray, and the air smelled like burning wood. In many places, the early morning sun appeared as a small glowing orange disc. New York City’s skyscrapers, which can be seen for miles away on a clear day, were rendered nearly invisible. The city’s schools were open for class on Wednesday, although outdoor events and activities, including a middle school graduation, were canceled, postponed or moved indoors. Canadian authorities on Wednesday issued a starker air-quality warning for the residents of the country’s financial capital Toronto due to several raging wildfires that have burned through a record area this year. While Canadian wildfires are common in the country’s western provinces, the eastern province of Nova Scotia is experiencing its worst-ever season. The federal government has sent the military to the region.