In pictures: Canadian wildfires impact US air quality
People look out at a hazy Manhattan skyline from the Rockefeller Center viewing deck Friday, June 30, in New York.
In pictures: Canadian wildfires impact US air quality
Smoke from wildfires in Canada shrouds the view of the Statue of Liberty on Friday in New York.
In pictures: Canadian wildfires impact US air quality
The setting sun is shrouded by smoke from Canadian wildfires in this photo taken from New York's 42nd Street on Thursday, June 29.
In pictures: Canadian wildfires impact US air quality
Haze hangs over downtown Pittsburgh and PNC Park as fans take their seats before a Major League Baseball game between the Pittsburgh Pirates and the San Diego Padres on June 29.
In pictures: Canadian wildfires impact US air quality
Traffic makes its way across the Chesapeake Bay Bridge in Stevensville, Maryland, on June 29.
In pictures: Canadian wildfires impact US air quality
Smoke and haze is seen from the Lincoln Memorial in Washington, DC, on Tuesday, June 27.
In pictures: Canadian wildfires impact US air quality
People in Chicago walk along the shoreline of Lake Michigan on June 27.
In pictures: Canadian wildfires impact US air quality
A Canadian soldier flies over a wildfire near Mistissini, Quebec, on June 12.
In pictures: Canadian wildfires impact US air quality
Wildfires burn in British Columbia in this aerial photo released by the BC Wildfire Service on June 9.
In pictures: Canadian wildfires impact US air quality
A jogger wears a face mask in Washington, DC, on June 9.
In pictures: Canadian wildfires impact US air quality
A man wears a protective face mask while walking through Times Square in New York on June 8.
In pictures: Canadian wildfires impact US air quality
An airplane takes off June 8 from Ronald Reagan Washington National Airport in Arlington, Virginia.
In pictures: Canadian wildfires impact US air quality
Traffic heads into Washington, DC, under hazy conditions on June 8.
In pictures: Canadian wildfires impact US air quality
Smoke from Canadian wildfires obscures the visibility in Pittsburgh on June 8.
In pictures: Canadian wildfires impact US air quality
Buildings in the Philadelphia skyline are shrouded in smoke on June 8.
In pictures: Canadian wildfires impact US air quality
A cyclist rides under a blanket of haze that was partially obscuring the US Capitol on June 8.
In pictures: Canadian wildfires impact US air quality
A woman in New York City wears a mask during the morning rush hour on June 8. The city saw slight air quality improvements, but levels were still considered "very unhealthy" for residents.
In pictures: Canadian wildfires impact US air quality
The One World Trade Center tower is seen in New York, shortly after sunrise on June 8.
In pictures: Canadian wildfires impact US air quality
A starting gate is unused at Belmont Park in Elmont, New York, on June 8.
In pictures: Canadian wildfires impact US air quality
Transit employee Shanita Hancle, left, hands out masks to commuters at a subway station in New York on June 8.
In pictures: Canadian wildfires impact US air quality
The Philadelphia skyline is shrouded in haze on June 8.
In pictures: Canadian wildfires impact US air quality
A smoky haze obscures M&T Bank Stadium in Baltimore on June 8.
In pictures: Canadian wildfires impact US air quality
Firefighters battle a wildfire in Evansburg, Alberta, on June 8.
In pictures: Canadian wildfires impact US air quality
A person on the New York City subway wears a mask as smoky haze blankets a neighborhood on June 7.
In pictures: Canadian wildfires impact US air quality
Workers chain up seats at Citizens Bank Park after the Philadelphia Phillies postponed a baseball game because of poor air quality on June 7. The New York Yankees also postponed a game that night.
In pictures: Canadian wildfires impact US air quality
Two men stand by the waterfront in Brooklyn, New York, on June 7.
In pictures: Canadian wildfires impact US air quality
Smoky haze affects the visibility of the Empire State Building in New York on June 7.
In pictures: Canadian wildfires impact US air quality
A man in Piermont, New York, attempts to photograph the sun obscured by smoke on June 7.
In pictures: Canadian wildfires impact US air quality
Smoke blankets the Lincoln Memorial Reflecting Pool and the National Mall on June 7.
In pictures: Canadian wildfires impact US air quality
A person in Fort Lee, New Jersey, talks on the phone near the George Washington Bridge on June 7.
In pictures: Canadian wildfires impact US air quality
Smoke obscures the view from the New York State Thruway, looking north from West Nyack on June 7.
In pictures: Canadian wildfires impact US air quality
People wear face masks as they walk in New York's Herald Square on June 7.
In pictures: Canadian wildfires impact US air quality
A woman jogs along the Hudson River as a smoky haze hangs over the New York City skyline shortly after sunrise on June 7.
In pictures: Canadian wildfires impact US air quality
A couple sits for lunch in Cincinnati on June 6. Smoke from the Canadian wildfires had drifted to the city, causing the air to appear hazy.
In pictures: Canadian wildfires impact US air quality
People at Toronto's CN Tower take photos of the smoky city on June 6.
In pictures: Canadian wildfires impact US air quality
The sky is discolored during a New York Yankees baseball game on June 6.
In pictures: Canadian wildfires impact US air quality
A smoky sky provides a muted backdrop June 6 at Rock Harbor in Massachusetts. Skies over Cape Cod were filled with smoke from the wildfires in Canada.
In pictures: Canadian wildfires impact US air quality
People in New York wear masks as they ride bikes on June 6. That morning, the city briefly had the world's worst levels of air pollution.
In pictures: Canadian wildfires impact US air quality
The Statue of Liberty is obscured by the air pollution in New York on June 6.
In pictures: Canadian wildfires impact US air quality
Wildfire smoke engulfs downtown Ottawa on June 5.
In pictures: Canadian wildfires impact US air quality
Smoke billows upwards from a planned ignition by firefighters who were tackling the Donnie Creek Complex wildfire south of Fort Nelson, British Columbia, on June 3.
In pictures: Canadian wildfires impact US air quality
Firefighter Jason Rock sprays hot spots in the Birchtown area while tackling wildfires in Shelburne County, Nova Scotia, on June 3.
In pictures: Canadian wildfires impact US air quality
An astronaut aboard the International Space Station took this photo of wildfire smoke near Shelburne, Nova Scotia, on May 29. Human-caused climate change has exacerbated the hot and dry conditions that allow wildfires to ignite and grow.
In pictures: Canadian wildfires impact US air quality
Smoke rises from a wildfire in Fort Nelson on May 27.
In pictures: Canadian wildfires impact US air quality
Firefighters stand on a truck while battling a blaze near Fort St. John, British Columbia, on May 14.
In pictures: Canadian wildfires impact US air quality
BJ Fuchs, a farmer who has lost some land and had to move his cattle due to the wildfires, stands in Shining Bank, Alberta, on May 11.