A pedestrian walks along the roadside amid heavy smoggy conditions in New Delhi. Delhi is considered the world's most polluted megacity, with a melange of factory and vehicle emissions exacerbated by seasonal agricultural fires. Money Sharma/AFP via Getty Images hide caption
Commuters make their way down a smoggy road in Lahore, Pakistan in 2022. Extreme heat waves make air pollution, like smog, worse. Arif Ali/AFP via Getty Images hide caption
The Statue of Liberty is seen June 7 through a haze-filled sky from the Staten Island Ferry in New York. The smoke from Canadian wildfires that drifted into the U.S. led to a spike in people with asthma visiting emergency rooms — particularly in the New York area. Yuki Iwamura/AP hide caption
Poor indoor air quality can contribute to health problems. Letting in fresh air, keeping air filters changed and using air purifiers can help. skaman306/Getty Images hide caption
Canada's smoke blanketed the Midwest less than three weeks ago. Scott Olson/Getty Images hide caption
Fireworks burst on the National Mall above the Lincoln Memorial, Washington Monument and U.S. Capitol on July 4, 2022. J. David Ake/AP hide caption
An aerial drone display by approximately 500 drones hovers over the Garonne River in Bordeaux, France on June 23, 2023. Some U.S. cities are planning their own drone shows as an alternative to Fourth of July fireworks. Romain Perrocheau/AFP via Getty Images hide caption
People at a viewing deck of the Rockefeller Center gaze at the Manhattan skyline on Friday during heavy smog brought by wildfire smoke from Canada. Ed Jones/AFP via Getty Images hide caption
Smoky air is filled with microscopic flakes of particulate matter that can get into the lungs and even into the blood stream. Timothy A. Clary/AFP via Getty Images hide caption
New York topped the list of most polluted major cities in the world on Tuesday night, as smoke from the wildfires in Canada continues to blanket the East Coast of the United States. David Dee Delgado/Getty Images hide caption
A person cycles past the skyline in Philadelphia shrouded in haze on Thursday. Matt Rourke/AP hide caption
The sun rises over a hazy New York City skyline as seen from Jersey City, N.J., Wednesday, June 7, 2023. Intense Canadian wildfires are blanketing the northeastern U.S. in a haze, turning the air acrid, the sky yellowish gray and prompting warnings for vulnerable populations to stay inside. Seth Wenig/AP hide caption
EPA Administrator Michael Regan stands near the Marathon Petroleum Refinery in Reserve, La., on Nov. 16, 2021. The Environmental Protection Agency on Thursday awarded grants for projects to monitor air quality in 37 states. Gerald Herbert/AP hide caption
A recent study by the American Lung Association found that more Americans are living in areas with increased levels of air pollution. Los Angeles has recorded the worst levels of ozone for all but one of the past 23 years. Mario Tama/Getty Images hide caption
Classroom in Winston-Salem, North Carolina, United States Mint Images/Getty Images hide caption
Open windows and portable air cleaners can help improve ventilation, but many schools in the U.S. are more than 45 years old and need a complete overhaul of their HVAC systems. Michael Loccisano/Getty Images hide caption
Interstate 405 cuts through neighborhoods near the Los Angeles International Airport, seen here in 2017. A new study found links between modern urban air pollution and historical redlining at the national level. Reed Saxon/AP hide caption
A thick haze hangs over Manhattan on Tuesday. Wildfires in the West, including the Bootleg Fire in Oregon, are creating hazy skies and poor air quality as far away as the East Coast. Julie Jacobson/AP hide caption
A field hospital tent for suspected COVID-19 patient triage was set up outside the emergency department of Martin Luther King Jr. Community Hospital in Los Angeles, California. A surge in deaths has prompted the county to lift environmental limits on the number of cremations that can be performed each month. Patrick T. Fallon/AFP via Getty Images hide caption
A firefighter battles the Glass Fire in Napa Valley, Calif., on Thursday. Noah Berger/AP hide caption
Rows of pinot noir grapevines sit under a blanket of dense smoke from wildfires last week in Oregon's Arlyn Vineyard. Brett Curtis/Arlyn Vineyard hide caption
Travis Hull says two of his four kids – including Talon, now a high school student – have asthma. Eilís O'Neill/KUOW hide caption
Smoke from bushfires shrouds the Melbourne city skyline. Hazardous breathing conditions prompted Australian Open officials to suspend practice sessions on Tuesday. Daniel Pockett/Getty Images hide caption