Smoke and ash from wildfires can travel thousands of miles.

Medical experts say breathing this smoky-air can cause multiple health problems including asthma attacks, heart attacks and strokes, and – with repeated exposure – lung cancer. One report shows that this unhealthy air could be affecting more Americans than ever before.

The American Lung Association says that more than 40% of Americans are experiencing more days of ‘very unhealthy’ and ‘hazardous’ air quality now. The report looked at Americans’ exposures to two types of air pollution: ozone, or ‘smog’, and particle pollution, or soot. It found that more than 63 million Americans are breathing soot-filled air, in large part due to increasing numbers of wildfires in the west. And not everyone is affected equally. People of color are 61% more likely than white people to live in a county with a failing grade for air quality.

Experts recommend that parents of children with asthma check the air quality index at airnow.gov and to avoid outdoor air pollution exposure when it reaches unhealthy levels.