More than 100 million Americans from Iowa to Maine started Thursday under air quality alerts as wildfire smoke from Canada continues to drift into the U.S.
The worst air quality was reported in a swath from eastern Iowa to western Pennsylvania. In some of those areas, officials said the air wasn't just unhealthy, but considered very unhealthy.
The wildfires have caused poor air quality in the U.S. off and on for weeks. Earlier this month, unhealthy air quality forced the cancellation of a number of outdoor events in the Northeast.
Air quality is gauged by measuring how prevalent fine particles, generally 2.5 µm or smaller, are in the air. That is then put on a scale from 0-500. This is known as the air quality index.
When the air quality index reaches 101, it is considered unhealthy for sensitive groups. When it reaches 151, it is considered unhealthy for all groups; and if it goes above 201, it is considered very unhealthy.
SEE MORE: How to further protect yourself from wildfire smoke while indoors
Chicago's air quality has been above 201 for the last three days. The air quality index in Cleveland exceeded 201 on Wednesday. Detroit's air quality index was an average of 200 on Wednesday.
The air quality index was 105 on Wednesday in Philadelphia and is expected to top 151 on Thursday.
"Fine particles can get deep into our lungs; they can bypass the body's defenses and even cross into the bloodstream, causing cardiovascular issues," said William Barrett, senior director of the Clear Air Initiative with the American Lung Association.
The U.S. Environmental Protection Agency offers tips on its website on what to do when there is smoke in the air.
The EPA recommends checking the air quality frequently as it can change throughout the day; wearing a mask that protects you from smoke; taking it easy while outside and avoiding strenuous activity; and rescheduling outdoor work tasks.
When indoors, the EPA recommends running central air conditioning to filter the air and using a portable air cleaner. It also recommends avoiding doing activities indoors that can kick up pollutants, such as vacuuming or frying foods.
You can check the air quality for your area on the EPA's website.
The forecast is calling for improving air quality throughout the Midwest and Northeast this weekend. However, with winds shifting, that will likely bring hot and humid conditions, which brings other health concerns.
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