Plumes of smoke are blanketing large swaths of the U.S. and parts of Europe , which presents risks for those breathing it.

The Midwest, Northeast, and Great Lakes regions of the U.S. have “very unhealthy” air quality, according to the Environmental Protection Agency, and experts say people should take precautions.

What counts as bad air?

The EPA’s Air Quality Index converts all pollutant levels into a single number. The lower the number, the better.

Anything below 50 is classified as “healthy.” Fifty to 100 is “moderate” while 100-150 is unhealthy for “sensitive groups,” and anything above 150 is bad for everyone. The Air Quality Index was around 160 in many parts of the Upper Midwest on Wednesday.

Sensitive groups include people with asthma, lung disease or chronic obstructive pu

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