Smoke from devastating wildfires in Canada is blanketing a large swath of the Midwest and Northeast this week, causing cities across the region to issue air quality warnings.

The extreme levels of smoke mean that even able-bodied adults would be wise to take some precautions to protect their health. The increasing severity of wildfires across the continent—driven in part to climate change—means that even places where blazes aren’t burning will still suffer with the impacts.

More than 100 fires are burning out of control across Canada as of Wednesday, with hundreds more being monitored or battled. The smoke has drifted south and east, turning skies hazy from Minnesota to New York. Particularly dramatic images have emerged from Toronto, where commuters went to work on Wednesday morning under orange skies. The region is also dealing with a heat wave, with temperatures well above 90 degrees Fahrenheit in many areas and an even higher heat index.

On Wednesday evening, the air quality index in New York City topped out at 180, putting the city's air squarely in the “unhealthy” category as defined by the US Environmental Protection Agency. Other places were even worse off, with Duluth, Minnesota, seeing AQI top out above 500 (anything over 301 is labeled “hazardous” and considered unsafe for anyone). Smoky conditions are expected to worsen in parts of the Northeast US on Thursday, including New York.

The wildfire smoke blanketing the area contains microscopic particles of matter known as PM2.5—shorthand for particles that are smaller than 2.5 micrometers, or 30 times smaller than the width of a human hair.

Exposure to PM2.5 can trigger or worsen a number of medical conditions, especially in vulnerable populations. Nicholas Nassikas, a pulmonologist and assistant professor of medicine at Harvard Medical School, says that he would tell his patients with preexisting conditions, like asthma and lung diseases, to limit their time outside in such smoky conditions. Children “have a faster breathing rate—they just breathe more,” says Nassikas, while the elderly, who often have compounding conditions and may live in less well-ventilated ...