Maps: Wildfire smoke forecast for the Midwest and northern U.S.

Hot summer days are getting stuffier for more than 100 million people in the U.S. as wildfire smoke drifts south and imposes dangerous risks to their health.

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Hundreds of wildfires burning in Canada have created a thick wave of smoke that’s shrouding swaths of the U.S. from Milwaukee to Washington, D.C., in unhealthy air.

As of Thursday, more than 800 wildfires are actively burning across Canada , with large clusters in Ontario near the border with Minnesota. A heat wave across North America is trapping the unhealthy air and carrying it south.

With smoky air comes certain health risks. The Environmental Protection Agency uses a measure called the Air Quality Index to assess how polluted the air is. Scores of 100 or more (out of 500) mean the air is unhealthy and could harm people’s health.

Some U.S. cities — including Chicago, Detroit, Milwaukee, Pittsburgh and Washington — had ratings of more than 200 and led the world in rankings for worst air quality on Friday, according to data from air pollution monitoring company IQAir. Parts of Michigan, Ohio and Wisconsin had AQI ratings in the 300s on Friday afternoon — levels high enough that experts advise avoiding outdoor activities.

In response, public health officials have issued air quality advisories and suspended some government operations. New York launched emergency operations Wednesday , offering free KN95 masks and opening shelter space. Due to deteriorating air quality, Philadelphia city officials suspended trash pickup and closed public pools Friday.

High levels of particle-laden air can exacerbate health concerns for those considered sensitive, including children, pregnant individuals, older adults and those with asthma or heart conditions. Higher levels are dangerous for any who breathe it, and people should be extra careful with wildfire smoke, said Suzanne Paulson, director of the Center for Clean Air at the University of California, Los Angeles.

“No particles are good to breathe, but wildfire particles seem to be more toxic than regular urban particles themselves,” Paulson told NBC News. “Whatever the [AQI] color is, consider that it’s the next color up if there’s a lot of wildfire smoke.”

Wildfire smoke consists of microscopic particulate matter, ash and trace minerals that, combined with water vapor, create the haze effect. Small particulate matter, called PM2.5, is the most dangerous because it can be deeply inhaled, penetrating the lungs and even entering the bloodstream. PM2.5 particles are smaller than a grain of sand and 30 times smaller than the width of a hair strand, said Serap Erdal, a professor at the University of Illinois, Chicago, and the project leader of Open Air Chicago , a citywide network of air sensors.

The best way to protect yourself is to stay indoors and avoid physical activity outside in areas with exceptionally high air quality indexes, experts say. Keeping children indoors limits their exposure to the pollutants and can reduce their level of activity, which decreases the amount of particles they breathe in, Paulson said.

Using an air purifier or setting your air conditioning on recirculate can also help recycle the air and clear out toxins, Erdal said. “Just because we’re indoors doesn’t mean we’re not inhaling outdoor air.”

When outside, wearing high-quality particle masks like N95 or KN95 can reduce exposure. A regular surgical mask can provide a slight barrier but will not filter out small particles, Erdal said.

“Although healthy people may recover quickly from wildfire smoke, many people can experience short-term, mid-term and long-term effects,” Erdal said. “It’s important that people take measures to protect themselves.”