Reading an Air Quality Index: How dangerous is U.S. air from Canada's wildfires?

With hundreds of wildfires still burning in Canada, a large swath of the U.S. Northeast continues to suffer under hazy skies and compromised air into Wednesday.

As of late Tuesday, Quebec's forest fire prevention agency reported that more than 150 blazes were active, including more than 110 deemed out of control, the Associated Press reported. A hot, dry summer is expected for the province and beyond.

Related: Air quality worsens in U.S. as Canada faces toughest wildfire season on record

The U.S. Environmental Protection Agency said its Air Quality Index registers above 151 in some areas of the northeastern U.S., spreading down into the Mid-Atlantic region. The upper Midwest reported concerning issues to start the week as well. Once an Air Quality Index reading clears 100, it's typically a warning to people who have respiratory conditions, including asthma, to take precautions.

The EPA established an AQI for five major air pollutants regulated by the 50-year-old Clean Air Act. The agency takes readings at more than 1,000 air-quality stations around the country and includes special sensors activated by smoke in particular, for real-time readings.

Each of these pollutants measured by the EPA requires a standard deemed important to public health:

Especially during wildfire season, fine particles in soot, ash and dust can fill the air. And because it's nearly summer, the combination of smoke and hotter temperatures can generate more ozone pollution, which can aggravate respiratory issues.

Related: Cheery climate news? Cancer-linked ozone hole blamed on hairspray and A/C continues to close.

The EPA says to think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern.

For example, an AQI value of 50 or below represents good air quality for essentially all the population. A reading above 100 typically means that the outdoor air remains safe for most, but seniors, pregnant people and children are at increased risk. Those with heart and lung disease may also be at greater risk. And an AQI value over 300 represents hazardous air quality that will impact to some degree nearly everyone exposed to the air, even healthy people.

Because remembering the severity of number ranges may be challenging, EPA has assigned a color to each range, with green and yellow representing the most favorable conditions, and orange, red, purple and maroon reflective of levels that are progressively worse, topping out at maroon or readings between 301 to 500.

For comparison, the record-setting wildfire years of 2020 and 2021 meant that outdoor air near Portland, Ore., on select days produced an AQI above 400.

Visit the government-run Air Now site for the latest readings.

You can also examine longer-term air quality by select region.

The EPA and public health officials warn citizens against regular exposure to fire-impacted air, especially for outdoor workers, even if local readings aren't especially dangerous.

The effects of air pollution can be mild, like eye and throat irritation. But, for some, those effects turn serious, including heart and respiratory issues. And pollutants might linger longer than hazy, discolored skies persist, causing inflammation of the lung tissue and increasing vulnerability to infections.

Lingering particle measurements are picked up when the AQI tracks PM 2.5, which quantifies the concentration of particles smaller than 2.5 micrometers. When inhaled, these nearly undetectable particles can increase the risk of heart attack, select cancers and acute respiratory infections, especially in children and older adults.

Smokers, including those using vape pens, can invite added health risk with wildfire smoke exposure, say public health officials.

Read: Non-smoking lung cancer is on the rise. Blame pollution, says American Lung Association.

Read more at the EPA's air-quality guide for particle pollution.

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Rachel Koning Beals is a MarketWatch news editor in Chicago.