Some 98 million people were under air quality alert conditions Wednesday as smoke from hundreds of wildfires in Eastern Canada ravaged northern states. Here is what to know about the health hazards of the smoke.

At least 18 states issued air quality alerts Wednesday, warning people to avoid going outside or limit their activities as the smoke continued to drift south from Canada, bringing with it a dull, gray pallor that settled over many city skylines and caused many to wake to the smell of smoke. As of Wednesday afternoon, multiple United States airports had ordered ground stops for their flights due to the low visibility.

Kaelan Deese/Washington Examiner

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The U.S. National Weather Service issued an air quality alert Wednesday for much of the Northeast and Mid-Atlantic region. "Wearing a mask may help limit irritation for those suffering from respiratory illnesses," it said on Twitter.

Meanwhile, millions of residents in states stretching as far West as Ohio and Kansas and as far south as South Carolina were urged to limit their outdoor activity as much as possible. Others in the Northeast were urged to remain indoors and keep windows closed due to the dangerous air quality.

Why it matters

Wildfire smoke carries a mix of toxic air pollutants and particulate matter, including dust, soot, and ash, among other materials. Particulate matter accounts for roughly 80% of smoke from wildfires.

The particulate matter can make its way into the eyes — accounting for many complaints of irritation — but it can also travel into the airways and settle in the lungs, causing irritation of the airways and other adverse effects, especially for people with underlying health conditions.

Exposure to wildfire smoke is “basically the same as tobacco smoke, without the nicotine,” Dr. John Balmes, a pulmonologist and professor of medicine at the University of California, Berkeley, told the Washington Examiner in an interview.

“So, the same harmful particulate matter and gases that are in tobacco smoke are in wildfire smoke," Balmes added.

Those with asthma, chronic obstructive pulmonary disease, or other lung conditions should be particularly cautious about their behavior, as their conditions can be exacerbated by the particulate matter.

Fine particulate matter that is inhaled can make it deep into the lungs and even into the bloodstream, causing inflammation and affecting the immune system.

"After a wildfire, you’ll see increases in emergency room visits and hospitalizations for respiratory events — things like asthma exacerbation, COPD exacerbation, or bronchitis and pneumonia. You see that in the very young — up to age five — as well as the elderly, and anybody with a preexisting condition,” Dr. Mary Prunicki, director of air pollution and health research at the Sean N. Parker Center for Allergy and Asthma Research at Stanford University, told the Washington Examiner in an interview.

Instances of heart attacks and heart arrhythmia are more common after wildfire exposure, particularly in elderly populations, and exposure has also been linked to instances of sudden cardiac death.

“If you have ischemic heart disease — that's what causes heart attacks — you can have increased angina or chest pain related to wildfire smoke exposure that can actually even lead to a heart attack,” Balmes, the pulmonologist and professor at UC-Berkeley, told the Washington Examiner.

Seth Wenig/AP

Pregnant women, particularly in their third trimester, should also be extra cautious about smoke exposure, as there is an association with low birth weight and premature birth. A 2021 study published in the Environmental Research Journal found that just one week of exposure to wildfire smoke was linked to a 3% rise in premature births.

Importantly, proximity doesn't matter when it comes to exposure or potential harm from wildfire smoke — meaning that those living in the U.S. many miles away from the wildfires can still be at risk, depending on the concentration of particulate matter.

“Unfortunately, because smoke can travel thousands of miles, you don’t have to be close to the fires [to be adversely affected] by wildfire smoke," Prunicki said.

"Smoke can blow thousands of miles. So it really doesn't matter if you're thousands of miles away, if that particulate matter is getting to you — depending on its concentration at that point — you can definitely be impacted," she added.

Those under air quality advisories should avoid going outside when possible, Balmes said.

Who is affected

Washington, D.C., New York City, and Philadelphia had air quality indexes ranging from the 150s to the 200s or 300s — ratings considered "unhealthy," "very unhealthy," or "hazardous" on the U.S. Air Quality Index, or the six-tier scale used by the EPA to measure air quality and particulate pollution in a given area.

Code Red includes air quality measuring between 151 and 200, according to the EPA. (For context, a “good” rating is defined as any value under 50.) The next tier, Code Purple, is measured at 201-301. A Code Purple is considered "very unhealthy" air conditions, with increased health risks for everyone, not just certain groups.

The sixth tier, Code Maroon, is an air quality index of 301 or higher. Conditions under that designation are hazardous to everyone, and those living in affected areas should both remain indoors and reduce activity levels.

As of mid-morning Wednesday, several cities in Pennsylvania, upstate New York, Delaware, and Washington, D.C. were under a Code Purple, with air quality index numbers surpassing 200.

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Syracuse, New York, and several surrounding cities recorded an air quality index of 400 or higher on Wednesday — a level NASA air quality scientist Ryan Stauffer described on Twitter as "unprecedented."

Air quality in Washington, D.C., New York City, and Philadelphia could get even worse in these three cities by tomorrow, according to modeling from the National Weather Service.