Why smoke from wildfires could cause a host of symptoms -- even in healthy people

Millions of Americans are at risk due to poor air quality conditions covering a large swath of the U.S. due to the wildfires burning in Canada.

Inhaling toxic smoke and ash from wildfires could weaken the immune system and cause damage to the body, including the lungs and heart, for anyone regardless of their health status.

Fine particulate matter, known as PM2.5, is 30 times smaller in diameter than a human hair and cannot be seen by the naked eye. Exposure to concentrated amounts of PM2.5 can cause both short-term effects such as irritation of the eyes, nose and throat; coughing, sneezing; and shortness of breath, and long-term effects such as worsening of conditions such as asthma and heart disease, according to the U.S. Environmental Protection Agency.

Fine particles are able to enter the body through the eyes and lungs. Not everyone feels the same symptoms, and the pollution can exacerbate existing health issues, such as asthma and allergies, Peter DeCarlo, associate professor of environmental health and engineering, told ABC News.

Prolonged length of time and amount of exposure could lead to lung inflammation and impaired lung function that lasts long after the wildfire has ended. Studies have also shown a link between poor air quality and cardiovascular disease, including strokes, heart attacks, heart failure and atrial fibrillation.

The smoke -- and fine particulate matter within it -- has billowed south from more than 100 fires burning in Quebec, bringing hazy skies and dangerous conditions that could affect the health of millions of residents in much of the Northeast, Midwest and even as far south as Georgia and Alabama.

PM2.5 particles found in wildfires are 10 times more harmful than the same type of air pollution coming from combustion activity, a 2021 study conducted in California found.

Typically when air quality is poor, alerts are issued for sensitive groups, including very young children, the elderly, pregnant women and those who are immunocompromised or have pre-existing conditions. But there is currently so much particulate matter in the air that the air quality is deemed unhealthy for the entire population in affected areas.

"Breathing high levels of air pollution affects everyone but it affects some groups disproportionately," DeCarlo said. "Like children with growing bodies and developing lungs, you definitely want to avoid their exposure -- and elderly people, whose bodies are in decline."

All populations should limit their time outdoors and limit exercise -- even indoors, because the fine particulate matter is able to filter inside as well. If it is necessary to venture outside, everyone -- especially vulnerable populations -- should wear a mask, ideally an N95, health experts advise.

"Minimize exercise and any activity that makes you breathe harder," DeCarlo said. "Obviously, the harder you breed the more air you're you're taking into your body with wildfire smoke to get in."

Residents currently under air quality advisories should also run their air conditioner or air filters, if they have them, to filter out as many particles as possible, DeCarlo said. Make sure to keep the fresh air intake closed and the filter clean to prevent bringing additional smoke inside.

Leaving the area may also be advisable for vulnerable populations to avoid heavy smoke for a prolonged period of time, health experts say.

Those who have asthma or other lung diseases should follow their doctor's directions about taking their medication and their management plans. Patients are also instructed to call their doctor should their symptoms worsen.

Lower socioeconomic neighborhoods are at a higher risk as well because they often do not have ability to get away from the smoke -- for both financial and social reasons, Kimberly Humphrey, a climate change and human health fellow at the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health, told ABC News.

Climate change and the role warming temperatures is playing in lengthening the annual fire season will likely make wildfire smoke a more prominent public health concern in the near future, Dr. Vijay Limaye, a climate and health scientist at the National Resources Defense Council, told ABC News.

ABC News' Mary Kekatos and Sony Salzman contributed to this report.