Canadian wildfire smoke is drifting across parts of the U.S., prompting doctors to warn that even healthy people may face risks from prolonged exposure to polluted air.
More than 180 wildfires are burning in Ontario alone, while nearly 850 are active across Canada, sending smoke into the Midwest and Northeast, according to NASA.
More than 20 states have been affected, prompting widespread air quality alerts from the Upper Midwest to the East Coast.
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The greatest concern comes from fine particulate matter (PM2.5), which is about 30 times smaller than the width of a human hair. According to the U.S. Environmental Protection Agency (EPA), these microscopic particles can penetrate deep into the lungs and even enter the bloodstream.
"The main concern is fine particle pollution, known as PM2.5," Dr. Fernando Safdie, director of endoluminal and airway surgery and a lung cancer specialist in Miami Beach, Florida, told Fox News Digital. "These particles are small enough to get deep into the lungs, and the smallest can pass into the bloodstream."
"Smoke also carries irritant gases," he added. "That combination is what triggers symptoms."
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The EPA considers an Air Quality Index (AQI) above 100 unhealthy for sensitive groups, while levels above 150 are unhealthy for everyone.
The most common effect of wildfire smoke is irritation to the respiratory system.
"Most people notice burning or watery eyes, scratchy throat, runny nose or coughing," said Safdie.
"It can progress to chest tightness, shortness of breath, wheezing, headaches, fatigue and a faster heartbeat."
For anyone with asthma, COPD or chronic bronchitis, smoke can set off a flare or make existing symptoms noticeably worse, the doctor cautioned.
Wildfire smoke can also strain the cardiovascular system, according to the American Heart Association. It has been linked to increased emergency room visits for ischemic heart disease, irregular heart rhythm, heart failure, pulmonary embolism and stroke.
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Short-term exposure to wildfire-related fine particulate matter can also worsen heart failure and contribute to premature death, the EPA notes.
Studies have linked long-term exposure to an increased risk of premature death, chronic heart and lung disease, lung cancer, cognitive decline and dementia, as well as pregnancy complications such as preterm birth and low birth weight, according to the EPA and recent research.
Children are at the highest risk because their lungs are still developing and they breathe more air for their body size, according to Safdie.
Older adults, pregnant women, and and anyone with a heart or lung condition, such as asthma, COPD or heart disease, are also especially vulnerable. Other high-risk groups include those with compromised immune systems and chronic kidney failure, the doctor said.
"Outdoor workers are exposed simply because of where they spend the day," Safdie added.
The doctor emphasized that outdoor air quality dangers may not always be visible.
"When you don't see the fires, you could still be exposed to the smoke and breathing that highly polluted air," he told Fox News Digital.
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The CDC and EPA shared the following protective measures to prevent smoke-related health risks.
Monitor air quality. Check air quality at AirNow.gov before spending time outdoors. Sensitive groups should limit outdoor activity when AQI exceeds 100, and everyone should reduce exposure at levels of 150 and higher.
Stay indoors when smoke levels are high. Keep windows and doors closed. If using air conditioning or central HVAC, set the system to recirculate to reduce the amount of smoky outdoor air entering the home.
Clean indoor air. Run a portable air purifier with a HEPA filter or use a highest-efficiency filter. Experts also recommend avoiding activities that worsen indoor air pollution, such as burning candles, smoking, frying food or vacuuming while smoke levels remain elevated.
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Wear an N95 respirator if necessary. If you must spend time outside when smoke levels are high, a well-fitting N95 respirator can filter out most harmful fine particles.
Reduce strenuous outdoor activity. Heavy exercise causes people to breathe faster and deeper, increasing the amount of fine particulate matter that reaches the lungs. Consider moving workouts indoors until air quality improves, experts advise.
"Don’t tough it out if breathing is genuinely hard," Safdie advised. "Get medical care if you have shortness of breath that doesn’t ease with rest, a cough that won’t settle, wheezing, chest tightness, dizziness or heart palpitations."
Those who have asthma or COPD should follow their doctor-provided action plan.
"Don’t wait if your rescue inhaler isn’t doing the job," Safdie said. "Call 911 for severe difficulty breathing, chest pain or confusion. When in doubt, get checked."
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Although short-term exposure is unlikely to cause long-term damage, "very intense" exposure can generate chronic issues, he said.
"For example, firefighters who are in the eye of the problem should definitely be very well-protected with the appropriate equipment, because the concentration of the smoke and the PM25 particles will be extremely high and could cause chronic problems," Safdie said.
"But for the general population, it is unlikely that they will have such an exposure. I think that it would be more of an acute problem."
