Air quality alerts: Symptoms and signs of health complications to know

Air quality advisories were in effect for 110 million people in the United States on Wednesday morning, ranging from New Hampshire to South Carolina and covering the Northeast, including Boston, New York City and Philadelphia.

In the Northeast, the poor air quality is the result of wildfires raging in eastern Canada, sending smoke wafting into the U.S. In some areas, the smoke is so dense that people can smell it and the skies appear hazy. About 90 million people in the U.S. are impacted by the smoke.

Beyond the Northeast, high concentrations of ground-level ozone — also known as smog — are driving unhealthy air quality levels in a number of urban areas across the country.

Follow live coverage on U.S. air quality conditions and Canada's wildfires

Poor air quality can be caused by any airborne "irritant" — a particle or substance in the air that is harmful to a person to breathe in, according to Dr. Purvi Parikh, an allergist and immunologist at the Allergy & Asthma Network, an advocacy group for people with asthma, allergies and related conditions.

Some examples are air pollution, including from vehicles and carbon emissions, as well as rising ozone levels, she said.

Natural disasters, like wildfires, often cause short-term spikes in poor air quality as the smoke, which contains carbon monoxide and other dangerous chemicals, enters the atmosphere.

Especially concerning is small particulate matter — tiny particles in the air that measure less than 2.5 micrometers in diameter, or roughly 4% of the width of a strand of hair. These particles are small enough to be inhaled deep into the lungs and can enter the bloodstream.

Here are a few options for face masks if you need to be outside: Disposable , KN95 and N95 .

The Environmental Protection Agency uses the air quality index to report air quality. It ranges from 0 to more than 300, with levels 50 and below considered the healthiest. When levels exceed 150, the general population may start to experience symptoms. Over 200 is considered "very unhealthy."

Many of the health issues people see from poor air quality, in general, can overlap with health issues people see from wildfire smoke, said Dr. Wynne Armand, a physician at Massachusetts General Hospital and associate director of the MGH Center for the Environment and Health.

Air pollution from wildfire smoke can make breathing difficult for anyone, but especially for young children, older adults, pregnant women and people with asthma or other pre-existing respiratory conditions, she said.

Dr. Aida Capo, a pulmonologist at Hackensack Meridian Health in New Jersey, said on Wednesday that she's already seen an influx of patients because of the poor air quality, including patients with worsening symptoms of asthma or emphysema.

"It's an almost immediate effect," Capo said. "If you're outside for any length of time, your symptoms can start and can worsen quickly."

In the short term, wildfire smoke can cause irritation to the eyes, nose, throat and lungs, as well as an increased risk of respiratory infection. Studies have also found that short-term exposure to small particulate matter increases the risk of a range of cardiovascular and respiratory diseases.

Longer term, exposure to air pollution is associated with several chronic health conditions, including:

Smoke can be especially dangerous for pregnant women because they usually have diminished lung capacities due to their growing bellies, Parikh said. Exposure to air pollution during the first and second trimesters may also be associated with gestational diabetes, according to a study published in March.

Additionally, air pollution can harm a developing fetus and increase the risk of low birth weight, miscarriage and stillbirth. A global analysis found that air pollution likely contributed to nearly 6 million premature births in 2019.

While smoke itself is not an allergen, it can irritate the nasal passages and airways.

"If you have allergies on top of that, you’ve got two different things causing symptoms at the same time," said Dr. Stokes Peebles, an allergy and pulmonary specialist at Vanderbilt University Medical Center in Nashville.

Symptoms may also worsen depending on what's burning. If a person is sensitive to an environmental allergen, such as trees or grass, breathing in the smoke that results from burning that allergen may cause a reaction.

What's more, heat can cause pollen to rise and be airborne for longer periods of time, spreading those particles even hundreds of miles away, Peebles said. "They can travel farther than they would ordinarily because they go higher into the atmosphere."

An N95 mask can help block those particles for people especially prone to environmental allergens, Peebles said.

Experts advise checking air quality alerts regularly. AirNow.gov, the EPA's website, allows people to track air quality by entering their ZIP codes. Many smartphones have apps that also track air quality.

Brady Scott, a fellow at the American Association for Respiratory Care, a professional organization for respiratory therapists, recommended that people stay indoors as much as they can, with doors and windows closed. That includes for activities like exercise, which can cause stress on the lungs.

People with respiratory-related health conditions, including asthma, should monitor their symptoms closely, he added. They should also make sure their medications, like inhalers, are available or not expired.

"People know their bodies really well. If they see some changes they believe are related to bad air, perhaps they need to contact a physician or advanced practice provider," Scott said.

Capo, of Hackensack Meridian Health, recommended that people with asthma use their rescue inhaler 15 minutes before they go outside.

Parikh advised people to keep their homes well ventilated. People who need to go outside can wear a mask, such as an N95, she said.

"Believe it or not, masking just like we did with Covid can be helpful in acting as a barrier between you and reduce the amount of particulate matter that you breathe in," she said.

Armand advised against dusting or mowing your lawn on days when the air quality is poor. Burning candles or using a gas stove can also contribute to poorer air quality on these days.

People should seek medical attention if they experience a cough, trouble breathing, chest pain or wheezing or hear a whistling sound in the chest, Parikh said.

Indoor air filters can help reduce or remove pollutants, including small particulate matter from wildfire smoke.

Indoor air filtration, including HVAC systems (heating, ventilation and air conditioning) and portable air purifiers can also help scrub pollutants that may have traveled inside homes and other buildings.

People can purchase portable air cleaners with replaceable HEPA filters that strain out small particulate matter, or PM2.5. HEPA filters are also available for homes outfitted with central heating and cooling systems. California's Environmental Protection Agency recommends using an indoor air cleaner anytime the air quality index hits "unhealthy" levels, or if people see or smell smoke in the air.

But people can also make their own indoor air cleaners by attaching an air filter to a box fan with tape, brackets or a bungee cord. If window air conditioning units, HVAC systems or portable air cleaners are not available, the U.S. Environmental Protection Agency said "DIY air cleaners" can serve as "a temporary alternative to commercial air cleaners." The agency cautioned that DIY air cleaners should not be used routinely, and that concerns have been raised about the potential fire or burn risk involved if box fans overheat.

The EPA added that there is limited research on the effectiveness of DIY air cleaners, but a study published in July 2021 in the journal Aerosol and Air Quality Research found that low-cost filtration methods, including attaching a filter to a box fan, "can have significant benefit for filtering submicron smoke particles and may reduce exposure to PM2.5 during wildfire smoke events."

Absolutely, according to Parikh.

"Other mammals, they suffer from many of the same lung conditions that humans do," she said.

Scott said people should keep their pets — such as cats and dogs — indoors as much as possible.

"If pets are outside, they’re running around outside or if they’re spending most of their time outside, it seems like it would probably create some irritation in their breathing and their airways and lungs as well," he said.

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Berkeley Lovelace Jr. is a health and medical reporter for NBC News. He covers the Food and Drug Administration, with a special focus on Covid vaccines, prescription drug pricing and health care. He previously covered the biotech and pharmaceutical industry with CNBC.

Denise Chow is a reporter for NBC News Science focused on general science and climate change.