Thinking about going outside? Here’s what to know about the risks from breathing wildfire smoke.

Philadelphia skies looked clearer Thursday, but air quality experts warn that wildfire smoke from Canada continues to circulate and post a significant health risk.

This kind of poor air quality event is unlike anything the region has seen in decades, said Arthur Frank, a professor of environmental and occupational health at Drexel University’s Dornsife College of Public Health.

Even people without underlying health conditions can expect to feel the effects of the wildfire smoke in the air today, he said.

Here’s what to know about how poor air quality affects peoples’ health and what precautions to take.

Outside, people might feel irritation in their mucous membranes — such as the eyes. They’ll smell smoke in the air and might experience shortness of breath or difficulty breathing, Frank said.

The particles in the air — known as PM (or fine particulate matter) 2.5 — are particularly dangerous because they are small enough that they can travel deep into the lungs and the bloodstream, instead of being filtered out in the nose, throat and lungs like a larger particle would be.

To eradicate them, the body mounts an inflammatory response, Frank said, directing cells to fight off the effects of a foreign material in the body. That can cause blood vessels to constrict and irritate breathing tubes in the lungs.

People with lung conditions, such as asthma, and heart disease, will have worse reactions to particulates in the air because of the inflammatory reactions their bodies will produce. People with asthma should stay inside and monitor their symptoms, Frank said. And they should seek medical care if they’re getting short of breath and not responding to their normal medication.

People with heart disease have a documented higher risk of heart attacks during elevated air pollution events, Frank said. They, too, should not go outside and should avoid outdoor exercise.

Children are also at risk because they breathe more quickly than adults. So are older adults, because they tend to have more underlying health conditions than the general population.

And pregnant people should also be careful, Frank said. Many already have breathing difficulties later in pregnancy because the fetus is pushing up on their diaphragm. “They already have a compromised ability to breathe, and this might make it worse,” he said.

People who work outside, such as police officers, firefighters, and agricultural workers, are also at a higher risk for adverse effects from the smoke, he said.

Well-fitted N95 masks and KN95 masks are best, said Frank.

A surgical mask is not designed to keep out the kind of particles in the air today, and doesn’t seal tightly around the face like an N95, said Leah Lande, a pulmonologist at Main Line Health. But it’s better than nothing, she said.

The city regularly hands out free N95 masks at its health department resource hubs.

It’s best to keep young children inside, Frank says. “I wouldn’t take them outside in the conditions yesterday, and probably today,” he said Thursday.

It depends on the level of pollution and the individual person’s health.

Jamie Garfield, a professor of thoracic medicine and surgery at Temple University Hospital, urged people to try to minimize potential harm by staying inside when possible and taking precautions when they must go out.

“If you don’t have to be outside, don’t be outside,” she said. “And if you do have to go outside, wear an N95 mask.”

The antidote for polluted air is clean air.

Normally, it’s possible to take breaks from hazardous air. For instance, people may inhale some smoke while grilling or preparing a campfire, but can step away for a deep breath of fresh air.

“But today and yesterday, you can’t,” Garfield said Thursday. “Every single breath is going to be one that is like breathing in a campfire.”

Garfield warns against assuming that air quality is safe just because the sky looks clearer. Check air quality levels at AirNow.gov.

Garfield recommends wearing a mask outside and even inside a motor vehicle, if the air conditioner is pulling air from outside.

“My recommendation is just spend as much time as you can breathing clean air,” she said.

Frank has studied poor air quality in China, India and other countries for years. Some cities in those countries deal with much more frequent, chronic air pollution, which can shorten life spans, he said.

“We don’t have the same situation here,” he said.

But Philadelphia does have neighborhoods where chronic pollution is more widespread, leading to high levels of asthma in children. Doctors in those neighborhoods said they were particularly concerned about asthma patients this week.

Still, there’s little research on the long-term effects of minimal exposure to PM 2.5 for otherwise healthy people, said Lande, the Main Line Health pulmonologist.

“Most of the data that exists around wildfire smoke is studies that have been done in the immediate area where the wildfire is, which is not our situation,” she said.

Frank said he doesn’t expect lingering health effects for people without chronic lung and heart conditions.

But people should still take precautions to minimize their exposure to the air as long as air quality measures continue to be poor, and people with underlying health conditions should be particularly careful, experts say.

Climate change increases the likelihood of poor air quality events in the future, Frank said.

“We’re just at the beginning of fire season — there are fires in New Jersey, everyone is saying we’re behind on our rain,” he said. “Health effects are tied to our general environment.”

To prepare for another poor air quality event, stock up on N95 masks. Consider purchasing a home air purifier, if you can afford it, Frank says.

People at greater risk, such as those with lung conditions, may consider upgrading their air filtration system, ensuring their windows are sealed well, and checking their air conditioner filters, Lande suggested.