Canadian wildfires are causing poor air quality across parts of the U.S., posing a health danger to millions of people. Experts say the air is especially harmful for more vulnerable populations, including older people and those with lung or heart issues.

The best thing to do to protect yourself? Stay inside and take other precautionary steps to limit your exposure, experts say.

"The particulate matter that's in this haze is significant because it does irritate the bronchioles, or the small tubes that go down into your lungs and connect to the alveoli, which are the sacs that allow you to breathe," Dr. Bob Lahita, director of the Institute for Autoimmune and Rheumatic Disease at St. Joseph's Health, told CBS News. "That gets irritated in people without asthma, but if you have asthma, chronic obstructive pulmonary disease, congestive heart failure or anything that involves your lungs or even your heart, you should stay indoors."

While indoors, keep all windows and doors shut, according to AirNow, a government-run site on air quality data.

AirNow's guide on reducing smoke exposure also advises not to contribute to indoor air pollution either. 

"Do not burn candles or use gas, propane, woodburning stoves, fireplaces or aerosol sprays," the guide reads. "Do not fry or broil meat, smoke tobacco products or vacuum. All of these can increase air pollution indoors."

Does wearing a mask help with bad air quality? 

If you need to be outside, experts advise wearing a mask.

N95 masks are the best option to reduce your exposure to pollutants, according to Lahita, but if you don't have those, surgical masks or even a scarf is better than nothing.

"Put a scarf over your nose and mouth so that the particulate matter does not go into your lungs," he says.

To avoid both large and small particles, the U.S. Environmental Protection Agency says dust masks aren't enough.

"Paper 'comfort' or 'dust' masks — the kinds you commonly can buy at the hardware store — are designed to trap large particles, such as sawdust," the EPA's website reads. "These masks generally will not protect your lungs from the fine particles in smoke."

Do air purifiers help with smoke?

Using a portable air cleaner can reduce indoor air pollution, according to AirNow.

"Make sure it is sized for the room and that it does not make ozone, which is a harmful air pollutant," the orginzation advises.

Dr. David Hill, a pulmonologist in Waterbury, Connecticut, and a member of the American Lung Association's National Board of Directors, told the Associated Press that people, especially those with underlying lung or heart disease, "should consider investing in in air purifiers for their homes."

Do air conditioners bring in unhealthy outside air?

If you have filters on your home HVAC system, Hill suggests making sure they're up to date and high quality.

Hill also recommends running the air conditioning on a recirculation setting to prevent outside air coming in. 

You can also recirculate the air in your car if you need to drive anywhere.

"Reduce smoke in your vehicle by closing the windows and vents and running the air conditioner in recirculate mode," AirNow's guide says. 

But experts say to use common sense and keep any eye on air quality data for your area, and try to avoid leaving your home in particularly polluted times. 

"Take it easier during smoky times to reduce how much smoke you inhale. If it looks or smells smoky outside, avoid strenuous activities such as mowing the lawn or going for a run," the guide adds. "Smoke levels can change a lot during the day, so wait until air quality is better before you are active outdoors."

Vladimir Duthiers, Anne-Marie Green and The Associated Press contributed to this report. 

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• Wildfire Smoke
• Wildfires