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MILWAUKEE (AP) — The eerie glow of an orange sun through thick wildfire smoke might be unnerving, but the biggest risk from the haze is something much harder to see.

Tiny particles in the air can cause a range of short-term and chronic health issues, experts say.

Wildfire smoke can settle over cities for days — like the recent blanket over parts of the U.S. from burning fires in Canada, which has created hazardous air for millions of people across multiple states.

Health officials advise people to stay indoors — and to take steps to keep the air in your home clean.

How to safely run your AC

If you have central air conditioning, it should clean the air in your home.

Make sure filters are replaced on schedule and are of the highest level of filtration your system can handle. Filters rated MERV 13 or higher are ideal. Set the air to recirculate and close any vents that pull air from outside if you can. The University of Oregon’s Center for Wildfire Smoke Research and Practice recommends running the system’s fan through the entire smoke event, not just when cooling.

If you buy a portable air filter, look for a HEPA air filter or one that says it traps small particles of pollution. Ensure it does not generate ozone, another harmful air pollutant.

This article is part of AP’s Be Well coverage, focusing on wellness, fitness, diet and mental health. Read more Be Well.

If you can only afford one portable air filter, use it to create a “clean room” in your house where you can spend most of your time.

“The basic message is that something is better than nothing,” said Amy Kalkbrenner, an environmental epidemiologist at the University of Wisconsin-Milwaukee.

The science behind DIY air filters

There’s good evidence that DIY filters made by duct-taping furnace air filters to a box fan work.

They’re simple, relatively affordable and scientists at the Environmental Protection Agency have found they are just as good at removing fine particles of pollution from the air as a small commercial air purifier.

“The good news is they really work,” said Kalkbrenner.

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All you need to have to make one is: four 20-by-20-inch (50.8-by-50.8-centimeters) furnace air filters rated MERV 13 or higher, a box fan, cardboard and duct tape. The EPA’s website shows how you can modify the design to only use one or two filters. The fan should blow out, and the arrows on the filters should point in. Make sure everything is taped up air-tight.

Using your window AC and cooling your air safely

Many window air conditioners, mini-split heat exchangers and portable air conditioners don’t have the filters needed to remove fine pollution particles. But heat can also be dangerous.

“When smoke and heat occur together, the impacts on your health are even greater than when they occur by themselves,” said Jess Downey, a researcher at the University of Oregon’s Center for Wildfire Smoke Research and Practice

Downey said the ideal scenario is running the AC unit and using an air purifier or a DIY box fan filter to clean the air. Make sure your AC is well-sealed in your window frame, she added.

Make sure the window units are set to recirculate air. Portable units with a single hose should be used sparingly, according to the University of Oregon wildfire smoke center.

If you don’t have AC, any reprieve from hot, smoky air is important. Head to the library, shopping mall or other space with clean, cool air.

Ways to seal up a home to keep hazardous air out

Whether it’s a towel, some tape, or something else — plug areas where air could enter from outside.

“Being from Chicago, I think you can liken it to trying to keep the cold air out,” said Dr. Khalilah Gates, a lung doctor with Northwestern Medicine. “Any areas in which air can come in, consider sealing up those areas to reduce the amount that’s coming into the home.”

When smoke levels are high, experts recommend avoiding anything that burns — candles, fireplaces, even gas stoves — so plan ahead with some meal prep.

How wildfire smoke affects the body

Exposure to wildfire smoke can lead to shortness of breath, chest tightness, headaches, and a burning sensation in the nose and eyes.

“When you ever start to feel those symptoms and they’re not severe, it’s important to acknowledge, ‘Oh, my body is responding to this. I need to move away from it. I need to move inside,’” Gates said.

If you’re wheezing, can’t catch your breath or are in distress, get to the emergency room immediately, she said.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.