(CNN) The deadliest wildfire in California history will hurt the health of people who may never have seen the flames.

Smoke from the Camp Fire, which has burned an area the size of Chicago, hangs heavy in parts of California, forcing schools to close and shutting down public transportation.

The air quality is so bad that San Francisco, Stockton and Sacramento became the world's three "most polluted cities" on Friday, worse than hot spots in China and India, and firefighters say it probably won't go away any time soon. They predict the wildfires won't be out until the end of the month

People in these areas should minimize the amount of time they spend outside, experts say

Exposure to wildfire smoke in the short term can irritate your eyes, your sinuses and your throat. It can make you cough or wheeze, give you a headache and stress your heart, even if you are generally healthy.

For children, who breathe faster and tend to take in more of the polluted air than adults, this can be especially aggravating to still-developing lungs. For the elderly, who typically have more chronic conditions than younger adults, the wildfires can exacerbate those conditions. People with heart or lung disease , as well as pregnant women , are also vulnerable to the health problems the smoke can bring.

Particle pollution in the smoke is what puts your health most at risk. Fires burn trees but also destroy homes and businesses; the Camp Fire has destroyed more than 10,500 homes. In those structures are chemicals, plastics, pesticides, metals and other hazardous materials that add to this tiny pollution.

The pollution is so tiny -- 1/20th of a width of a human hair -- that it can travel past your body's usual defenses , and instead of getting breathed out, it can embed in your lungs, causing serious irritation.

"What's in the smoke from wildfires is, milligram per milligram, more toxic than tobacco smoke, although I certainly don't advocate smoking cigarettes, either," said Matt Kadlec, a senior toxicologist with the Washington Department of Ecology who has been studying the impact of smoke from wildfires. "The nice thing about the lungs is, at least in healthy people, most of that pollution gets cleared out eventually. It might take about a month or so if you are healthy. If you have COPD or some other chronic respiratory illness, it can take longer."

The other problem is that these tiny particles can go into circulation in your body.

"We don't know enough long-term what the impact is if they get into your vessel walls, if it maybe can lead to early heart disease," said Dr. Brian Christman, a volunteer national spokesman for the American Lung Association and professor and vice chairman for clinical affairs at Vanderbilt University Medical Center.

Christman said research shows that exposure to smoke from wildfires can cause thousands of excess deaths a year and can lead to more hospital admissions for respiratory problems and cardiovascular issues.

"Definitely limit your time outside, and if you do have to go outside, the air tends to be a little better when it's cooler in the morning and gets worse during the day, but stay inside as much as you can," Christman said. "With the air quality numbers we are seeing right now, even normal healthy people are going to have trouble."

There are other things you can do to minimize the impact of this smoke on your health.

Keeping doors and windows closed helps , but the tiny particles can get into even well-sealed homes. To cut down on pollution, run central air conditioning if you have it, but keep the fresh-air intake closed and make sure the filter is clean. Also use a high-efficiency (HEPA) air-cleaning filter, but make sure you don't use one that makes ozone.

Washington Department of Ecology spokesman Andy Wineke said when that state's wildfires were bad this year, the agency advised people who couldn't get an air purifier with a HEPA filter to attach a standard furnace filter to a box fan to help clean the air.

Do not burn candles, fry or broil meat or even vacuum, according to the US Environmental Protection Agency. All these activities can increase indoor air pollution.

If you have to go outside , use an N95 or P100 respirator, available in most hardware stores. Other dust masks and bandanas don't work with this kind of pollution.

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Make sure the mask seals tightly to your face, because a gap means pollution can get in. The EPA says the masks don't work well for men with beards, because a good seal isn't possible with facial hair. If it's making it hard to breathe, talk to your doctor about using one.

In your car, make sure to keep the windows up and turn your air conditioner to the setting that recirculates air to reduce your exposure.

When the smoke clears, the EPA suggests airing out your home to reduce indoor pollution.

Because the number of fires has grown over the years, scientists say they will continue to study their impact on human health.

"I'm afraid the consequence of climate change mean that we are going to see more of these fires," Christman said. "This is going to be a growing concern for more of us in the future."