San Francisco faced multiple days of poor air quality in recent months. Artur Debat / Getty Images
  • Smoke from wildfires have left millions of Americans trapped inside for days — sometimes weeks — on end during fire season.
  • When wildfires ignite, they release tiny particulate matter called PM2.5. 
  • These particles can hang in the air and when inhaled, travel deep in the lungs.

As wildfires rage on in the West, clean air has become somewhat of a precious resource. 

A clear, blue sky is becoming a rarer sight for people living in areas like Napa Valley, Denver, San Francisco, Los Angeles, Portland, and Seattle.

Smoke from wildfires have left millions trapped inside for days, sometimes weeks, on end. 

To figure out when it’s safe to go outside, people should check their local air quality index (AQI) levels, a measurement that tells us what the current nearby air quality is like. 

For many, reading and interpreting AQI is a bit of a guessing game. The AQI scale ranges from 0 to 500 and includes different tiers associated with various health risks for different groups of people.

Scientists are still uncovering all the short- and long-term health issues that come with inhaling wildfire smoke, but there’s enough evidence that shows a strong link between air pollution and respiratory and cardiovascular health problems. 

How poor air quality impacts our health

When wildfires ignite, they release tiny particulate matter called PM2.5. 

These minuscule particles hang out in the air, and when inhaled, can get lodged deep inside the throat and lungs, according to Dr. Stephanie Christenson, an assistant professor in the division of pulmonary, critical care, allergy, and sleep medicine at University of California, San Francisco. 

When that happens, the particles can cause a lot of adverse effects on our health. 

In the short term, people who inhale air with high PM2.5 levels may experience difficulty breathing, a cough, runny nose, eye irritation, sore throat, and heart palpitations. 

People with underlying health issues — like asthma, chronic obstructive pulmonary disease (COPD), or heart disease — have a higher risk, as PM2.5 can cause direct injury to the airways and lead to a lot of inflammation, exasperating these conditions and sending some to the hospital. 

Evidence suggests prolonged exposure to wildfire smoke can up the risk of pregnancy complications like high blood pressure, gestational diabetes, low birth weight, and preterm birth. 

Otherwise healthy people can feel the effects of the poor air quality, too. 

“Even a normal person, if you have enough particulate matter in the air and are breathing enough in, you can get a lot of symptoms immediately or within hours to days,” Christenson said. 

Here’s how to make sense of AQI

AQI charts — which look at the amount of PM2.5 and other pollutants like ozone in the air — are typically displayed in different tiers:

  • 0–50. The air is good. 
  • 51–100. The air is moderate, and people who are unusually sensitive may experience symptoms.
  • 101–150. The air is unhealthy for sensitive groups, like those with asthma or COPD.
  • 151–200. The air is unhealthy for people with conditions and sensitive groups; some people in the general public may experience health effects, too.
  • 201–300. Health alert; risk factors are increased for everyone.
  • 301+. The air is hazardous. 

AQI charts can be confusing to navigate, since the tiers are wide and the particles can impact different people in different ways, according to Christine Wiedinmyer, PhD, the associate director for science at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

The 0–50 range is the green zone. The air is clean and this is the safest window for people with underlying conditions to go out and exercise.

Above that tier is the yellow zone, which ranges from 51 to 100. 

“With 50 to 100, we usually think it’s probably OK to open your windows and go outside and exercise, but some people may be particularly sensitive,” said Christenson. 

In the 100 to 150 range — the orange range — it gets a bit riskier, especially for people with underlying conditions. 

“The common thought process is most people who are healthy are probably OK to go for a run,” Christenson said. 

Some people, even those who are healthy, might feel some symptoms minutes, to hours, to days after exercising outside in the 100 to 150 range, but it’s really different from person to person. 

This tier is a bit of a gray zone, according to Christenson, so you’ve really got to listen to your body. 

If you start to feel anything — which is not always shortness of breath but sometimes headache, fatigue, dizziness, or palpitations — your body is telling you it needs clean air. 

Above 150, the air is unhealthy for everyone. 

Quick activities such as going to the car to run an errand or taking the dog out should be OK, but you should really limit how much time you spend outdoors, said Christenson.

Spending more time outside, like an hour or two, or exercising outdoors, will probably be challenging and trigger some breathing issues. 

“Try to limit exerting yourself too much,” Christenson said of the 100 to 150 range. 

When the AQI spikes above 150, indicating the air is hazardous, everyone should avoid the outdoors and wear a N95 or P100 mask if you must go out. 

Play it safe because the long-term effects aren’t yet known

Now, here comes the most important thing to understand about AQI charts: They’re based on the short-term health effects, not the long-term risks. 

Scientists don’t have much data on the health issues caused by long-term exposure. 

This makes it hard to gauge what’s truly safe to do outside at moderate and higher AQI levels, especially as wildfires last longer and ignite more frequently compared to past years. 

“We don’t have good answers, like at what air quality index is it safe to go outside or do a hike or safe to exercise,” Wiedinmyer said.

While it might be OK to run outside when the air is moderate once or twice, doing so for days and weeks on end during wildfire season year after year could have major implications on your health down the road. 

What kind of implications? Again, it’s unclear — but based on studies looking at air pollution in general, health experts assume long-term exposure to consistently high AQI levels may increase a person’s risk for developing asthma, COPD, heart disease, and some cancers. 

It’s also thought that kids, whose immune systems aren’t fully developed, may experience diminished lung functioning later on in life, said Christenson. 

Wiedinmyer has studied people who cook with open fires in developing countries and found that regular smoke exposure is linked to premature mortality and cardiovascular issues. 

“The more you’re exposed, the worse things get,” Christenson said. 

What precautions can you take?

The number one thing to do is limit outside time when the AQI is high. 

“If I can smell it, I stay inside. If the colors are orange, I stay inside,” Wiedinmyer said of her own strategy. 

Pregnant people along with those who have asthma, respiratory problems, or heart conditions should take extra precautions. 

Close and seal the windows and doors and bring your exercises inside. 

Indoors, you’ll want to ensure the air quality is as good as possible, too, as air pollution can creep inside, especially in older rental units. 

A portable air purifier with filters, which can be bought online or made at home, can help clear the air in your home. 

Christenson also recommends cutting out other sources of pollution — candles, smoking, fires, even vacuuming, as it could kick up dust. 

Lastly, keep in mind that the areas directly surrounding the wildfires aren’t the only areas experiencing poor air quality. 

“We’ve seen smoke from California go all the way to the east coast and to the Atlantic,” Wiedinmyer said, adding that everyone could benefit from checking the AQI in their area.

It’s not just a local issue, as polluted air has the potential to travel far, from state to state and across the country.

The bottom line

For people living near wildfires, reading and interpreting air quality index (AQI) charts is a bit of a guessing game.

The AQI scale ranges from 0 to over 300 and includes different tiers associated with various health risks for different groups of people.

In general, the higher the AQI and the more you’re exposed to polluted air, the worse the health outcomes are. 

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