The Air Quality Index Explained: What It Means and How to Stay Safe

Here’s what we know about pollution and a few tips to protect yourself.

Smoky skies in downtown Portland, Ore., last September.
Smoky skies in downtown Portland, Ore., last September.Credit…Amanda Lucier for The New York Times
Adeel Hassan

By Adeel HassanJune 17, 2021

As dense plumes of wildfire smoke thickened the air in Salem, Ore., last September, residents could barely see down the street, the morning sky glowed an ominous shade of orange and the Air Quality Index soared to almost 500, the worst reading in the United States for the entire year.

But it is not only in such extreme conditions that air pollution can make it hazardous to venture outside: A reading of 100, for instance, serves as a warning to people who have respiratory conditions to take precautions.

So having an understanding of the Air Quality Index can help you protect yourself from the harmful effects of air pollution. Here’s a guide to how it works:

The Air Quality Index measures the density of five pollutants: ground-level ozoneparticulatescarbon monoxidenitrogen dioxide, and sulfur dioxide. It was established by the Environmental Protection Agency as a way to communicate to Americans the cleanliness of the air they are breathing each day.

The index runs from 0 to 500; the higher the number, the greater the level of air pollution. If it’s registering a number that is less than 100, then air pollution is below the level known to cause adverse health effects.

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When the index hits 101 or above, the outdoor air remains safe for many, but older adults and children are at increased risk. Those with heart and lung disease may also be at greater risk. A number above 200 is considered “very unhealthy.”

The index is also divided into six color-coded categories, with green and yellow representing the best conditions, and orange, red, purple and maroon indicating levels that are progressively worse.

Wildfires are a common cause of extended periods of unhealthy air. In 2020, smoke from wildfires pushed the index above 400 around Portland, Ore., while the levels in the San Francisco Bay Area hovered between 200 and 300.

But no city had a worse sustained reading last year than Salem, according to Robert Rohde, the lead scientist at Berkeley Earth, an independent organization focused on environmental data science.

AirNow is a website and app run by the E.P.A. It has a separate fire and smoke map that uses portable sensors to track smoke plumes, a useful tool to help you and your family avoid foul air during wildfires.

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Even if the level of daily air quality is not listed as dangerous, some experts say that you may still feel negative effects. The effects of air pollution can be mild, like eye and throat irritation, or serious, including heart and respiratory issues. They can also linger even after the air has cleared, as pollution can cause inflammation of the lung tissue and increase the vulnerability to infections.

During wildfire season, fine particles in the soot, ash and dust can fill the air.

The Air Quality Index tracks this pollution with a measurement known as PM 2.5, which quantifies the concentration of particles smaller than 2.5 micrometers. When inhaled, these tiny specks can increase the risk of heart attacks, cancer, and acute respiratory infections, especially in children and older adults.

Ash-covered foliage is seen last September in Silver Falls State Park outside Sublimity, Ore. During wildfire season, fine particles in the soot, ash and dust of wildfire smoke make up particle pollutants.
Ash-covered foliage is seen last September in Silver Falls State Park outside Sublimity, Ore. During wildfire season, fine particles in the soot, ash and dust of wildfire smoke make up particle pollutants.Credit…Kristina Barker for The New York Times

And some research suggests that wildfire smoke may be more toxic to the lungs than standard urban air pollution as it contains a distinct mix of particulates that activate inflammatory cells deep in the lungs while hindering other cells that can dampen the inflammatory response later.

Also during summer, the combination of smoke pollutants and hotter temperatures can generate more ozone pollution.

  • Stay indoors if you can, with the windows and doors closed.
  • If you have air conditioning, run it continuously, not on the auto cycle. It’s also helpful to close the fresh air intake so that smoke doesn’t get inside the house. If your system allows for it, install a high efficiency air filter, classified as MERV 13 or higher.
  • Portable air cleaners can also reduce indoor particulate matter in smaller spaces. Wirecutter, a New York Times company, has recommendations.
  • Avoid frying food, which can increase indoor smoke.
  • Avoid strenuous outdoor activities like exercising or mowing the lawn.
  • Don’t smoke cigarettes.
  • And though exercising outdoors can be a great way to stay healthy, the 101-150 range on the Air Quality Index is probably the highest level at which it remains safe to do so.

A little. An N95 respirator mask, worn properly, can filter out some of the particles. But it does little to protect against harmful gases in wildfire smoke, like carbon monoxide. The looser facial coverings some of us wear now to lower our risk of spreading or becoming infected with the coronavirus will provide negligible protection against wildfire pollutants.

According to E.P.A. data from 2019, about 82 million Americans — 25 percent of the population — lived in counties with air quality concentrations above the safe level for one or more of the air quality pollutants, like particulates and ozone. And researchers have found that people of color are disproportionately exposed to more pollution from nearly every source.

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The country’s average air quality index in 2020 was 40, according to IQAir, a Swiss air quality technology company. Twenty-two countries had cleaner air than the United States, and 83 fared worse.

Overall, though, there was less air pollution last year, with the coronavirus pandemic slowing down factories, industry and travel. In early June, cities in California were dominating IQAir’s index of the most polluted cities for the United States.

Covid-19 and smoke are a dangerous combination, as both affect the respiratory system, making those exposed to the virus more vulnerable.

Studies have also shown that in areas with poor air quality, people are more likely to die if they contract the coronavirus. And coughing, difficulty breathing and headaches are symptoms that both the virus and wildfire smoke exposure can cause, making it more difficult to know which may be the source.

Smoke can also make people more vulnerable to the coronavirus because the lungs lose some of their ability to fight infection.

Adeel Hassan is a reporter and editor on the National Desk. He is a founding member of Race/Related, and much of his work focuses on identity and discrimination. He started the Morning Briefing for NYT Now and was its inaugural writer. He also served as an editor on the International Desk.  @adeelnyt

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