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

Warmer temperatures and bright sun often bring along an unwanted guest: poor air quality.

A measure called the Air Quality Index gauges just how bad the air has become in a particular place. Readings of 100 or higher usually serve as a warning to people who have respiratory conditions to take precautions.

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.

What is the Air Quality Index?

The Air Quality Index measures the density of five pollutants in the air: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide, and sulfur dioxide. It was established by the Environmental Protection Agency as a way to communicate to Americans the state of the air they are breathing each day. There are pollution monitors at more than 1,000 locations across the country.

How do you read the Air Quality Index?

The index runs from 0 to 500; the higher the number, the worse the air. If the index is registering less than 100, then the air pollution at that location is below the level known to cause adverse health effects.

When the index climbs above 100, the outdoor air may remain safe for many people, but older adults and children would be at some risk, as would people of any age with heart or lung disease. An. index number above 200 is considered “very unhealthy.”

Index readings fall into one of six color-coded categories; green represents the best conditions, yellow a bit less so, and orange, red, purple and maroon indicating progressively worse air.

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.

No city had a worse sustained reading in 2021 than Salem, Ore., where the index almost reached 500 — the top of the scale — according to Robert Rohde, the lead scientist at Berkeley Earth, an independent organization focused on environmental data science.

What is an air quality alert?

Officials put out air quality alerts to protect public health by notifying the public about unsafe air pollution levels. An Air Quality Index number over 100 will trigger an alert for vulnerable groups; levels over 300 will prompt alerts about very hazardous conditions for everyone.

What is the best way to track the latest pollution readings?

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

What are some of the harmful effects of poor air quality?

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, because pollution can cause inflammation of the lung tissue and can increase your vulnerability to infections. Even if the air quality is not listed as dangerous on a particular day, some experts say that you may still feel negative effects from pollutants ion the air.

During wildfire season, fine particles of soot, ash and dust can billow high in the air and be blown by the wind for hundreds of miles.

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

Some research suggests that wildfire smoke may be more toxic to the lungs than standard urban air pollution is, because the wildfire smoke contains a distinct mix of particulates that activate inflammatory cells deep in the lungs while hindering other cells from dampening the inflammatory response later.

During summer, the combination of smoke pollutants and hotter temperatures can also increase levels of ozone pollution.

How do I stay healthy when air quality is poor?

• Stay indoors if you can, with windows and doors closed.

• If you have air conditioning, run it continuously, not on the auto cycle, so the air will be passing through the system’s filter all the time. 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.

• Though exercising outdoors can usually 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.

Do face masks help?

A little bit. 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 that many people wear now to try to avoid catching or spreading the coronavirus will provide negligible protection against wildfire pollutants.

How is the air quality generally in the United States?

According to E.P.A. data from 2022, about 85 million Americans — about 25 percent of the population — lived in counties where one or more of the air quality pollutants, like particulates and ozone, tend to be present above the safe level. Researchers have found that people of color are disproportionately exposed to pollution from nearly every source.

The national average air quality index in 2023 was 38, according to IQAir, a Swiss air quality technology company. More than 100 nations had cleaner air, on average, than the United States, and 32 fared worse.