What is the Air Quality Index? Here’s How to Stay Safe

Hundreds of wildfires across Canada are polluting the air and making it hazardous for residents there to venture outside. But smoke from those fires has also drifted south, leading to air quality alerts in parts of New York, Connecticut, Massachusetts, Vermont and Minnesota. The resulting haze has even veiled the Manhattan skyline.

The Environmental Protection Agency said on Tuesday that many areas of the Northeast had unhealthy air, with Air Quality Index readings above 151. (A reading of 100 usually serves 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: 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 cleanliness 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 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.

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.

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

What is an air quality alert?

Air quality alerts are critical to protecting public health. They notify the public about unsafe air pollution levels. An Air Quality Index number over 100 triggers an alert for vulnerable groups. Levels over 300 mean very hazardous conditions for all.

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 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.

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

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.

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.

How do I stay healthy when air quality is poor?

• 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.

Do masks help?

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.

What is the air quality like in general 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 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.

The country’s average air quality index in 2022 was 37, according to IQAir, a Swiss air quality technology company. Thirty-two countries had cleaner air than the United States, and 98 fared worse.