The AQI Is Falling. Is It Safe to Go Outside?

The smoke that blanketed the East Coast this week is beginning to clear, and you may be wondering if it’s safe to go outside without a mask or whether you should crack a window to air things out.

The answer is likely yes, it's safe to go outside — with a few caveats. But you might want to wait before opening your windows.

The good news is, the Air Quality Index across much of the Northeast is now registering at under 100, putting most places in the “good,” or “moderate” category. But to make smart decisions about your health, it’s helpful to understand what those AQI numbers actually mean.

The categories are pegged to the National Ambient Air Quality Standards, which are set by the Environmental Protection Agency and cover various major pollutants. The Clean Air Act requires that these standards protect public health with an "adequate margin of safety.”

When you check the AQI on the EPA’s website or your weather app, what you’re seeing is a measure of five pollutants: particulate matter, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. When the AQI is below 100, that means none one of those pollutants are present at a concentration above the levels deemed protective by the EPA. But when any of the pollutants ticks above those levels, the AQI will change to “unhealthy for sensitive groups,” (orange), “unhealthy” (red), “very unhealthy” (purple), or “hazardous,” (maroon) depending on how bad it is.

For example, the standard for PM2.5, the primary health hazard in wildfire smoke, is a 24-hour average of 35 micrograms per cubic meter. When the AQI is below 100, that means PM2.5 levels are below that concentration. At the height of the smoke crisis on Wednesday, average PM2.5 levels in New York City were at 326 micrograms per cubic meter. That brought the AQI into the “hazardous” range, under which the EPA advises:

Everyone should avoid all physical activity outdoors; people with heart or lung disease, older adults, children, and people of lower socioeconomic status should remain indoors and keep activity levels low.

By contrast, at this moment my AQI in Brooklyn is registering at 54, or “moderate.”

At that level, EPA advises: “Unusually sensitive people should consider reducing prolonged or heavy exertion.”

If you look at the average AQI for the month of June in the New York City metropolitan area over the last decade, it has always hovered around 70. By that measure, today is looking pretty good.

I spoke to Jon Samet, a pulmonologist and epidemiologist and the dean of the Colorado School of Public Health, about how to make sense of these numbers. He said that to some extent, it’s up to the individual to use their best judgment.

“If you go below 100, the world may not be risk free, but it's in a range where the risks have been found to be acceptable,” he told me. “The caveat I would offer is that for those people who have heart and lung disease, or any other condition that makes them potentially susceptible, then they should think carefully.” The same goes for children with asthma, who may be particularly susceptible to air pollution.

The bad news is that it’s basically impossible to make statements like, “when the AQI goes below X number, it’s safe to take your asthmatic kid to the playground,” because it’s an imperfect measurement.

“Part of the challenge right now is that epidemiological studies show that even when you make your way below the standard, there's still evidence of adverse effects,” said Samet. “So far, we haven't found risk-free levels of air pollution.”

The best advice he could offer is that if the AQI is below 100, but you go outside and the visibility is poor, or you start coughing, or your eyes burn, that’s a signal to go back inside or pull out your N95 mask. “Pay attention to your own response,” he said.

Personally, looking at the historical averages in New York City makes me feel comfortable going outside without a mask.

As for opening windows to air out any smoke particles that made it inside, John Volckens, an air quality expert and professor of mechanical engineering at Colorado State University, told me it was a good idea to wait until we’re back in the “Good” zone. “While ‘moderate’ isn’t going to be harmful over the short-term for healthy individuals (i.e., those without respiratory or cardiovascular disease), most of the PM contributing to the moderate level will still be from wildfires, so you won’t really be ‘airing out’ your apartment at that point,” he said.

Guess I’ll have to wait a little bit longer to flush out my apartment — but in the meantime, I’m going for a walk.

Editor's Note: This article was updated to include new comments from John Volckens.