An orange sky and wildfire smoke above San Francisco.

How unhealthy is the air outside right now? You don’t need your own sensor to find out. Here’s how to find out how bad the smoke, pollen, and pollution in your local area—or anywhere else.

Understanding Air Quality Index (AQI) Numbers

These services all show Air Quality Index numbers. The higher the number, the more pollution in the air right now.

Note that different countries use different Air Quality Index systems. If you’re not in the USA, you will have to look up your country’s Air Quality Index standard.

In the USA, here’s what the numbers mean:

Color Air Quality Index (AQI) Level of Health Concern
Green 0 to 50 Good
Yellow 51 to 100 Moderate
Orange 101 to 150 Unhealthy for Sensitive Groups
Red 151 to 200 Unhealthy
Purple 201 to 300 Very Unhealthy
Maroon 301 to 500 Hazardous

Timely Crowd-Sourced Reports: PurpleAir

While there are a number of ways to get official numbers from standardized government sensors—and we’ll explain those in a bit—there’s one way to get more timely results.

PurpleAir shows more localized air quality numbers. They’re much more current but are crowd-sourced from sensors set up by individuals. Individual sensors in an area may not be positioned accurately. But, if you look at an area with multiple sensors, the average will show you a fairly complete picture.

Just head to the PurpleAir map and search for your local area (or another area you want information about.)

If a particular sensor is too high or low compared to its surrounding ones, you should ignore it. That said, the sensors in your area should point to an average reading that’s reasonably accurate in the moment. They may even suggest differences from neighborhood to neighborhood in your city.

AQI measurements on a PurpleAir map for Eugene, OR.

PurpleAir sensors show US AQI numbers even when they’re located in other countries, so you can look across the globe and compare readings in multiple countries, if you like.

Slower Official Numbers: AirNow and Smartphone Apps

You can also check the official numbers reported by your government. In the US, these numbers are provided by the EPA and its partners. They’re updated hourly and come from a fewer number of more precisely accurate sensors.

That’s convenient, but they won’t show you the most timely air quality details if it’s changing quickly or how the air quality differs from neighborhood to neighborhood in your city.

To find these numbers in the USA, Canada, and Mexico you can use the AirNow website. Plug in a location and you’ll see the air quality in your area. (For other countries, you will need to find a website with data for your country.)

AirNow showing "Hazardous" air quality.

AirNow also offers a map that shows air quality across North America.

An AirNow map showing poor air quality on the west coast due to wildfire smoke.

For convenience, you can quickly find these numbers via smartphone apps. On your iPhone, both the Maps and Weather apps show air quality information. You can see your local air quality information or view another area in the map and see its local air quality. (This is not available in all countries.)

On Android, you can ask Google Assistant about the air quality for your local area or another area. (Again, Google Assistant does not support this in every country.) You can also install a third-party app that displays this information.

Both Apple and Google’s apps show the same official information you’d find on AirNow and on government websites. For crowd-sourced information, turn to PurpleAir.

Air quality numbers shown in Apple Maps on an iPhone.

RELATED: How to Check Your Local Air Quality Index on iPhone or iPad

If you’d like to learn more, Wired did a good job of explaining the difference between the numbers on PurpleAir and AirNow and how they’re measured.

The numbers generated by the government are the canonical historical, official data for an area—but, with conditions changing quickly and varying from location to location even within a particular city, PurpleAir’s numbers may be more useful in the moment.

Profile Photo for Chris Hoffman Chris Hoffman
Chris Hoffman is Editor-in-Chief of How-To Geek. He's written about technology for over a decade and was a PCWorld columnist for two years. Chris has written for The New York Times and Reader's Digest, been interviewed as a technology expert on TV stations like Miami's NBC 6, and had his work covered by news outlets like the BBC. Since 2011, Chris has written over 2,000 articles that have been read more than one billion times---and that's just here at How-To Geek.
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