PurpleAir Map: Air Quality From the Sensor on Your Neighbor's Roof

PurpleAir Map is a world map covered in colored dots, and every dot is a real air-quality sensor that somebody bought and bolted to their house. Zoom into a city during a wildfire and you can watch the smoke plume move down individual streets. Zoom in on a normal day and you can see which side of the highway you would rather live on.

It is free to look at, requires no account, and is one of the clearest examples of citizen sensing actually working at scale.

What is PurpleAir?

Official air-quality monitoring is excellent and extremely sparse. A metropolitan area of several million people might have a handful of regulatory-grade stations, which is fine for reporting a regional average and useless for answering “is the air bad on my block right now.”

PurpleAir sells a small, relatively cheap particulate sensor that anyone can mount outside and connect to their WiFi. The device measures fine particulate matter — PM2.5, the stuff from smoke, traffic and industry that gets deep into your lungs — and reports its readings to the network continuously. In exchange, the owner’s data goes on the public map.

The result is a network with a completely different shape from the official one: not evenly distributed, but dense exactly where people cared enough to buy a sensor. Wildfire-prone parts of the American West are practically wallpapered. Some cities have hundreds of sensors; some countries have a scattered dozen. Schools, community groups and local air districts have added their own.

That density is the whole point. Pollution is not uniform across a city. It pools near freeways, near ports, near industrial sites, in valleys during inversions. A regional average smooths all of that away. A map with a sensor every few blocks shows it directly, which is why community air-quality campaigns lean on this data so heavily.

The tradeoff is honesty about precision. These are optical sensors, not laboratory instruments, and they tend to over-read in high humidity. The map handles this by offering conversion options, including a correction developed by the US EPA specifically to make PurpleAir readings line up better with reference monitors. Use them.

What can you do with the PurpleAir map?

• Check your own street. Zoom to your address and see whether a sensor is nearby, and what it is reading right now.
• Watch a smoke event unfold. During a wildfire the map becomes the fastest available picture of where the plume actually is, hour by hour.
• Switch the averaging period. Toggle between short real-time windows and longer averages to distinguish a passing spike from sustained bad air.
• Apply a conversion. The EPA and other correction options adjust raw readings, which matters most in humid conditions.
• Click a sensor for history. Individual sensors expose their own trend charts, so you can see the daily rush-hour pattern or a week of degrading air.
• Compare indoor and outdoor sensors. Some owners run both, which is the most direct test of whether closing the windows and running a filter is doing anything.
• Look at other pollutants and conditions. Many sensors also report temperature, humidity and pressure alongside particulates.
• Add your own sensor. Buy one, install it, and your block becomes part of the map for everyone else.

Tips to get the most out of it

Turn on the EPA conversion before drawing conclusions. Raw readings on a foggy morning can look alarming and largely be water. The correction exists precisely because this failure mode is common and predictable.

Look at several nearby sensors, not one. A single dot showing something dramatic while its neighbors are calm is usually a faulty or badly sited sensor — mounted near a barbecue, a dryer vent, or a dusty driveway.

Match the averaging window to your question. “Should I go for a run in the next hour” is a ten-minute-average question. “Is this a bad week to have the windows open” is a 24-hour question. The same data answers both differently.

Watch it during an inversion, not just a fire. Cold, still winter mornings in valley cities produce some of the most striking maps of the year, and they are the pollution events people forget about.

Use it to make a case, carefully. Community groups have used dense sensor data effectively, but pair it with official monitor readings when it matters. Low-cost data plus reference data is much harder to dismiss than either alone.

Consider siting before you buy. A sensor in a sheltered corner or right above a road tells you about that corner or that road. Mount it in open air, away from obvious local sources, if you want it to represent the neighborhood.

If you like PurpleAir Map, also try…

• Windy: the wind layer explains where the smoke on the PurpleAir map is heading next.
• Blitzortung Lightning Map: another global map built entirely from volunteer-owned sensors.
• earth :: nullschool: global atmospheric flow, including particulate layers, on a spinning globe.
• Our World in Data: the long-run statistical picture of air pollution behind today’s readings.

More live measurements in Data & Research, and more atmosphere in Maps & Exploration.