You can monitor air quality using a combination of free online tools for outdoor conditions and affordable sensors for the air inside your home. The key metric to understand is the Air Quality Index (AQI), a 0-to-500 scale where anything above 100 starts affecting sensitive groups and anything above 150 can bother anyone. Here’s how to track both outdoor and indoor air quality with confidence.
Understanding the AQI Scale
The EPA calculates the AQI based on five major pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each pollutant gets its own score, and the highest individual score becomes the overall AQI for that area.
The scale breaks down into six color-coded levels:
- Green (0 to 50): Good. Air pollution poses little or no risk.
- Yellow (51 to 100): Moderate. Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including people with asthma, older adults, and children.
- Red (151 to 200): Unhealthy. The general public may start experiencing symptoms.
- Purple (201 to 300): Very unhealthy. Health risk increases for everyone.
- Maroon (301 to 500): Hazardous. Emergency conditions.
For fine particle pollution (PM2.5), which is the pollutant most relevant during wildfires and smog events, a 24-hour average of 0 to 12.0 micrograms per cubic meter of air falls in the “Good” range. Once you hit 35.5 micrograms per cubic meter, you’re in “Unhealthy for Sensitive Groups” territory. Anything above 55.5 is unhealthy for everyone. These numbers matter if you’re reading raw data from a home sensor rather than a pre-calculated AQI score.
Free Tools for Outdoor Air Quality
The fastest way to check outdoor air quality is AirNow.gov, the EPA’s official platform. Enter your zip code and you’ll see the current AQI along with forecasts for the next day. Most weather apps also pull AQI data, so checking is as easy as checking the temperature. Apple Weather, Google Maps, and The Weather Channel all display air quality scores.
For more localized data, PurpleAir maintains a network of thousands of low-cost sensors installed by individuals and organizations across the country. These sensors fill geographic gaps between official EPA monitoring stations, which can be spaced far apart in rural or suburban areas. The tradeoff: consumer-grade sensors can incorrectly estimate pollutant levels compared to regulatory monitors. The EPA tested PurpleAir sensors at more than 70 locations and developed a correction equation to adjust the readings, making them more comparable to official data. The AirNow Fire and Smoke Map now integrates these corrected sensor readings, giving you an apples-to-apples comparison without needing to interpret raw data yourself.
In areas with no ground monitoring stations at all, satellite-derived estimates can fill the gap. Satellites measure the atmospheric column above a location, then mathematical models convert that data into surface-level air quality estimates. Several apps and platforms use this approach to provide forecasts in remote or underserved regions.
Choosing an Indoor Air Quality Monitor
Indoor air can be two to five times more polluted than outdoor air, and you can’t assess it by smell or visibility alone. A dedicated indoor air quality monitor gives you real-time readings for the pollutants that matter most inside your home.
At minimum, look for a monitor that tracks PM2.5 (fine particles from cooking, candles, dust, and outdoor air infiltration) and carbon dioxide (CO2), which rises with poor ventilation. Many mid-range monitors also measure volatile organic compounds (VOCs), the gases released by cleaning products, paint, new furniture, and building materials. There are no federal standards for acceptable VOC levels in homes, so most monitors display a relative scale from low to high rather than a specific safe threshold.
CO2 levels are a practical proxy for how well-ventilated a room is. Outdoor air sits around 420 ppm. A well-ventilated indoor space typically stays below 800 ppm. Once levels climb above 1,000 ppm, it generally means the room isn’t getting enough fresh air relative to the number of people in it. ASHRAE, the professional body that sets ventilation standards, notes that higher CO2 concentrations correspond to lower outdoor air ventilation rates and potentially increased risk of airborne pathogen transmission, though CO2 alone doesn’t capture the full picture of infection risk.
Where to Place Your Sensor
Placement matters more than most people realize. The EPA recommends positioning indoor sensors at breathing zone height, between 3 and 6 feet off the floor. Below 3 feet, readings can be skewed by ground-level effects like dust kicked up by foot traffic or splashing water.
Keep the sensor away from windows, doors, and HVAC vents. These spots introduce rapidly changing temperature and humidity, which can throw off readings. Air quality near an open window or a duct outlet reflects what’s coming through that opening, not the average conditions in the room. For the same reason, avoid placing sensors near high-voltage power lines or large electronics that could cause electronic interference. A bookshelf or side table in the middle of a room, away from direct airflow, is ideal.
If you’re monitoring multiple rooms, prioritize the bedroom (where you spend roughly a third of your day) and the kitchen (the biggest source of indoor PM2.5 in most homes).
Monitoring for Radon
Radon is an invisible, odorless radioactive gas that seeps into homes from the ground. It’s the second leading cause of lung cancer, and the only way to know your exposure level is to test. The EPA’s action level is 4 picocuries per liter (pCi/L). Above that, mitigation is strongly recommended.
Short-term radon tests run from 2 to 90 days and give you a snapshot. They’re useful for a quick check, especially during a home purchase. Long-term tests run for more than 90 days and provide a more accurate picture of your actual average exposure, since radon levels fluctuate with weather, soil moisture, and ventilation patterns. Continuous radon monitors are also available for permanent installation, giving you ongoing readings you can check anytime. If you’ve never tested your home, a short-term kit is a good starting point. They’re available at most hardware stores for under $20.
Building a Monitoring Routine
Checking air quality doesn’t need to be a daily chore, but a few habits make a big difference. Set up AQI alerts on your phone through the AirNow app or your default weather app so you’re notified when outdoor conditions deteriorate. This is especially valuable during wildfire season, high-ozone summer days, or if you live near industrial areas.
For indoor monitoring, glance at your sensor’s readings when you wake up and after activities that generate pollution: cooking on a gas stove, running a fireplace, using spray cleaners, or painting. If PM2.5 spikes above 35 micrograms per cubic meter or CO2 creeps past 1,000 ppm, open windows (assuming outdoor air is clean) or run an air purifier and exhaust fans. Over time, you’ll start to see patterns. Maybe your PM2.5 spikes every evening when you cook, or your CO2 climbs overnight in a bedroom with the door closed. Those patterns tell you exactly where to improve ventilation or filtration.
Most indoor monitors log data over weeks or months, letting you spot seasonal trends. Winter often brings worse indoor air because windows stay shut and heating systems circulate dust. Summer may bring outdoor ozone or wildfire smoke indoors. Reviewing this data periodically helps you adjust your habits and equipment before problems become chronic.