The fastest way to check outdoor air quality is AirNow.gov, the EPA’s free tool that reports a real-time Air Quality Index (AQI) score for your location. You can also check indoor air with consumer monitors or DIY test kits, though accuracy varies significantly depending on what you’re measuring. Here’s how to make sense of all your options.
What the AQI Score Means
The Air Quality Index is a standardized scale from 0 to 500 that tracks five major pollutants: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The reported AQI reflects whichever pollutant is highest at that moment. The scale breaks into six color-coded categories:
- 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. People with asthma, heart disease, or lung conditions may experience symptoms. Most others are fine.
- Red (151 to 200): Unhealthy. The general public may start to feel effects, and sensitive groups face more serious risk.
- Purple (201 to 300): Very unhealthy. Health risk increases for everyone.
- Maroon (301 and above): Hazardous. Emergency conditions where everyone is likely affected.
For context, most U.S. cities hover in the green-to-yellow range on a typical day. Wildfire smoke, heat waves, or industrial events can push readings into orange or red. If you see purple or maroon, limit your time outdoors and keep windows closed.
Free Online Tools for Outdoor Air
AirNow.gov is the primary government resource, pulling data from thousands of EPA-regulated monitoring stations across the country. You can search by city, by specific monitor, or use the interactive map to see conditions across a region. The site also offers a Fire and Smoke Map that overlays wildfire data with real-time PM and ozone readings, which is especially useful during fire season.
If you want alerts sent to you automatically, AirNow’s EnviroFlash email service notifies you when air quality in your area reaches a level you care about. There’s also an AirNow mobile app that provides the same data on the go. For international locations, IQAir and the World Air Quality Index Project aggregate sensor data from monitors worldwide, though readings from less-regulated networks can be less reliable than EPA stations.
How to Check Indoor Air Quality
Indoor air is a different challenge because no government network monitors it for you. Your main options are consumer electronic monitors, passive test kits, and professional inspections.
Consumer monitors are plug-in or battery-powered devices that display real-time readings for things like fine particulate matter (PM2.5), carbon dioxide (CO2), volatile organic compounds (VOCs), humidity, and temperature. Some higher-end models also detect formaldehyde. These devices give you a general sense of trends: you can see whether cooking spikes your particle levels, whether your CO2 climbs overnight in a closed bedroom, or whether a new piece of furniture is off-gassing chemicals.
That said, the EPA notes there is limited information on the accuracy and precision of low-cost monitors currently on the market. Factors like placement in the room, temperature, humidity, how long the device has been in use, and even the presence of multiple contaminants can all skew readings. Accuracy also varies across manufacturers and even within product lines from the same company. The EPA does not endorse or recommend specific consumer monitors for indoor use. So treat these devices as useful for spotting patterns and relative changes rather than as lab-grade instruments.
When DIY Test Kits Work (and When They Don’t)
Passive collection kits let you test for specific pollutants by exposing a collection device to your air, then either reading results yourself or mailing a sample to a lab. Their reliability depends heavily on what you’re testing for.
Radon is the standout success story for DIY testing. Short-term charcoal canister kits absorb radon over two to seven days and get mailed to a lab for analysis. When placed correctly under stable conditions, they provide a reasonable estimate of radon levels. The EPA recommends testing all homes and taking action at 4 picocuries per liter or higher. These kits typically cost under $20 and are widely available at hardware stores.
Mold settling plates, on the other hand, are the most commonly sold and arguably the least reliable DIY option. A settling plate gives you no baseline, no measured air volume, and no comparison to outdoor levels. You end up with a petri dish of mold colonies and no way to know whether what grew is normal for your region or a sign of an actual problem. Professional technicians use calibrated air sampling pumps that capture a precise volume of air, collect indoor and outdoor samples simultaneously, and identify mold to the species level. That comparison between indoor and outdoor counts is what actually matters. A settling plate simply cannot provide it.
Physical Signs Your Air May Be Poor
Your body can serve as a rough air quality detector. High concentrations of pollutants, whether outdoor or indoor, can irritate your eyes, nose, throat, and lungs. Common symptoms include coughing, wheezing, eye and throat irritation, a burning sensation in the chest, and a feeling of tightness or shortness of breath. If you have asthma, poor air quality often triggers more frequent or more severe attacks.
These symptoms overlap with colds and allergies, so pay attention to timing. If your throat irritation disappears when you leave the house and returns when you come back, that points to an indoor air issue. If you feel fine indoors but start coughing during outdoor exercise on a high-AQI day, that’s the outdoor air. Children are particularly vulnerable: persistent coughing, unusual sleepiness, trouble breathing, or loss of interest in eating or drinking during a poor air quality event all warrant a call to their pediatrician.
Key Pollutants Worth Tracking
PM2.5 (Fine Particulate Matter)
These are tiny particles 2.5 micrometers or smaller, about 30 times thinner than a human hair. They come from vehicle exhaust, wildfires, cooking, and burning candles or wood. PM2.5 is the pollutant most closely linked to serious long-term health effects because the particles are small enough to pass deep into the lungs and even enter the bloodstream. The EPA recently tightened its annual PM2.5 standard to 9.0 micrograms per cubic meter, down from the previous 12.0, reflecting stronger evidence that lower exposures still cause harm.
CO2 (Carbon Dioxide)
Outdoor CO2 levels sit around 420 parts per million. Indoor levels in a well-ventilated room typically stay under 1,000 ppm. Once concentrations climb above 1,000 to 1,500 ppm, you may notice drowsiness, difficulty concentrating, and headaches. CO2 itself isn’t toxic at these levels, but a high reading signals poor ventilation, which means other pollutants are also building up. Opening windows or adjusting your HVAC system usually solves the problem quickly.
VOCs (Volatile Organic Compounds)
These are gases released by paints, cleaning products, new furniture, adhesives, and building materials. Some VOCs cause short-term irritation, while others, like formaldehyde and benzene, carry long-term health risks with chronic exposure. Consumer monitors that detect “total VOCs” give you a combined reading rather than identifying specific chemicals, which limits how actionable the data is. If your monitor shows a persistent spike you can’t explain, a professional indoor air quality assessment can pinpoint the source.
Practical Steps for Better Readings
If you’re using AirNow or a similar outdoor tool, check it in the morning before deciding on outdoor exercise. Ozone levels tend to peak in the afternoon on hot days, while PM2.5 from wildfires can spike overnight and linger into morning hours. A reading taken at 6 a.m. may not reflect conditions at noon.
For indoor monitors, placement matters. Keep the device at breathing height, away from windows, doors, and cooking surfaces. Placing it directly next to a stove will give you dramatic but misleading numbers that don’t represent the air in the rest of your home. Let a new monitor run for at least 24 to 48 hours before drawing conclusions, since many sensors need a calibration period to stabilize their readings.
If you’re troubleshooting a specific concern like musty smells, unexplained respiratory symptoms, or visible mold, a professional indoor air quality inspection will give you far more reliable data than any consumer product. Professionals use calibrated equipment, control for outdoor baselines, and can identify specific contaminants rather than just flagging that “something is elevated.”