A healthy air quality index is 0 to 50, labeled “Good” on the EPA’s color-coded scale. At this level, air pollution poses little to no risk, and you can comfortably spend time outdoors without concern. Once the AQI climbs above 50, certain groups start to feel effects, and above 100, the air becomes a health issue for a wider population.
How the AQI Scale Works
The Air Quality Index runs from 0 to 500, divided into six categories. Each category is assigned a color so you can quickly gauge conditions in your area:
- 0 to 50 (Green, “Good”): Air quality is satisfactory. No health risk.
- 51 to 100 (Yellow, “Moderate”): Acceptable for most people, but unusually sensitive individuals may notice symptoms.
- 101 to 150 (Orange, “Unhealthy for Sensitive Groups”): People with asthma, older adults, children, and anyone exercising outdoors may experience respiratory effects.
- 151 to 200 (Red, “Unhealthy”): Everyone may begin to feel effects. Sensitive groups face more serious risk.
- 201 to 300 (Purple, “Very Unhealthy”): Health alert for the entire population.
- 301 to 500 (Maroon, “Hazardous”): Emergency conditions. Everyone is at risk of serious health effects.
An AQI of 100 is a critical threshold. It marks the upper boundary of “Moderate” and the point above which the EPA begins cautioning at-risk groups. Think of 100 as the line between “probably fine” and “start paying attention.”
What the AQI Actually Measures
The AQI isn’t tracking a single substance. The EPA calculates it for five major pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (both fine particles called PM2.5 and coarser particles called PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Whichever pollutant scores highest at a given monitoring station becomes the AQI value reported for that location.
Of these five, two dominate the readings in most U.S. cities: ground-level ozone (the main ingredient in smog, formed when sunlight reacts with vehicle and industrial emissions) and fine particle pollution. Fine particles, or PM2.5, are tiny enough to pass deep into your lungs and even enter your bloodstream. Exposure to PM2.5 is linked to heart attacks, strokes, and premature death, which is why it drives so many air quality warnings.
Who Needs to Worry First
The phrase “sensitive groups” comes up a lot in AQI forecasts. It refers to specific populations: people with lung diseases like asthma, older adults, children and teenagers, and anyone who spends long periods exercising or working outdoors. If you fall into any of these categories, an AQI in the Moderate range (51 to 100) is your signal to start monitoring how you feel, and anything above 100 is a reason to limit prolonged outdoor exertion.
Children breathe faster than adults relative to their body size, pulling in more pollutants per pound. Older adults are more vulnerable because their hearts and lungs handle the added stress less efficiently. And people exercising outdoors inhale more deeply and rapidly, delivering a bigger dose of whatever is in the air straight to their lungs. Being “active outdoors” doesn’t just mean running. It includes anyone doing physical labor, from construction workers to landscapers.
Tightening Standards for Fine Particles
What counts as “healthy” has shifted over time as research uncovers harm at lower and lower pollution levels. In 2024, the EPA lowered its annual standard for fine particle pollution from 12.0 to 9.0 micrograms per cubic meter, acknowledging that the previous limit was not adequate to protect public health. The administrator cited growing evidence that even modest, long-term exposure to PM2.5 causes cardiovascular and respiratory damage.
The World Health Organization goes further. In 2021, the WHO cut its recommended annual PM2.5 guideline from 10 down to 5 micrograms per cubic meter, the most aggressive target any major body has set. Most regions worldwide, including large parts of the United States, still exceed even the older 10 microgram guideline. The gap between these benchmarks matters: a day that registers as “Good” on the U.S. AQI scale might still exceed what the WHO considers safe for long-term exposure.
AQI Scales Differ by Country
If you’re checking air quality while traveling, keep in mind that not every country uses the same scale. China, India, and the European Union all have their own versions. China’s AQI, for instance, uses different methods to calculate particulate matter readings. A comparative analysis found that China’s system uses 24-hour average concentration breakpoints for particulate matter where the U.S. uses 1-hour averages, which can make pollution levels appear more severe. China’s breakpoints for PM2.5 are also less strict than the U.S. system when the index is below 200, meaning an AQI of 80 in Beijing does not represent the same air quality as an AQI of 80 in Denver. Always check which country’s scale a reading uses before comparing numbers.
Protecting Yourself When AQI Is High
When the AQI rises above 100, and especially during wildfire season or high-ozone days, a few practical steps make a meaningful difference.
N95 respirators are by far the most effective personal protection. Modeling research found that a properly fitted N95 reduces fine particle exposure by more than a factor of 14, meaning you’re breathing roughly one-fourteenth the pollution you would without it. Surgical masks, despite filtering over 90% of particles in lab settings, allow so much air to leak around the edges that they only cut exposure roughly in half. Cloth masks made from natural fibers perform similarly to surgical masks, offering a protection factor of about 1.9. The takeaway: if air quality is genuinely bad, an N95 is the only mask worth relying on.
Indoors, keeping windows and doors shut helps, but your home still isn’t airtight. An air purifier with a HEPA filter is the gold standard, but even a DIY setup can make a real difference. EPA research tested simple box-fan-and-filter designs for wildfire smoke and found that a single box fan fitted with a 4-inch MERV-13 filter and a cardboard shroud achieved a clean air delivery rate of about 248, enough for a 250-square-foot room. The general recommendation from the Association for Home Appliance Manufacturers is to match your purifier’s clean air delivery rate to the square footage of the room. A Corsi-Rosenthal box, which uses four MERV-13 filters taped around a box fan, reached a delivery rate of about 400, covering a larger living space for under $100 in materials.
How to Check AQI in Your Area
The EPA’s AirNow.gov website and app provide real-time AQI readings and forecasts for locations across the United States, updated hourly. Most weather apps also display AQI data. You can search by zip code to see which pollutant is driving the reading and whether conditions are expected to improve or worsen. On days when the AQI is forecast to exceed 100, many local governments issue air quality alerts through the same emergency notification systems used for severe weather.
Checking the AQI before outdoor exercise is especially useful in summer, when ground-level ozone peaks in the afternoon heat. The current federal standard for ozone is 0.070 parts per million averaged over eight hours. Morning workouts typically mean lower ozone exposure because the photochemical reactions that produce smog haven’t ramped up yet.