Air quality is a measure of how clean or polluted the air around you is, based on the concentration of specific pollutants that affect human health. In the United States, it’s expressed as a single number called the Air Quality Index (AQI), which runs from 0 to 500. The lower the number, the cleaner the air. An AQI of 50 or below is considered good, while anything above 150 starts posing risks for everyone, not just people with existing health conditions.
The Six Pollutants That Define Air Quality
The EPA tracks six pollutants to determine air quality. These are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. Each one has a federally set concentration limit, and the AQI reflects whichever pollutant is highest in your area at a given time.
Two of these pollutants cause the most day-to-day air quality problems for most people: particulate matter and ground-level ozone.
Particulate matter refers to tiny particles suspended in the air, classified by size. PM10 particles (dust, pollen, mold) are small enough to inhale but tend to deposit in the upper airways of the lungs. PM2.5 particles, which are less than 2.5 microns across (roughly 30 times smaller than a human hair), travel much deeper and settle on the surfaces of the lung’s smallest structures. That deeper penetration is what makes PM2.5 the more dangerous of the two.
Ground-level ozone isn’t emitted directly. It forms when nitrogen oxides from car exhaust and industrial emissions react with volatile organic compounds in the presence of sunlight. This is why air quality tends to be worst on hot, sunny afternoons in urban areas, and why ozone alerts are far more common in summer than winter.
How the AQI Scale Works
The AQI is divided into six color-coded categories, each tied to a range of health concern:
- Green (0 to 50), Good: Air poses little or no risk. A great day to be active outside.
- Yellow (51 to 100), Moderate: Acceptable for most people. Those with asthma or other lung conditions may notice mild symptoms.
- Orange (101 to 150), Unhealthy for Sensitive Groups: Healthy people can still be active, but anyone with respiratory or heart conditions should avoid prolonged outdoor exertion lasting more than an hour.
- Red (151 to 200), Unhealthy: Everyone should limit strenuous outdoor activities to under an hour. People in sensitive groups should avoid unnecessary time outside.
- Purple (201 to 300), Very Unhealthy: Move activities indoors. Everyone should avoid prolonged outdoor exertion.
- Maroon (301+), Hazardous: Avoid all unnecessary outdoor activity.
These numbers are calculated from monitoring stations that use standardized sampling instruments to measure pollutant concentrations. The EPA designates specific methods for each pollutant. For particulate matter, for instance, air is pulled through a filter over 24 hours, and the collected particles are weighed. The resulting concentration is then converted to the 0-to-500 AQI scale so the public doesn’t need to interpret raw numbers in micrograms per cubic meter.
What Poor Air Quality Does to Your Body
In the short term, breathing polluted air irritates the eyes, throat, nose, and lungs. Common symptoms within hours of exposure include coughing, wheezing, a burning sensation in the chest, shortness of breath, and eye irritation. People with asthma often experience more frequent or more severe attacks on high-AQI days.
Long-term exposure is where the serious damage accumulates. Years of breathing elevated levels of fine particulate matter are linked to increased rates of lung cancer, cardiovascular disease, and chronic obstructive pulmonary disease (COPD). The numbers are striking: for every 10 micrograms per cubic meter increase in long-term PM2.5 exposure, lung cancer mortality rises 15 to 21 percent, and cardiovascular mortality increases 12 to 14 percent. Researchers believe sustained lung inflammation from particle exposure drives much of this cardiovascular risk, as the inflammatory response doesn’t stay confined to the lungs.
Who Is Most at Risk
Certain groups are more vulnerable to air pollution than others. Children breathe faster relative to their body size, pulling in more pollutants per pound. Older adults are more susceptible because lung function naturally declines with age. People with existing lung diseases like asthma, chronic bronchitis, or emphysema face compounded effects. And anyone who works or exercises outdoors for extended periods gets a higher dose simply through increased breathing rate, even if they’re otherwise healthy.
If you fall into one of these groups, the AQI threshold that matters for you is lower. The “orange” category (101 to 150) is specifically designed as a warning level for sensitive individuals, while most healthy adults won’t notice effects until the red range.
Global Standards and Real-World Gaps
The World Health Organization set its most recent guideline for annual PM2.5 exposure in September 2021, recommending a limit of 5 micrograms per cubic meter. That’s half the previous guideline of 10, reflecting growing evidence that fine particles cause harm even at very low concentrations.
The gap between that guideline and reality is enormous. Over 90 percent of the world’s population breathes air exceeding the 5 microgram target, and more than 75 percent exceeds even the older, more lenient standard of 10. In some regions, natural background levels from dust and wildfires alone push concentrations above the WHO recommendation, making it effectively unattainable even with aggressive pollution controls.
Practical Steps Based on AQI Levels
You can check your local AQI in real time through the EPA’s AirNow website or app, most weather apps, and local news forecasts. The number updates throughout the day, so checking in the afternoon matters more than in the morning, especially during summer when ozone peaks.
At green and yellow levels, you don’t need to change your routine. Once the AQI hits orange, pay attention to how you feel during outdoor activity. If your eyes water, your throat gets sore, or you feel short of breath, head inside. At red and above, shift workouts indoors, close windows, and run air conditioning or an air purifier with a HEPA filter if you have one. At the hazardous level, treat outdoor air as something to actively avoid.
Indoor air quality can be significantly better than outdoor air on high-pollution days, but only if you’re not pulling in outside air. Keeping windows shut and recirculating air through a filter makes a measurable difference, especially for PM2.5, which is small enough to stay airborne for hours and penetrate buildings through open doors and gaps in older construction.