Air quality is a measure of how clean or polluted the air around you is, based on the concentration of specific harmful substances. In the United States, it’s tracked using the Air Quality Index (AQI), a numerical scale from 0 to 500 where lower numbers mean cleaner air. The AQI is calculated from the measured concentrations of six major pollutants regulated under the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead.
How the AQI Scale Works
The AQI translates raw pollutant concentrations into a single number that’s easy to interpret. It’s broken 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, including people with asthma, heart disease, or lung conditions.
- Red (151 to 200): Unhealthy. The general public may start experiencing health effects.
- Purple (201 to 300): Very unhealthy. Health risk increases for everyone.
- Maroon (301 and above): Hazardous. Emergency-level conditions affecting the entire population.
When you check an air quality reading on a weather app or a site like AirNow.gov, the number you see reflects whichever pollutant is highest at that moment. A city might have an AQI of 85 driven primarily by ozone on a hot afternoon, or an AQI of 160 driven by particulate matter during wildfire season.
The Pollutants That Matter Most
Not all air pollutants behave the same way. Some are emitted directly from a source. Others form in the atmosphere through chemical reactions. Here are the ones most likely to affect your daily life.
Particulate Matter (PM2.5 and PM10)
Particulate matter refers to tiny solid particles and liquid droplets suspended in the air. They’re classified by size: PM10 particles are 10 microns or smaller in diameter (roughly one-seventh the width of a human hair), while PM2.5 particles are 2.5 microns or smaller. The size difference matters because it determines how deep the particles travel into your lungs. PM10 tends to deposit in the upper airways, while PM2.5 reaches the deepest parts of the lung tissue. From there, the smallest particles can enter the bloodstream.
Sources include vehicle exhaust, power plants, wildfires, construction dust, and industrial operations. PM2.5 is widely considered the most dangerous common air pollutant because of its ability to penetrate deep into the body.
Ground-Level Ozone
Ozone in the upper atmosphere protects us from ultraviolet radiation, but ozone at ground level is a lung irritant. It isn’t emitted directly. Instead, it forms when nitrogen oxides (from car exhaust and power plants) react with volatile organic compounds (from gasoline vapors, solvents, and industrial chemicals) in the presence of sunlight. That’s why ozone levels peak on hot, sunny days in urban areas, though they can also climb during colder months.
Other Criteria Pollutants
Carbon monoxide comes primarily from burning fuel in vehicles and machinery. Sulfur dioxide is released by coal-fired power plants and industrial processes. Nitrogen dioxide is produced by combustion engines and power generation. Lead exposure has dropped dramatically since leaded gasoline was phased out, but it still enters the air from metal processing and some aviation fuels.
How Poor Air Quality Affects Your Health
Air pollution triggers oxidative stress and inflammation in cells throughout the body. Over time, this creates the conditions for a wide range of chronic diseases. The list is long: cardiovascular disease, respiratory illness, lung cancer, diabetes, obesity, and neurological and immune system disorders all have established links to polluted air.
Fine particulate matter specifically can impair blood vessel function and accelerate the buildup of calcium deposits in arteries. For older adults, traffic-related air pollution can lower levels of HDL (the protective form of cholesterol), raising cardiovascular risk. Short-term spikes in nitrogen dioxide exposure have been linked to increased stroke risk in postmenopausal women.
The effects on development are particularly striking. Breathing PM2.5, even at relatively low levels, may alter the size of a child’s developing brain, potentially increasing the risk of cognitive and emotional problems in adolescence. Prenatal exposure to fine particulate matter has been associated with low birth weight, an increased risk of cerebral palsy, and ADHD-related behavior problems. Women exposed to high levels of PM2.5 during pregnancy, especially in the third trimester, may face up to twice the risk of having a child with autism. A long-term study spanning 2000 to 2016 also found an association between lung cancer rates and regional reliance on coal for energy.
Indoor Air Can Be Worse Than Outdoor Air
Most people spend the majority of their time indoors, where pollutant concentrations can be surprisingly high. Volatile organic compounds, the gases released by paints, cleaning products, adhesives, air fresheners, and building materials, are consistently found at levels up to ten times higher inside homes than outside. Thousands of household products emit these compounds, both during use and while sitting in storage.
Other indoor sources include tobacco smoke, gas stoves and heaters (which produce nitrogen dioxide and carbon monoxide), mold, and even automobile emissions that drift in from attached garages. Benzene, a known carcinogen, enters indoor air through tobacco smoke, stored fuels, and paint supplies.
Global Standards Are Getting Stricter
The World Health Organization updated its air quality guidelines in 2021, cutting the recommended annual limit for PM2.5 in half, from 10 micrograms per cubic meter down to just 5. The annual limit for nitrogen dioxide dropped even more sharply, from 40 to 10 micrograms per cubic meter. A new long-term ozone guideline was also introduced, recommending peak-season concentrations stay below 60 micrograms per cubic meter. These tighter limits reflect growing evidence that harm occurs at pollution levels once considered safe. Most of the world’s population lives in areas that exceed even the older, more lenient guidelines.
How to Monitor Air Quality Yourself
Free tools like AirNow.gov provide real-time AQI readings based on data from government monitoring stations. Most weather apps now include air quality data as well. For more localized information, consumer-grade air quality monitors use optical sensing, estimating particle concentrations by measuring how light scatters off particles in a small chamber. These devices work on the same basic principle as professional-grade monitors but use mass-produced sensors that aren’t as precisely calibrated. They’re most useful when calibrated to a specific pollution source, such as wildfire smoke, and Berkeley Lab research has found them genuinely useful for tracking indoor particulate levels during fire events.
Reducing Your Exposure
On high-pollution days, staying indoors with windows closed is the simplest step. But keeping indoor air clean requires some attention to filtration. If you have a central HVAC system, upgrading to a filter rated MERV 13 or higher makes a meaningful difference. A MERV 13 filter captures at least 50% of particles in the 0.3 to 1.0 micron range and 85% or more of particles between 1.0 and 3.0 microns. That covers most of the PM2.5 range.
Standalone air purifiers with HEPA filters go further. HEPA filters capture 99.97% of particles at 0.3 microns, and they’re even more efficient at both larger and smaller sizes. For a room-level solution during wildfire smoke or high-pollution events, a portable HEPA purifier is the most effective option available.
Beyond filtration, reducing indoor pollution sources helps: ventilating when using paints or cleaning products, avoiding air fresheners and aerosol sprays when possible, and never idling a car in an attached garage. These steps won’t eliminate exposure entirely, but they can substantially reduce the total amount of pollution your lungs process each day.