Air becomes unhealthy when concentrations of certain pollutants rise high enough to cause harm with normal breathing. The U.S. EPA tracks this through the Air Quality Index (AQI), a scale from 0 to 500. An AQI above 150 is classified as “unhealthy” for everyone, while readings between 101 and 150 are considered unhealthy for sensitive groups like children, older adults, and people with heart or lung disease. The pollutants behind those numbers come from a mix of sources, some obvious and some surprisingly close to home.
Fine Particles: The Most Widespread Threat
The pollutant most often responsible for unhealthy air days is fine particulate matter, known as PM2.5. These particles are less than 2.5 micrometers in diameter, roughly 30 times smaller than the width of a human hair. Their size is what makes them dangerous. While larger particles (PM10, under 10 micrometers) can irritate your airways and worsen respiratory disease, PM2.5 particles are small enough to slip past the nose’s filtering system, travel deep into the lungs, and reach the tiny air sacs where oxygen enters your blood.
Once lodged in lung tissue, PM2.5 irritates and corrodes the walls of those air sacs, impairing lung function over time. But the damage doesn’t stop there. These particles have a large surface area relative to their size, which means they can carry toxic chemicals along for the ride. Through the oxygen exchange in your lungs, those substances can enter your bloodstream and trigger inflammation throughout your body. This process generates reactive oxygen species, molecules that damage cells and are linked to cardiovascular disease, respiratory disorders, and lung cancer.
PM2.5 comes from vehicle exhaust, power plants, industrial emissions, wood-burning stoves, and wildfires. The World Health Organization tightened its recommended annual exposure limit in 2021, cutting it in half from 10 to just 5 micrograms per cubic meter of air, citing growing evidence that even low levels cause measurable health harm. Many cities worldwide still exceed even the older, less strict guideline.
Ground-Level Ozone and Summer Smog
Ozone high in the atmosphere protects us from ultraviolet radiation. At ground level, it’s a lung irritant and a major component of smog. Ground-level ozone isn’t released directly from any source. Instead, it forms when two types of pollutants, nitrogen oxides and volatile organic compounds, react together in the presence of sunlight. Cars, power plants, refineries, and chemical plants all emit these precursor chemicals.
This is why ozone-driven air quality problems tend to peak on hot, sunny afternoons in the summer months. The sunlight essentially acts as a catalyst, cooking those emissions into ozone. Cities with heavy traffic and strong sunshine are especially vulnerable. On high-ozone days, you may notice chest tightness, coughing, or shortness of breath even during moderate outdoor activity.
Carbon Monoxide: An Invisible Danger
Carbon monoxide (CO) is a colorless, odorless gas produced by burning fuel. Vehicle exhaust is the largest outdoor source, but it also accumulates indoors from gas stoves, furnaces, and generators. What makes CO especially dangerous is its chemistry: hemoglobin, the protein in red blood cells that carries oxygen, binds to carbon monoxide 200 to 300 times more readily than it binds to oxygen.
When CO latches onto hemoglobin, it doesn’t just block that one binding site. It also changes the shape of the hemoglobin molecule so that the remaining oxygen it carries gets held more tightly, making it harder for your tissues to actually receive that oxygen. The result is that even when your blood technically carries some oxygen, your muscles, brain, and organs are starved of it. At low levels this causes headaches and fatigue. At high levels it can be fatal.
Indoor Air Can Be Worse Than Outdoor
People often think of air quality as an outdoor problem, but many of the chemicals that make air unhealthy are concentrated inside your home. Volatile organic compounds (VOCs) are released by an enormous range of household products: paints, varnishes, cleaning sprays, disinfectants, air fresheners, moth repellents, adhesives, hobby supplies, stored fuels, and even dry-cleaned clothing. These chemicals off-gas while you use the products and, to a lesser extent, while they sit in storage.
Short-term exposure to VOCs can cause eye, nose, and throat irritation, headaches, dizziness, nausea, and visual or memory problems. Longer-term exposure raises the stakes. Benzene, found in tobacco smoke, stored fuels, paint supplies, and car exhaust that drifts in from attached garages, is a known human carcinogen. Methylene chloride, used in paint strippers and aerosol spray paints, is converted to carbon monoxide inside your body and causes the same oxygen-deprivation symptoms. Perchloroethylene, the primary chemical in dry cleaning, causes cancer in laboratory animals.
Ventilation makes a significant difference. Opening windows, running exhaust fans while using cleaning products, and storing chemicals in detached sheds or garages rather than living spaces all reduce indoor VOC levels.
Who Is Most at Risk
Air pollution doesn’t affect everyone equally. The EPA defines several sensitive groups who experience health effects at lower pollution levels than the general population. Children under 18 are vulnerable because their lungs are still developing and they breathe faster relative to their body size, taking in more pollutants per pound. Older adults (generally 65 and up) face higher risk because of natural declines in lung and heart function.
People with existing heart disease, lung conditions like asthma or COPD, and diabetes are also more susceptible. Research consistently shows that people with lower socioeconomic status bear a disproportionate burden as well, often because they live closer to highways, industrial facilities, or other pollution sources and have less access to air filtration or medical care. For these groups, the AQI threshold for concern starts at 101, not 151.
How the AQI Scale Works
The AQI translates raw pollution measurements into a single number that tells you how clean or polluted the air is right now. It tracks five major pollutants: PM2.5, PM10, ground-level ozone, carbon monoxide, and sulfur dioxide. The highest reading among those pollutants determines the overall AQI for your area.
- 0 to 50 (Good): Air quality poses little or no risk.
- 51 to 100 (Moderate): Acceptable for most people, though unusually sensitive individuals may notice effects.
- 101 to 150 (Unhealthy for Sensitive Groups): Children, older adults, and people with heart or lung conditions should reduce prolonged outdoor exertion.
- 151 to 200 (Unhealthy): Everyone may begin to experience health effects. Sensitive groups face more serious risk.
- 201 to 300 (Very Unhealthy): Health alert for the entire population.
- 301 to 500 (Hazardous): Emergency conditions. Everyone is likely to be affected.
You can check your local AQI in real time through the EPA’s AirNow website or app, most weather apps, and many smart home devices. On days when the AQI climbs above 100, limiting time outdoors, keeping windows closed, and using air purifiers with HEPA filters can meaningfully reduce your exposure.