Air quality is considered “bad” when the Air Quality Index (AQI) rises above 100, the point where pollution starts affecting people’s health. Below that threshold, most people won’t notice any issues. Above it, the risks climb quickly, especially for children, older adults, and anyone with asthma or heart disease. You can check your local AQI in real time on sites like AirNow.gov, which reports conditions by zip code.
How the AQI Scale Works
The AQI runs from 0 to 500 and is color-coded so you can gauge risk at a glance. Each range corresponds to a different level of health concern:
- 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 mild effects.
- Orange (101 to 150): Unhealthy for sensitive groups. Children, older adults, and people with lung or heart conditions should limit prolonged time outdoors.
- Red (151 to 200): Unhealthy. Everyone may start to experience symptoms like coughing, throat irritation, or chest tightness.
- Purple (201 to 300): Very unhealthy. The entire population faces increased health risk.
- Maroon (301+): Hazardous. This is emergency-level pollution, and outdoor activity should be avoided entirely.
These numbers are calculated from five major pollutants: fine particulate matter (PM2.5), ground-level ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The AQI reflects whichever pollutant is highest at a given time, so a reading of 160 might be driven by wildfire smoke one day and ozone the next.
What Makes Air Quality Dangerous
The two pollutants most often responsible for bad air days are fine particulate matter and ground-level ozone.
Fine Particulate Matter (PM2.5)
PM2.5 refers to particles smaller than 2.5 micrometers, roughly 30 times thinner than a human hair. They’re produced by vehicle exhaust, power plants, wildfires, and industrial processes. Because they’re so small, they bypass the natural filters in your nose and throat and travel deep into your lungs. Once there, they irritate the tissue lining the air sacs and can trigger inflammation throughout the body.
These particles can also carry toxic compounds on their surface. When they reach the deepest parts of the lungs, they generate free radicals that damage cells and trigger inflammatory responses. Over time, this contributes to chronic conditions like heart disease, reduced lung function, and buildup of cholesterol in artery walls. The World Health Organization set its recommended annual PM2.5 limit at just 5 micrograms per cubic meter in 2021, a level that most of the world’s population exceeds.
Ground-Level Ozone
Ozone forms when pollutants from cars and industry react with sunlight, which is why bad ozone days peak during hot, sunny afternoons in summer. Even a few hours of exposure can cause coughing, throat irritation, chest pain when breathing deeply, and shortness of breath. The relationship between ozone and health problems is strongest during warm months, with much smaller effects in winter. People with poorly controlled asthma are especially vulnerable. Even a small bump in ozone levels can cause a significant spike in lung inflammation, with symptoms often worsening one to two days after exposure.
Nitrogen Dioxide
Nitrogen dioxide comes primarily from burning fuel, especially from cars, trucks, buses, and power plants. Short-term spikes can aggravate asthma and lead to coughing, wheezing, and difficulty breathing. Longer exposure over months or years may contribute to developing asthma in the first place and can increase susceptibility to respiratory infections. People who live near busy roads face the highest concentrations.
Short-Term Spikes vs. Long-Term Exposure
A single bad air day can cause immediate symptoms: burning eyes, scratchy throat, chest tightness, and worsening of existing asthma or heart disease. These short-term exposures are linked to increased emergency room visits and, during severe spikes, even premature deaths among vulnerable people.
Chronic exposure over years is a different kind of threat. It doesn’t always announce itself with obvious symptoms. Instead, it gradually damages lung tissue, accelerates hardening of the arteries, and reduces overall life expectancy. Research has found that children growing up near heavy traffic pollution can experience measurably impaired lung development, while long-term exposure in older adults is associated with cognitive decline. You don’t need to live next to a factory for this to matter. Years of breathing moderately elevated pollution add up.
Who Is Most at Risk
On orange AQI days and above, certain groups feel the effects sooner and more severely. Children are vulnerable because their lungs are still developing and they tend to spend more time playing outdoors. Older adults face increased risk of stroke and heart attack when pollution rises. People with asthma, chronic lung disease, or cardiovascular conditions are the most sensitive of all, sometimes reacting to pollution levels that wouldn’t bother a healthy adult.
Pregnancy also increases vulnerability, as does working outdoors for extended periods. If you’re in any of these categories, start paying attention at the orange level and consider staying indoors during red days or higher.
Your Indoor Air Might Not Be Much Better
Americans spend about 90% of their time indoors, and indoor air can actually be two to five times more polluted than outdoor air. Common indoor culprits include gas stoves and fireplaces (which produce carbon monoxide and particulate matter), mold, pet dander, volatile organic compounds from cleaning products and building materials, and even certain air fresheners.
During wildfire events or high-pollution days, keeping windows closed helps. But if your home has its own pollution sources, sealing it up without filtration can make things worse. A portable air purifier with a HEPA filter is the most effective indoor solution. To size one correctly, look for a Clean Air Delivery Rate (CADR) of about 65 cubic feet per minute for every 100 square feet of room space. A typical bedroom of 200 square feet needs a CADR of at least 130, while a 400-square-foot living room needs 260 or higher. These estimates assume standard 8-foot ceilings; higher ceilings need more capacity.
How to Protect Yourself on Bad Air Days
When the AQI climbs above 100, simple steps can significantly reduce your exposure. Limit vigorous outdoor exercise, since heavy breathing pulls more pollutants deeper into your lungs. If you need to be outside during very unhealthy or hazardous conditions, an N95 respirator filters more than 95% of fine particles in the PM2.5 size range. Standard surgical masks, dust masks, and bandanas do essentially nothing to block wildfire smoke or fine particulates.
Indoors, run your air purifier and keep windows shut. If you have central air conditioning, make sure the filter is clean and rated to capture fine particles. Avoid adding to indoor pollution by skipping candles, wood fires, and frying at high heat on smoky days. Running the exhaust fan over your stove also helps.
On moderate days (AQI 51 to 100), most people don’t need to change their routines. The air isn’t pristine, but it’s within a range that healthy lungs handle without trouble. The goal isn’t to live in a bubble. It’s to recognize when conditions cross the line from manageable to harmful and adjust accordingly.