When the air outside looks hazy or a weather app flashes an orange or red warning, the Air Quality Index is what’s behind it. The AQI is a color-coded scale from 0 to 500 that tells you how polluted the air is and what that pollution could mean for your health. Higher numbers mean more pollution and greater concern.
The scale runs from good air at the low end to hazardous air at the top. Six color categories mark the waypoints. The number itself is driven by whichever pollutant is highest at that moment, not an average of them all.
What Does Unhealthy Air Quality Mean AQI Explained
“Unhealthy” on the AQI scale means the air has reached a level where some people can feel effects right away, and everyone may be at risk with enough exposure. It begins at an AQI of 151 and runs to 200, marked in red.
At this level, the official guidance is that everyone should reduce prolonged or heavy exertion outdoors. People with heart or lung conditions, older adults, and children should limit outdoor exertion and move activities indoors or reschedule them. This is the point where the air is no longer just a concern for sensitive groups.
It helps to understand what the number is measuring. The AQI is not a single substance. It is a reporting tool built from several pollutants tracked under the Clean Air Act: ground-level ozone, particle pollution (often called PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each pollutant, the Environmental Protection Agency has established breakpoints that convert a measured concentration into a common 0 to 500 scale. The reported AQI is the highest of those individual values.
That last detail is worth pausing on. A single pollutant can push the whole number up even when the others are low. On a hot, sunny afternoon, ozone can spike and drive the AQI into unhealthy territory while fine particles stay modest. On a smoky day from wildfire, fine particles do the driving. The number tells you how bad the air is, but the pollutant behind it changes what you are breathing.
What Do the Six AQI Colors and Levels Mean?
Each color band corresponds to a range of numbers and a set of health messages. Here is how the categories break down.
| AQI Range | Category | Color | What It Means |
|---|---|---|---|
| 0–50 | Good | Green | Air quality is satisfactory; little risk. |
| 51–100 | Moderate | Yellow | Acceptable, though a small number of unusually sensitive people may notice effects. |
| 101–150 | Unhealthy for Sensitive Groups | Orange | Sensitive groups may feel effects; the general public is less likely to be affected. |
| 151–200 | Unhealthy | Red | Some people may feel effects; sensitive groups may feel more serious effects. Everyone should reduce exertion. |
| 201–300 | Very Unhealthy | Purple | Health alert: everyone may experience more serious effects. |
| 301–500 | Hazardous | Maroon | Emergency conditions; everyone is at risk. |
The “sensitive groups” label is not vague. It refers to people with asthma, chronic obstructive pulmonary disease (COPD), heart disease, and other conditions that make the lungs or heart more vulnerable. It also includes older adults, children and teens, and pregnant women. Children take in more air relative to their body size and their lungs are still developing, which is part of why they are grouped here.
Why Does the AQI Number Change So Much From Day to Day?
Air quality is a moving target because pollution depends on weather, traffic, industry, and geography all at once. A still, sunny day can let ozone build up. A windy day can clear it out. A wildfire hundreds of miles away can send smoke into a city with no fire in sight.
Fine particle pollution, or PM2.5, is the pollutant most consistently linked to health effects, and it is small enough to travel deep into the lungs. Particles this size can come from combustion — vehicle exhaust, wood smoke, power plants — and from chemical reactions in the air. Because they are so small, they can stay suspended and drift far from their source.
Ozone works differently. It forms when sunlight reacts with certain pollutants from vehicles and industry, so it tends to peak on hot afternoons and in warmer months. This is why the same city can have clean mornings and unhealthy ozone by late afternoon in summer.
Indoor air is not automatically a safe haven either. Outdoor pollution can seep indoors, and some sources — cooking, candles, fireplaces — add to it. On high-AQI days, keeping windows closed and running a filter can reduce indoor exposure, though the evidence for specific devices varies.
How Does Air Pollution Affect Your Body?
The health effects of air pollution are well established, especially for the heart and lungs. Fine particles and ozone can irritate the airways, trigger inflammation, and worsen existing conditions like asthma and heart disease. Short-term exposure can cause coughing, shortness of breath, and chest discomfort, and can increase the risk of heart attacks and strokes in vulnerable people.
Long-term exposure is linked to more serious outcomes. Research has consistently associated sustained exposure to fine particle pollution with increased risk of cardiovascular disease and reduced lung function. The World Health Organization recognizes air pollution as a major environmental risk to health and has published guidelines for acceptable pollutant levels.
What the evidence shows clearly is that people with pre-existing heart or lung disease are more vulnerable. What is less certain is exactly how much a single bad day matters for an otherwise healthy person. The honest position is that short exposures carry some risk, but the size of that risk depends on the pollutant, the concentration, and the individual.
What Should You Do on an Unhealthy Air Quality Day?
The practical steps on a red or worse day are straightforward and grounded in public health guidance.
- Reduce prolonged or heavy outdoor exertion. A brisk walk is different from a long run or heavy yard work.
- Move exercise indoors or reschedule it to a time when the AQI is lower.
- Keep windows and doors closed if outdoor air is worse than indoor air.
- Use a high-efficiency filter in your home HVAC system or a portable air cleaner if you have one.
- People with asthma should keep rescue medication on hand and follow their care plan.
- Check the AQI before planning outdoor activities, especially for children and older adults.
These steps reduce exposure, but they are not a guarantee of protection. The evidence that specific indoor air cleaners meaningfully lower health risk is limited, even though they can reduce measured indoor particle levels. It is reasonable to use them, but not to assume they erase the risk of a bad air day.
Where Should You Check the AQI?
The AQI is reported by government air quality agencies and is widely available through weather apps, local news, and the EPA’s AirNow system. Many apps show a current number and a forecast, which helps you plan around a spike.
One clarification that often gets lost: the AQI is a local reading from monitoring stations, not a precise measurement of the air at your exact address. Pollution can vary block to block, especially near busy roads or industrial sites. Treat the number as a useful guide, not a guarantee of what is in the air where you stand.
The scale is also not the same everywhere. The United States uses the EPA’s 0 to 500 index. Other countries use different scales and breakpoints, so a number reported abroad may not mean the same thing as the same number at home. If you are traveling, check which system the local reading uses.
Frequently Asked Questions
What AQI level is considered unhealthy?
The “Unhealthy” category begins at an AQI of 151 and runs to 200, shown in red. At this level, everyone should reduce prolonged or heavy outdoor exertion.
Is AQI 150 bad?
An AQI of 150 sits at the top of the “Unhealthy for Sensitive Groups” range, just below the red “Unhealthy” threshold. Sensitive groups should limit exertion, while the general public is less likely to be affected.
What does it mean when the air quality is unhealthy for sensitive groups?
It means people with heart or lung conditions, older adults, children, and pregnant women may feel effects, but the general public is less likely to be affected. This is the orange range from 101 to 150.
Can you go outside when the AQI is unhealthy?
You can go outside, but you should reduce prolonged or heavy exertion and move strenuous activity indoors. People with heart or lung conditions should limit outdoor time and follow their care plan.
