How Much Pollution Is In The Air Aqi Explained? Key Facts

You check the weather app, see an air quality number, and wonder what it actually means. The Air Quality Index, or AQI, is a scale from 0 to 500 that translates pollutant levels into one simple number. The higher the number, the more pollution is in the air and the greater the health concern. An AQI of 50 or below is considered good. Anything above 100 starts to affect sensitive groups, and above 150 the air is unhealthy for everyone.

What Does the AQI Actually Measure?

The AQI does not measure one single thing. It measures five major air pollutants regulated under the Clean Air Act, and it reports whichever one is highest at a given time and place.

Those five pollutants are ground-level ozone, particulate matter (both PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The EPA calculates a separate index value for each pollutant, then reports the highest one as the overall AQI. That highest value is called the “dominant pollutant.”

This matters because the number you see does not tell you which pollutant is driving it. On a hot sunny afternoon, ozone is often the culprit. On a smoky day from wildfires, PM2.5 usually dominates. Both can produce the same AQI number, but they behave differently in your body and in the air around you.

Particulate matter is worth understanding on its own. PM2.5 refers to particles 2.5 micrometers in diameter or smaller. For comparison, a human hair is roughly 70 micrometers wide. These particles are small enough to travel deep into the lungs and into the bloodstream. That is why PM2.5 is often considered the pollutant with the strongest link to serious health effects.

What Do the AQI Colors and Numbers Mean?

The AQI uses six color-coded categories. Each category corresponds to a specific range of numbers and a specific level of health concern.

An AQI of 100 generally corresponds to the level of the national air quality standard for that pollutant. Below 100, air quality is considered acceptable for most people. Above 100, the concern grows.

The “sensitive groups” category is not vague. It includes people with asthma, people with heart disease or lung disease, older adults, children, and pregnant women. Children breathe more air per pound of body weight than adults and their lungs are still developing, which is one reason they face higher risk from the same air.

How Is the AQI Calculated?

For each pollutant, the EPA uses a formula that converts the measured concentration into a number on a common 0-to-500 scale. This lets you compare ozone to particulate matter even though they are measured in different units.

The formula uses breakpoints. Each pollutant has a table of concentration ranges that map to AQI ranges. For example, a specific concentration of ground-level ozone in parts per billion maps to a specific AQI value. The same is done for each of the other pollutants.

Once all five values are calculated, the highest one becomes the reported AQI. So if ozone scores 85 and PM2.5 scores 130, the AQI you see is 130, and PM2.5 is the dominant pollutant.

One detail people often miss: the AQI is a tool for reporting, not a direct measurement of harm. It is a standardized communication scale. Two cities with the same AQI may have different mixes of pollutants, and the health implications can differ depending on which pollutant is driving the number and who is breathing it.

How Much Pollution Is in the Air at Different AQI Levels?

There is no single conversion from AQI to “amount of pollution” because each pollutant has its own breakpoints. But the categories give you a practical sense of what is happening.

• Good (0–50): Air quality is satisfactory and poses little or no risk.
• Moderate (51–100): Air quality is acceptable, though there may be a concern for people who are unusually sensitive to certain pollutants.
• Unhealthy for sensitive groups (101–150): Members of sensitive groups may experience health effects. The general public is less likely to be affected.
• Unhealthy (151–200): Some members of the general public may experience health effects. Sensitive groups may experience more serious effects.
• Very unhealthy (201–300): Health alert. The risk of health effects is increased for everyone.
• Hazardous (301–500): Emergency conditions. Everyone is more likely to be affected.

What actually changes between categories is the concentration of pollutants in the air. At higher AQI levels, there are more fine particles, more ozone, or higher levels of other pollutants. More pollution means more exposure for your lungs and heart.

Who Is Most at Risk From Poor Air Quality?

Some people face greater harm from the same air. This is well established in public health research.

People with asthma may notice worsening symptoms at lower AQI levels than others. The same is true for people with chronic obstructive pulmonary disease (COPD) and other lung conditions. For people with heart disease, fine particulate matter has been linked to cardiovascular effects, including heart attacks and strokes, in a large body of research.

Children are at higher risk because their lungs are still growing, they breathe faster, and they spend more time outdoors. Older adults are at higher risk because lung and heart function naturally decline with age, and because underlying conditions are more common.

Pregnant women are also considered a sensitive group. Research has linked air pollution exposure during pregnancy to outcomes such as preterm birth and low birth weight, though the exact mechanisms and the size of the effect are still being studied.

People who work outdoors, exercise heavily outdoors, or live near highways and industrial areas also face higher exposure. Where you are matters as much as what the AQI says, because air quality can vary block to block.

How Can You Use the AQI to Protect Your Health?

The AQI is designed to help you make decisions. The main one is whether to adjust your outdoor activity.

When the AQI is in the moderate range, most people can go about their normal activities. If you are in a sensitive group, it may be worth paying attention to how you feel and considering a shorter or less intense outdoor workout.

At the unhealthy for sensitive groups level, sensitive groups should reduce prolonged or heavy outdoor exertion. That means shorter runs, lighter bike rides, or moving the workout indoors. The general public can usually continue normal activity.

At the unhealthy level and above, everyone should consider reducing prolonged or heavy outdoor exertion. Sensitive groups should avoid it. If you must be outside, doing less intense activity for less time reduces your exposure.

Indoor air quality matters too. Keeping windows closed on high AQI days, using a high-efficiency particulate air (HEPA) filter, and avoiding activities that add indoor pollution — like burning candles or frying at high heat — can help reduce your total exposure. The evidence for portable air cleaners reducing indoor particulate levels is reasonably consistent, though the direct health benefits are still being studied.

One practical point: check the AQI at the time you plan to be outside, not just in the morning. Ozone tends to peak in the afternoon on hot sunny days, while particulate matter from traffic tends to be higher during rush hour. Timing your activity can make a real difference.

What the AQI Does Not Tell You

The AQI is a useful tool, but it has limits. It reports the highest pollutant value, not the combined effect of all pollutants together. It does not capture every harmful substance in the air, such as some volatile organic compounds or certain metals.

It also does not account for how long you are exposed or how hard you are breathing. Twenty minutes of hard running outdoors at an AQI of 120 is a different exposure than sitting on a porch at the same AQI. Your personal risk depends on the pollutant, your health, your activity level, and how long you are outside.

And the AQI is based on outdoor monitors. It does not tell you what is happening inside your home, which is where most people spend the majority of their time.