Planning a picnic? You check the weather forecast. But have you checked the air you'll be breathing?
That's where the Air Quality Index (AQI) comes in. It's a scale from 0 to 500 used by the U.S. Environmental Protection Agency (EPA) to report daily air quality. Simply put, it tells you how clean or polluted your local air is and what health effects you might need to watch out for. A low number is great news, while a high number means there's a greater health risk.
With wildfires, urban smog, and seasonal allergens on the rise, knowing your local AQI is more important than ever. This guide will explain what the numbers mean, how they're measured, and what you can do to protect yourself when the air quality takes a dip.
AQI 0–50 is "Good," meaning air quality is satisfactory and poses little to no risk.
AQI over 100 signals that air quality is unhealthy for sensitive groups, like children and those with asthma.
AQI over 150 is unhealthy for everyone, regardless of medical history.
Particulate matter (PM2.5) and Ozone are two of the most common pollutants driving AQI numbers.
Indoor air quality becomes crucial when outdoor levels rise; keeping windows closed and using proper filtration helps.
0–50: Good (Safe for everyone)
51–100: Moderate (Acceptable, some sensitivity possible)
101–150: Unhealthy for Sensitive Groups
151–200: Unhealthy
201–300: Very Unhealthy
301–500: Hazardous
For a quick, simple definition of air quality:
Air quality refers to how clean or polluted the air is and whether it is safe to breathe. It depends on the levels of pollutants, such as particulate matter and ozone, present in the atmosphere at any given time.
Think of it like water quality; just as you wouldn't drink water filled with dirt or chemicals, you shouldn't breathe air filled with smoke or exhaust.
When scientists measure air quality, they aren't just looking at "dust." They are looking for specific chemical indicators regulated by environmental laws. The primary indicators include:
PM2.5 (Fine Particulate Matter): Tiny particles that can penetrate deep into the lungs and enter the bloodstream.
PM10 (Coarse Particulate Matter): Larger particles like dust and pollen that irritate the eyes, nose, and throat.
Ozone (O3): Good up high in the atmosphere, but harmful at ground level where it damages lung tissue.
Nitrogen Dioxide (NO2): A marker for traffic-related air pollution.
Sulfur Dioxide (SO2): An indicator of industrial pollution.
Carbon Monoxide (CO): A gas that reduces oxygen delivery to the body's organs.
While all are measured, PM2.5 is currently one of the most concerning indicators for health officials because its microscopic size allows it to cause systemic inflammation in the body.
When you see a specific number on your weather app, you might wonder what is actually being measured. The meaning of the Air Quality Index (AQI) goes beyond "dirty air." It is a standardized system used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become.
The AQI measures short-term pollution exposure risk, typically over hours or days, rather than lifetime exposure. It tracks five major pollutants regulated by the Clean Air Act:
Ground-level Ozone: Often called "smog," this is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight.
Particulate Matter (particle pollution): This includes PM2.5 (fine particles) and PM10 (coarse dust). This is often the primary driver of poor AQI during wildfire seasons.
Carbon Monoxide: An odorless, colorless gas released from combustion processes like vehicle exhaust.
Sulfur Dioxide: Produced by burning fossil fuels and industrial processes.
Nitrogen Dioxide: Highly reactive gases formed by emissions from cars, trucks, and buses, power plants, and off-road equipment.
In most regions, the AQI is updated hourly. The highest pollutant concentration at that moment determines the reported AQI. For example, if ozone levels are low (AQI 40) but smoke from a distant fire raises particulate matter levels to 130 (AQI 130), the reported AQI for the day will be 130.
To make air quality reports easy to understand, the EPA AirNow system uses a standardized scale. The air quality index, simply put, is a tool that converts complex scientific data, such as micrograms per cubic meter, into a user-friendly number.
The scale runs from 0 to 500. Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern.
Under 50: Air quality is good.
Under 100: Generally satisfactory air quality.
Above 100: This corresponds to the national air quality standard for the pollutant, which is the level the EPA has set to protect public health.
Above 300: Hazardous air quality.
Because pollutants vary in toxicity, the EPA uses specific formulas to convert pollutant concentrations to the standardized 0-500 scale. This ensures that an AQI of 120 means roughly the same level of public health risk whether the cause is ozone or particulate matter.
The EPA has divided the AQI into six color-coded categories to make it easy for people to understand quickly. This chart is the visual centerpiece of most weather reports.
| AQI Range | Category | Color | What It Means | Health Effects |
|---|---|---|---|---|
| 0–50 | Good | Green | Clean air | No risk |
| 51–100 | Moderate | Yellow | Acceptable | Minor irritation possible for unusually sensitive people |
| 101–150 | Unhealthy for Sensitive Groups | Orange | Reduced air quality | Asthma/heart conditions may worsen; general public is okay |
| 151–200 | Unhealthy | Red | Everyone affected | Limit outdoor exposure; sensitive groups avoid outdoors |
| 201–300 | Very Unhealthy | Purple | Health alert | Serious health effects for everyone |
| 301–500 | Hazardous | Maroon | Emergency conditions | Severe health risk for the entire population |
When viewing this chart, note that the "Orange" and "Red" zones are the tipping points at which behavior should change. Once the color reaches red, pollution is high enough that everyone may experience adverse health effects, and members of sensitive groups may experience more serious effects.
If you are checking your local forecast, you want to see an AQI between 0 and 50. This range is considered "Good" and is represented by the color green. At this level, air pollution poses little or no risk.
The "Moderate" range (51–100) is also generally acceptable for the vast majority of people. However, there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution. For example, people who are extremely sensitive to ozone may experience respiratory symptoms.
So, while 0–50 is ideal, anything under 100 typically allows for normal outdoor activities without special precautions for most healthy adults.
While everyone’s tolerance is different, air quality generally becomes a public health issue once the index exceeds 100. This is the threshold where air quality is deemed "Unhealthy for Sensitive Groups."
However, significant harm for the general population usually begins once the AQI surpasses 150.
101–150 (Orange): Sensitive groups, such as children, older adults, and people with lung disease, may experience coughing, mild irritation, or shortness of breath. The general public is less likely to be affected.
151–200 (Red): The atmosphere is unhealthy for everyone. You may notice breathing difficulty during exertion.
201+ (Purple): This triggers a health alert. The risk of serious health effects increases for everyone. Symptoms can include significant chest tightness, wheezing, fatigue, and irritation of the eyes and throat.
An AQI of 500 is rare but dangerous. It represents the maximum value on the standard scale and indicates Hazardous conditions. This level is typically seen only during extreme events, such as severe industrial accidents or intense wildfire smoke that settles into a valley.
If the AQI reaches or approaches 500:
It is considered an emergency condition.
The entire population is likely to be affected by serious health effects.
Everyone should avoid all outdoor physical activity.
Staying indoors with filtered air is mandatory for safety.
In these scenarios, indoor air protection becomes critical. Sealing drafts and running high-efficiency air purifiers are necessary to keep the indoor environment safe while the outdoor air is toxic.
Air pollution does not treat everyone equally. Some groups are at far higher risk even at lower AQI levels (such as the Orange 101–150 range).
Children: Their lungs are still developing, and they breathe more air per pound of body weight than adults. They are more active outdoors, increasing exposure.
Older Adults: They may have undiagnosed heart or lung disease, making them more susceptible to pollution-induced stress.
Asthma Sufferers: Even moderate pollution can trigger asthma attacks.
Heart Patients: Particle pollution has been linked to heart attacks and arrhythmias in people with heart disease.
Pregnant Individuals: Exposure to high pollution levels can impact fetal development and is linked to low birth weight.
Staying informed is the best defense against air pollution. You can check your local AQI through several reliable sources:
EPA AirNow: The official U.S. government site for air quality data.
Weather Apps: Most modern smartphone weather apps include a section for "Air Quality" or "AQI."
Smart Home Devices: Devices like Amazon Echo or Google Nest can verbally tell you the current AQI if asked.
Local News: During pollution events, local weather stations will broadcast air quality alerts.
Check your local AQI now to see if you should adjust your outdoor plans.
When the numbers climb into the unhealthy ranges (Red, Purple, or Maroon), taking action can protect your long-term health.
Stay Indoors: Reduce the amount of time you spend outside.
Close Windows: Prevent outdoor pollution from entering your home.
Run HVAC Systems: Use your central air conditioning to filter the air. Ensure the "recirculate" setting is on to avoid pulling in outdoor air.
Upgrade Filters: Use higher-efficiency air filters, such as MERV 13 from Filterbuy, which are often recommended during wildfire smoke events to capture fine particles.
Limit Exertion: Avoid heavy exercise outdoors. The harder you breathe, the more pollutants you inhale deep into your lungs.
Your home should be a sanctuary, but pollution is sneaky. When outdoor AQI is poor, particulate matter can infiltrate homes through cracks, doors, and natural ventilation. If your indoor air quality isn't managed, the air inside can eventually become nearly as bad as the air outside.
This is why indoor air quality matters immensely during pollution events. By ensuring your home is well-sealed and using proper HVAC filtration, you can significantly reduce the fine particles you breathe while sleeping or relaxing. Check out Filterbuy for high-quality filters to help keep your home's air clean and safe.
The Air Quality Index (AQI) is a standardized system used to report daily air quality. It tells you how clean or polluted the air is on a scale of 0 to 500. It focuses on health effects you may experience within a few hours or days after breathing polluted air.
An AQI value between 0 and 50 is considered "Good." This is represented by the color green on the AQI chart. At this level, air pollution poses little or no risk to the population, allowing everyone to enjoy outdoor activities freely.
If the AQI reaches 500, the air quality is considered "Hazardous." This is the top of the standard scale and signals emergency conditions. Everyone is likely to experience serious health effects, and you should avoid all outdoor exertion and remain indoors with filtered air.
Air quality refers to the condition of the air within our surroundings. It is a measure of how free the air is from pollutants that can harm human health or the environment. Good air quality is clean and clear; poor air quality contains high levels of pollutants.
Air generally becomes harmful for sensitive groups (like those with asthma) when the AQI exceeds 100. For the general public, air becomes harmful when the AQI rises above 150. Above 200, the air is considered "Very Unhealthy" and poses serious risks to everyone.
The five major pollutants used to calculate AQI are ground-level ozone, particle pollution (also known as particulate matter, including PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. These are established by the EPA under the Clean Air Act.
