June 22, 2026 · 9 min read
Millions of people check the weather every morning. However, far fewer check the air quality — even though polluted air harms your health just as fast as extreme temperatures. So what is AQI, and why does it matter? AQI stands for Air Quality Index, a standardised number that tells you how clean or polluted your local air is right now. Governments worldwide use it to turn complex pollution data into one simple, actionable score.
In this guide, you will learn exactly what is AQI, how it is calculated, what each level means for your health, and how to protect yourself on high-pollution days.
AQI stands for Air Quality Index. The United States Environmental Protection Agency developed the AQI scale in the 1970s to give the public a consistent way to understand air pollution. Today, more than 90 countries use similar scales, though exact breakpoints vary by region.
The AQI is not a direct measurement — it is a calculated score. Environmental agencies measure the concentration of specific pollutants. They then convert those concentrations into sub-index values. The highest sub-index value among all measured pollutants becomes the final AQI. In the US, six pollutants feed into the score:
In most cities, PM2.5 drives the AQI the most often. Vehicle exhaust, wildfire smoke and industrial emissions all produce fine particles. You can check the AQI of your location by ZIP code to see which pollutant is driving the number in your area right now.
The AQI scale runs from 0 to 500 and divides into six colour-coded categories. Each category comes with a specific health message for the public.
Air quality is satisfactory and poses little or no risk. Consequently, all outdoor activities are safe for everyone, including children, elderly adults and people with respiratory conditions.
Air quality is acceptable for most people. However, a small number of unusually sensitive individuals may experience mild symptoms. Furthermore, people with severe asthma should monitor how they feel during prolonged outdoor time.
Children, elderly adults and people with asthma or heart disease may experience health effects at this level. Therefore, sensitive groups should reduce prolonged or heavy outdoor exertion. The general public is unlikely to be affected at this range.
Everyone may begin to experience health effects. Additionally, sensitive groups may suffer more serious effects. Reduce outdoor exercise and consider wearing an N95 mask when going outside.
Health alerts apply to the entire population. As a result, everyone should avoid prolonged outdoor activities. Keep windows closed and run an air purifier with a HEPA filter indoors.
Emergency conditions. Everyone should avoid all outdoor physical activity. Stay indoors with filtered air. This level occurs most often during severe wildfires or industrial accidents.
PM2.5 refers to particles smaller than 2.5 micrometres in diameter — roughly 30 times thinner than a human hair. These particles penetrate deep into the lungs and can enter the bloodstream. Long-term exposure to PM2.5 is linked to lung disease, heart disease, stroke and premature death. According to the WHO, PM2.5 is responsible for millions of deaths per year globally.
To understand just how harmful high PM2.5 air is, the cigarette equivalent calculator converts PM2.5 levels into cigarettes-per-day — the results often surprise people.
PM10 particles range from 2.5 to 10 micrometres. Road dust, pollen and construction debris are the most common sources. These particles lodge in the upper airways and can trigger asthma attacks, bronchitis and eye irritation.
Ground-level ozone forms when sunlight reacts with pollutants from vehicles and industry. Ozone irritates the airways, causes chest pain and worsens asthma. Levels typically peak on hot, sunny afternoons — which is why AQI often rises during summer heatwaves.
NO2 comes primarily from vehicle exhaust and power plants. It irritates the respiratory system and can aggravate existing lung conditions. Cities with heavy traffic — such as Delhi, Beijing and Los Angeles — frequently see elevated NO2 during rush hours.
Checking your local AQI takes less than five seconds. The fastest method is to allow location access in your browser. The AQI of My Location homepage auto-detects your GPS position and instantly shows the real-time AQI, PM2.5, PM10 and NO2 readings for your exact area.
Alternatively, you can search by ZIP code or postal code using the AQI by ZIP code tool. It supports the US, India, UK, Canada, Australia and 30+ other countries. If you want to compare air quality between two places, the compare two cities AQI tool shows a live side-by-side verdict with PM2.5, PM10 and health scores for both locations.
A Good AQI is any value between 0 and 50. At this level, air quality poses little or no risk to anyone. An AQI of 25 is generally considered excellent — comparable to clean rural air on a clear day after rainfall.
An AQI between 51 and 100 is Moderate. Most healthy adults will not experience symptoms in this range. However, people with asthma or allergies may notice mild irritation. In contrast, anything above 100 warrants action — especially for those who exercise outdoors, have children or manage a respiratory condition.
The US AQI is the most referenced internationally, but other countries use their own systems. India’s National AQI also runs from 0 to 500 but uses eight pollutants with slightly different breakpoints. The European CAQI (Common Air Quality Index) uses a 0-to-100+ scale. China uses the same 0-500 framework as the US but with locally adjusted standards.
When comparing AQI readings across countries, always verify which scale is being used. An AQI of 100 in India is classified as Satisfactory, whereas 100 on the US scale sits at the upper boundary of Moderate. Therefore, raw number comparisons between countries can mislead without this context.
The WHO air quality guidelines recommend an annual PM2.5 limit of just 5 µg/m³ — a threshold that most major cities exceed every single day. Consequently, checking your local AQI daily is one of the simplest health habits you can build.