What is the Air Quality Index (AQI)? A Complete Guide

Before you head out for a run, open the windows, or send the kids to play outside, there is one number worth checking: the Air Quality Index, or AQI. It turns complex pollution data into a single, easy-to-read score so you can make smarter decisions about your day.

Whether you live in Johannesburg, London, New York, or Sydney, AQI works the same way — and understanding it is one of the simplest things you can do to protect your health.

What is the AQI and why does it matter?

The Air Quality Index is a standardised scale that reports how clean or polluted the air is at a given time and place. Instead of asking you to interpret separate readings for ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide, and particulate matter, the index rolls everything into one number.

That matters because air pollution is often invisible. You cannot always see smog, smell ozone, or feel fine particles in your lungs — yet poor air quality contributes to millions of premature deaths worldwide each year. AQI gives you an at-a-glance warning when conditions are risky.

Why a single number helps

Different pollutants affect health in different ways. AQI simplifies the picture by focusing on whichever pollutant poses the greatest risk right now. If PM2.5 is high but ozone is low, the index reflects the more dangerous reading. That is why the dominant pollutant can change throughout the day.

How AQI is calculated

AQI is not a direct measurement of one substance. It is derived from monitored concentrations of key pollutants, each of which has its own sub-index. The overall AQI value is typically the highest of those sub-indices — the "worst offender" approach.

Each pollutant concentration is mapped onto a standardised scale using breakpoint tables defined by environmental agencies. For example, a PM2.5 concentration of 35 µg/m³ might correspond to a moderate AQI reading, while 75 µg/m³ could push the index into unhealthy territory.

Data sources behind the number

Ground monitoring stations provide the most accurate local readings where they exist. In areas without dense sensor networks, modelled data and satellite estimates fill the gaps. Platforms like Aether combine multiple sources so you can check conditions for your city even when you are on the move.

The AQI scale breakdown

Most countries use a similar six-category scale. Here is what each band means in plain language:

Good (0–50)

Air quality is satisfactory and poses little or no risk. Enjoy outdoor activities as normal. This is the range most people aim for year-round.

Moderate (51–100)

Air quality is acceptable for the general public. Unusually sensitive people — such as those with mild asthma — may notice minor effects during prolonged outdoor exertion.

Unhealthy for Sensitive Groups (101–150)

Members of sensitive groups may experience health effects. Children, older adults, and people with heart or lung disease should reduce prolonged or heavy outdoor exertion.

Unhealthy (151–200)

Everyone may begin to experience effects. Sensitive groups should avoid prolonged outdoor activity, and the general public should limit time outside, especially during exercise.

Very Unhealthy (201–300)

Health alert: the risk of serious effects increases for everyone. Outdoor exertion should be avoided. Schools and sports organisers often cancel events at this level.

Hazardous (301–500)

Emergency conditions. The entire population is likely to be affected. Stay indoors, keep windows closed, and follow local health advisories. This range is associated with wildfire smoke, severe industrial events, or extreme winter inversions.

What each level means for your daily life

AQI is most useful when you connect it to real decisions. Here is a practical guide:

• Exercise: At moderate levels, most healthy adults can jog or cycle as usual. Above 100, consider moving workouts indoors or to early morning when pollution often dips.
• Children at play: Kids breathe faster than adults relative to their body size, so they absorb more pollution per kilogram. Limit playground time when AQI exceeds 100.
• Windows and ventilation: Open windows when AQI is good to refresh indoor air. Close them when outdoor pollution rises above moderate, especially if you live near busy roads.
• Commuting: Pollution peaks during rush hour. If AQI is elevated, consider delaying travel, using less congested routes, or keeping car windows closed in traffic.

Who is most at risk from poor air quality?

Air pollution does not affect everyone equally. Some groups are especially vulnerable even at moderate AQI levels:

• Children: Developing lungs are more susceptible to damage from fine particles and ozone. School-age children also spend more time active outdoors.
• Older adults: Age-related declines in lung and heart function make it harder to cope with polluted air.
• Asthma and COPD sufferers: Inflamed airways react strongly to irritants. AQI above 100 can trigger symptoms, coughing, and attacks.
• Heart disease patients: Fine particles can enter the bloodstream and contribute to cardiovascular stress.
• Pregnant people: Emerging research links sustained exposure to adverse pregnancy outcomes, making caution advisable during high-AQI periods.

How to check AQI and what to do when it is high

Checking AQI takes seconds. Use a trusted real-time tracker, enter your city or allow location access, and look at both the current reading and the hourly forecast. A single snapshot is helpful; a trend is even better.

When AQI is elevated, try these steps:

1. Reduce strenuous outdoor activity, especially in the afternoon.
2. Keep windows closed and use air conditioning on recirculate if available.
3. Wear a well-fitted N95 or KN95 mask if you must be outside for extended periods.
4. Check on vulnerable family members and neighbours.
5. Follow official health advisories from your local environmental agency.

Remember that AQI is a planning tool, not a medical diagnosis. For personal health decisions, consult a healthcare professional — especially if you have a chronic condition.

Track AQI in real time with Aether

Aether is a free air quality platform that shows live AQI, PM2.5 and PM10 levels, hourly forecasts, and health context for cities worldwide. Whether you are comparing conditions across South Africa, checking London before a weekend trip, or monitoring seasonal changes at home, Aether puts the data you need in one place.