Air Quality and the AQI Index

The air you breathe is not always clean. Alongside the weather forecast, it is increasingly useful to check air quality, especially in big cities, on heavy-traffic days, or when there are wildfires nearby. The AQI sums up in a single number how healthy the air is at that moment.

What the AQI is

AQI stands for Air Quality Index. It is a scale that converts the concentrations of several pollutants into a value that is easy to interpret, usually paired with a colour. The higher the number, the worse the air quality and the greater the risk to health.

The idea is straightforward: not everyone has time to interpret micrograms per cubic metre of each pollutant, so the AQI translates them into clear categories that anyone can understand at a glance.

The main pollutants

The air can contain several harmful substances. These are the ones monitored most closely:

• PM2.5: fine particles smaller than 2.5 microns. They are the most dangerous because they penetrate deep into the lungs and even pass into the bloodstream. They come from traffic, fuel burning and wildfires.
• PM10: larger particles up to 10 microns, such as dust and pollen. They irritate the airways.
• O₃ (ground-level ozone): forms with sunlight and heat from other pollutants. It is typical of sunny summer afternoons and affects the lungs.
• NO₂ (nitrogen dioxide): comes mainly from traffic and combustion. It is linked to respiratory problems and worsens with urban congestion.

The AQI usually reflects the worst of these pollutants at any given time, since just one high reading is enough to make the air unhealthy.

The health categories

The index is divided into levels, generally from green to maroon, each with a recommendation:

• Good: the air is clean and there is no risk. You can do any outdoor activity.
• Moderate: acceptable for most people, but very sensitive individuals might notice something.
• Unhealthy for sensitive groups: children, older adults and people with asthma or heart conditions should take care.
• Unhealthy: everyone may start to feel effects; it is wise to cut back on outdoor exertion.
• Very unhealthy: a serious health risk; avoid outdoor activities.
• Hazardous: a health alert; it is best to stay indoors.

Who should take extra care

Although polluted air affects everyone, some groups are more vulnerable and should watch the AQI more closely. These include young children, older adults, pregnant women and anyone with asthma, allergies, lung diseases such as COPD, or heart conditions. For them, even moderate levels can cause discomfort such as coughing, itchy eyes or difficulty breathing.

How to reduce your exposure

When the index is high, a few simple steps make a difference:

• Limit intense outdoor exercise and choose the least polluted hours, usually early in the morning.
• Keep windows closed during pollution peaks and, if you have one, use a purifier with a HEPA filter.
• On high-ozone days, avoid going out during hot, sunny afternoons.
• If there is wildfire smoke or very high levels, consider an FFP2 or N95 mask when you go out.
• Check the AQI just as you check the weather: it helps you plan your day and protect the most sensitive people in your home.

Checking air quality becomes a habit as natural as looking to see if it will rain. With a quick glance you know whether it is a good time to go for a run or whether it is better to stay inside a little longer.