Air Quality Index (AQI) Explained: PM2.5, PM10, and Winter Inversion Layers

I spent three winters doing air quality monitoring in Ahmedabad and then Delhi, and the thing that struck me most wasn't the numbers — it was watching schoolchildren play cricket in a playground at 8 AM with a PM2.5 reading of 280 μg/m³. That's almost 12 times the WHO safe limit. The air was visibly grey. And the kids were sprinting and breathing hard. Nobody thought to stop them because nobody fully understood what that AQI number on the government app actually meant for their lungs.

The Air Quality Index is genuinely important to understand, especially if you live in any Indian metro in winter. Here's what you need to know.

The Pollutants That Matter Most

India's AQI system tracks six pollutants: PM2.5, PM10, ozone, NO2, SO2, and CO. The sub-index for each is calculated separately, and the overall AQI is whichever sub-index is highest — the "worst pollutant wins" approach. In most Indian cities most of the time, especially in winter, PM2.5 is the controlling pollutant.

PM2.5 refers to particulate matter with diameter less than 2.5 micrometres — roughly 1/30th the width of a human hair. These particles are small enough to bypass the nose and throat's filtering mechanisms entirely, reach the deep lung, and in some cases pass through the lung tissue into the bloodstream. Long-term exposure at elevated levels increases risk of cardiovascular disease, lung cancer, and respiratory conditions. Short-term spikes cause immediate throat and eye irritation, worsen asthma, and in vulnerable populations can trigger hospital-level events.

PM10 particles (up to 10 micrometres) are more like dust — construction dust, road dust, soil. They're filtered more effectively by the upper respiratory tract and are less health-critical than PM2.5, but still relevant for people with existing respiratory conditions.

Why Winters Are So Much Worse — The Inversion Layer

The basic meteorology here: under normal conditions, air near the ground is warmer than air higher up. Warm air rises, taking pollutants with it and dispersing them through the upper atmosphere. During winter nights, the ground radiates heat rapidly into a clear sky and cools down quickly. The air immediately above the ground cools with it, while air higher up remains relatively warmer. Now you have a temperature inversion — cold air near the ground trapped under a warmer layer above.

This warm air layer acts like a lid. Pollutants from vehicles, construction, burning fields (the stubble burning from Punjab and Haryana that reaches Delhi), and industrial activity can't rise above the inversion layer. They stay near the ground, accumulating hour after hour. This is why a Delhi AQI that might be 120 at 10 PM can be 400 by 7 AM the next morning. Nothing changed except that emissions accumulated in a shrinking air volume all night.

The inversion breaks when the sun heats the ground enough to warm the near-surface air past the temperature of the layer above. In deep winter this might not happen until 10–11 AM. During the worst fog events, it might not break fully at all for several days in a row.

Making Practical Decisions With AQI

A few simple rules that I apply myself and share with families asking about this:

If AQI is below 100, normal outdoor activity is fine for most people. Between 100 and 200, people with asthma, COPD, heart conditions, or very young children should reduce prolonged outdoor exposure. Between 200 and 300, limit outdoor exercise for everyone — this is especially important because breathing hard during exercise means you're inhaling much more air per minute. Above 300, stay indoors if possible, especially in the mornings.

Check AQI in the morning before planning outdoor exercise or activities for children. In Delhi and other northern Indian cities between November and February, the highest-risk window is typically 6–10 AM — early morning exercise routines can be the highest dose of pollution in a person's day. Post-noon is generally better as inversions break down. Use N95 or equivalent masks if you're in high-AQI conditions and need to be outside — cloth masks filter almost nothing at PM2.5 scale.

The AQI number isn't just an environmental metric. It's a direct input to your health decisions, and treating it that way is worth the effort.