Why is the air quality bad today?
Usually one of three things: hot sun turning exhaust into ozone, smoke or dust blowing in, or still air letting pollution pile up instead of blowing away.
Not much you feel on your skin; ozone can sting eyes.
Pollutants go straight to your lungs with every breath.
Show me why1 min
The number: AQI
The US Air Quality Index puts several pollutants on one 0-to-500 scale: 0–50 good (green), 51–100 moderate (yellow), 101–150 unhealthy for sensitive groups (orange), 151–200 unhealthy (red), 201–300 very unhealthy (purple), and above 300 hazardous (maroon). The reading is set by whichever pollutant is worst, usually ozone or fine particles.
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Sunny days make ozone
Ozone high in the sky shields us from UV. Down here it irritates the lungs, and nothing emits it directly: it forms when sunlight drives reactions between gases in exhaust from cars, power plants and industry. Hot, bright, still days are ozone days, which is why air quality alerts often arrive with heat waves and why ozone tends to peak in the afternoon.
Still air lets it pile up
Wind and rising warm air normally carry pollution up and away. Under a big high-pressure system the air sinks, winds go light, and an inversion can cap the lowest layer like a lid. Pollution, smoke and dust build up day after day until a front or storm clears the air out. Valleys and basins are especially prone.
Questions people ask
- What does AQI 100 mean?
- 100 is the top of the moderate range. Above it the air is unhealthy for sensitive groups: people with asthma or heart or lung disease, children and older adults.
- Why is air quality worse in summer?
- Strong sunlight and heat speed up the reactions that make ozone, and summer brings wildfire smoke to many places.
- Why is air quality bad in winter?
- Cold, calm nights form strong inversions that trap wood smoke and exhaust near the ground, especially in valleys.