Ozone

O₃ · µg/m³ · 8h average

A city skyline in afternoon haze — Afternoon haze over a city skyline. Photo: Ernst Halberstadt, public domain, via the Digital Public Library of America.

What it is

Ozone at ground level, made from traffic exhaust and heat in sunlight. Peaks mid-afternoon.

How it affects breathing

Evidence. Shown in lab studies: lung function drops and airways inflame after hours at about 120 µg/m³ (micrograms per cubic meter of air), below the US standard of about 140, and exercise multiplies the dose because you breathe more of it. The effect lags by hours.
How likely. A summer-afternoon problem, worst downwind of big cities, and winter has none.
What helps. Ozone is a gas, so an ordinary mask does nothing, but it is low indoors and low in the morning, so hard exercise before noon or inside avoids most of the dose. A daily controller inhaler blunts the inflammation; a rescue inhaler treats the tightness after the fact.

How Breathing Index measures it

Your Breathing Index uses the average of the last 8 hours of measurements, since the damage builds over hours of breathing it and shows up hours later; the US standard uses the same 8-hour span.

Where the number comes from

A monitor when one is within about 50 miles: an instrument run by your state environmental agency, published through the EPA’s AirNow.
Otherwise a model: a computer estimate of the air from satellites, weather and emissions data on a 45-kilometer (28-mile) grid. A model is a prediction, not a measurement, and it can miss a plume a monitor would catch. For ozone the model runs high in the eastern US in summer.

Breathing Index is a breathing lens on public air data — not medical advice. Trust your symptoms and your asthma action plan over anything on this screen.