How to Read the Air Quality Index (AQI): What the Number Actually Means

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The AQI turns a soup of pollution measurements into one number and one colour — here is what each band means for your lungs, why the same "150" is not the same everywhere, and how to act on it.

From Messy Chemistry to a Single Number

The air over a city is not one thing. It is a mixture of dozens of pollutants, and the ones that matter most for health are tracked separately: particulate matter (tiny solid and liquid specks called PM2.5 and PM10), ground-level ozone (O₃, formed in sunlight rather than emitted), nitrogen dioxide (NO₂, largely from diesel), sulphur dioxide (SO₂) and carbon monoxide (CO). Reporting five different concentrations in micrograms per cubic metre is useless to a person trying to decide whether to go for a run.

So meteorologists and public-health agencies compress all of that into the Air Quality Index, a single scale from 0 upward. The trick is a breakpoints table: each pollutant's measured concentration is mapped onto a 0–500 number, and the pollutant that gives the highest sub-index becomes the headline AQI for that moment. A day with clean air but a dust spike is reported as a dust day, because the worst component is what should govern your behaviour.

The Bands, and What Each One Feels Like

Most people learn the scale through the colours, and the number tells you roughly how long a healthy adult can be outside before it becomes a problem.

• 0–50 (Green, "Good"): air is clean. No restrictions.

• 51–100 (Yellow, "Moderate"): fine for almost everyone; a very small number of unusually sensitive people may notice irritation.

• 101–150 (Orange, "Unhealthy for Sensitive Groups"): the line that actually matters. Children, older adults, people with asthma or heart disease should cut down strenuous outdoor activity. Everyone else is usually fine.

• 151–200 (Red, "Unhealthy"): everybody starts to feel it — eyes, throat, breathing. Prolonged exposure is discouraged.

• 201–300 (Purple, "Very Unhealthy"): health alert; the sensitive-group threshold has been blown past and even healthy people should stay indoors and wear a proper mask if outside.

• 301+ (Maroon, "Hazardous"): emergency conditions, sometimes driven by wildfire smoke or a dust storm; outdoor activity is genuinely risky.

The number is not linear in harm: the jump from 40 to 80 is minor, while 150 to 250 is a large change in real exposure. That is why the bands are unequal in width.

Why PM2.5 Matters Out of Proportion

Of all the pollutants, the smallest particles do the most damage, because size controls how deep they travel. PM10 (particles up to 10 micrometres) is caught mostly in the nose and upper airway. PM2.5 — up to 2.5 micrometres, about thirty times narrower than a human hair — slips past those defences into the alveoli, and the finest fraction can cross into the bloodstream. That is the mechanism behind the well-documented links between sustained PM2.5 and cardiovascular and respiratory disease, which is why many health economists treat long-term PM2.5 as one of the biggest environmental risks to life expectancy. When you check an AQI, checking the PM2.5 line underneath it is usually the single most informative thing you can do.

The Same Number Is Not the Same Everywhere

A common source of confusion: AQI is not one global standard. The US EPA scale, India's National AQI, China's AQI and the European Union's CAQI all use the same idea but different breakpoints, different pollutant weighting and different scale ranges. The European CAQI runs 1–100 in five bands, so a "60" there is nowhere near a "60" on the US scale. Even two cities reporting "AQI 150" can have quite different underlying pollution. When comparing places, always look at the raw PM2.5 concentration alongside the index — micrograms per cubic metre is the one figure that means the same thing on both sides of a border.

Turning the Number Into a Decision

The practical use of an AQI is not to memorise bands but to answer three questions: should I exercise hard outside, should I ventilate my home, and do I need a real mask? On orange days, moving a run to early morning (before traffic and before photochemical ozone builds in the afternoon sun) genuinely lowers your dose. On red days and above, a fitted respirator rated for fine particles helps where a cloth or surgical mask barely does, because those are designed to block droplets, not PM2.5. Indoors, a HEPA cleaner cuts the particulate load; ozone, by contrast, is worse indoors on a hot still day, so "clean air at home" depends on which pollutant you are fighting.

You can see current readings, the categories explained in plain language and the science of what your body actually does with the air on our own air-quality pages — and follow the breathing and carbon explainers to understand what those particles and gases are doing once they are in your lungs.

See also

• Why You Get Jet Lag: The Body Clock Explained
• Why There Are So Few Top Predators — The 10% Rule of Energy
• What Your Blood Pressure Numbers Actually Mean
• What Sleep Debt Actually Does to Your Brain and Body
• How to Read a Nutrition Label in Two Minutes
• BMI Explained: What the Number Really Measures — and Where It Fails