AQI 100, 150, and 200 Explained — What Each Number Means

You open a weather app and see a number — 42, 87, 156 — next to "Air Quality Index." But what does AQI mean in plain English? Is AQI 100 safe? When is AQI 150 dangerous? And what should you actually do when the reading hits 200?

This guide breaks down the most common AQI numbers so you can protect your health without memorising chemistry.

What AQI is — in one sentence

The Air Quality Index is a standardised score from 0 to 500+ that tells you how polluted the air is right now. It combines readings for pollutants like PM2.5, ozone, and nitrogen dioxide into one easy number based on whichever poses the greatest risk at that moment.

Why one number instead of many

Raw pollution data is messy. AQI distils it so you can answer a simple question: is the air safe for me and my family today? Higher numbers mean more pollution and more reason to adjust outdoor plans.

The six AQI colour bands

Most countries use a similar scale. Here is what each range means:

0–50 — Good (Green)

Air quality is satisfactory. Little or no health risk for the general public. Enjoy outdoor life as normal.

51–100 — Moderate (Yellow)

Acceptable air quality. Unusually sensitive people — those with mild asthma, for example — may experience minor effects during prolonged outdoor exertion.

101–150 — Unhealthy for Sensitive Groups (Orange)

Sensitive groups should reduce prolonged outdoor exertion. Children, older adults, and people with heart or lung disease are most affected.

151–200 — Unhealthy (Red)

Everyone may begin to experience health effects. Sensitive groups should avoid prolonged outdoor activity. The general public should limit time outside.

201–300 — Very Unhealthy (Purple)

Health alert. Significant risk for everyone. Avoid outdoor exertion entirely.

301+ — Hazardous (Maroon)

Emergency conditions. Stay indoors, keep windows closed, and follow official health warnings.

Is AQI 100 safe?

Yes — for most healthy adults, AQI 100 sits at the top of the Moderate band and is generally safe for everyday outdoor activities like walking, cycling, and gardening. It is not perfect air, but it is acceptable.

However, AQI 100 is not ideal for everyone. If you have asthma, COPD, heart disease, or you are pregnant, you may notice effects during longer or harder exercise. Sensitive groups should treat 100 as a yellow flag rather than a green light for strenuous sport.

AQI 100 vs AQI 50

Both are in acceptable ranges, but AQI 50 means noticeably cleaner air. If you have a choice — morning run at 45 or afternoon at 95 — the lower reading is the better one for lung health.

When is AQI 150 dangerous?

AQI 150 falls in the Unhealthy for Sensitive Groups band (101–150) or just at the edge of Unhealthy (151+). At 150, pollution is genuinely risky for children, older adults, and anyone with respiratory or cardiovascular conditions.

Healthy adults may tolerate brief outdoor time at AQI 150, but prolonged exercise, school sports, and playground sessions should be shortened or moved indoors. This is the level where many cities issue public health advisories.

Signs the air is affecting you at this level

• Coughing or throat irritation after time outside
• Shortness of breath during normal activity
• Stinging or watery eyes
• Headache that eases when you go indoors

AQI 200 — what to do

At AQI 200, you are firmly in the Unhealthy range. This is when what AQI is dangerous becomes personal for nearly everyone — not just sensitive groups.

Immediate steps

1. Cancel or postpone strenuous outdoor activities.
2. Keep windows and doors closed.
3. If you must go outside, limit time and avoid busy roads where pollution concentrates.
4. Wear a well-fitted N95 or KN95 mask during extended outdoor exposure — especially during smoke events.
5. Check on elderly neighbours and family members with asthma.
6. Monitor the hourly forecast — conditions may improve or worsen.

Common causes of AQI 200 include winter smog trapped by weather inversions, heavy traffic under stagnant air, and regional wildfire smoke. The right response is the same: reduce exposure until readings fall.

What AQI is dangerous long term?

Single-day spikes matter, but repeated exposure to moderate and high AQI also adds up. Living in an area where AQI regularly exceeds 100 is linked to higher rates of asthma, heart disease, and reduced lung function over years — not just hours.

That is why tracking trends on a free platform like Aether helps you spot patterns: winter smog season, fire season, or daily rush-hour peaks. Good AQI vs bad AQI is not just about today — it is about understanding your local air over weeks and months.

Track AQI numbers on Aether

Aether shows live AQI with colour-coded health bands, PM2.5 readings, and hourly forecasts so you always know what the number means for your plans. Check before you leave home, before the kids play outside, and before you lace up for a run.