What Air Quality Index Is Bad? AQI Numbers Explained

On the US Air Quality Index, air quality becomes bad above 100. From 101 to 150 it is "Unhealthy for Sensitive Groups", which means children, older adults and people with heart or lung disease should take it easy outdoors. From 151 to 200 it is "Unhealthy" for everyone, 201 to 300 is "Very Unhealthy", and above 300 is "Hazardous". Anything from 0 to 50 is Good, and 51 to 100 is Moderate, which is fine for most people.

So an AQI of 150 is at the top of the sensitive-groups band, and 160 has crossed into Unhealthy. This guide explains where those numbers come from, which pollutant is behind them, and how to check the air where you are.

Check air quality by city

The AQI scale, band by band

The US AQI is the Environmental Protection Agency's index for reporting daily air quality. It has six colour-coded categories (AirNow):

The breakpoint that matters most is 100. AirNow explains that "for each pollutant an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard" (AirNow). Under 100, the air meets the standard. Over it, it doesn't.

Is AQI 150 bad? Is 160 bad?

Taking the numbers people search for most:

• AQI 50 or below. Good. Go outside.
• AQI 60 to 100. Moderate. Fine for nearly everyone. If you are unusually sensitive, you may want to shorten long, hard exercise.
• AQI 101 to 150. Bad for sensitive groups. Children and teens, older adults, pregnant people and anyone with asthma, heart or lung disease should cut back on long or heavy exertion outdoors. Everyone else is usually fine.
• AQI 151 to 200. Unhealthy. Everyone should reduce long or heavy exertion outdoors. Sensitive groups should avoid it.
• AQI 201 to 300. Very unhealthy. Everyone should avoid long or heavy exertion outdoors; sensitive groups should stay in.
• AQI 301 and up. Hazardous. Stay indoors with filtered air if you can.

Who counts as "sensitive" depends on the pollutant, and EPA's reporting rules require the pollutant-specific sensitive groups to be named for any AQI over 100 (40 CFR Part 58, Appendix G). The advice in the list above is what GodsView shows for each band on its city pages.

![img:aqi-bands](The "What the US AQI numbers mean" table on a GodsView city page, with the advice for each band from Good (0 to 50) to Hazardous (301 to 500), above the advice for sensitive groups.)

One number, several pollutants

The AQI is not a single measurement. EPA sets AQIs for five common pollutants: particle pollution (PM2.5 and PM10), ground-level ozone, carbon monoxide, nitrogen dioxide and sulfur dioxide (EPA). Each gets its own index value. The one you see is the worst: under EPA's rules, "the reported AQI corresponds to the pollutant with the highest calculated AQI" (40 CFR Part 58, Appendix G).

That has two practical effects:

1. Different days, different culprits. The same AQI of 160 can be driven by fine particles one day and ozone another. The headline number doesn't tell you which.
2. Different fixes. GodsView's advice for sensitive groups notes that a well-fitted N95 or FFP2 mask cuts particle exposure but does not filter ozone. Knowing which pollutant is high tells you whether a mask helps.

GodsView's city pages show the pollutants separately. On November 3, 2026, Delhi's page read US AQI 189, Unhealthy, with PM2.5 at 136 µg/m³ and PM10 at 357 µg/m³.

, PM2.5 136 µg/m³, PM10 357 µg/m³ and ozone 64 µg/m³, with advice for the Unhealthy band.)

What changed in 2024

In February 2024, EPA tightened the annual standard for fine particles (PM2.5) to 9.0 µg/m³ and changed several AQI breakpoints to match. The changes took effect on May 6, 2024 (EPA fact sheet):

EPA says many areas "can expect to see more days in the Moderate (Code Yellow) category" because Moderate now starts at 9 µg/m³ rather than 12. It also says the change was made because of new health science, not because air was getting worse. During wildfires, the new upper breakpoints may move some days from Unhealthy to Very Unhealthy, or from Very Unhealthy to Hazardous.

So if your app shows more yellow days than it used to, that's partly the scale, not just the air.

Why your app and the news show different numbers

Two apps can show different AQIs for the same city at the same time, and both can be right. Common reasons:

• Different scales. The UK's Daily Air Quality Index runs from 1 to 10, in four bands: 1 to 3 Low, 4 to 6 Moderate, 7 to 9 High and 10 Very High (UK-AIR, Defra), and it is set by the highest of five pollutants. Other countries have their own scales too. GodsView shows the US AQI everywhere so cities can be compared on one scale.
• Different time windows. The official AQI is a daily figure. EPA's AirNow also shows a near-real-time "NowCast AQI" built from hourly monitoring data from nearly 900 areas across the US (EPA fact sheet).
• Monitor or model. A reading from a monitor down the road is a measurement. GodsView's city values are modelled by CAMS (Copernicus) and served by Open-Meteo, so they are estimates for the area around the city centre, not readings from an official monitor. Each city page compares the model with the nearest measured PM2.5 from sensor networks (Sensor.Community and OpenAQ) when one is within 25 km.

When in doubt, trust the official monitor nearest you, and use a model forecast to plan the next day or two.

How to check air quality where you are

1. Open Air quality and press Air quality near me, or pick your city from the list.
2. Read the answer box at the top: the band, the US AQI and the advice.
3. Check the pollutant tiles to see whether PM2.5, PM10 or ozone is driving the number.
4. Scroll to the 48-hour forecast and the Best time to be outside section if you are planning a run or a school trip.
5. If wildfire smoke is mentioned, open the live wildfire map to see where the fires are. The NASA FIRMS guide explains the hotspots.

See air quality near me

In the US, EPA's AirNow website and app, and the AirNow Fire and Smoke Map it runs with the US Forest Service, carry the official numbers. EPA says many state and local air agencies also publish the NowCast AQI on their own websites.

Getting a heads-up before the air turns bad

Checking every morning gets old. GodsView's My places watches the places you add and alerts on unhealthy PM2.5 readings, along with earthquakes, storms, floods and wildfires within 50 km. The disaster alerts guide covers the setup, and the alerts tutorial walks through it with screenshots.