How do you read an air quality index, and when to act?

Short answer. Read your own country's index and act at its first level written for sensitive people: 101 on the U.S. AQI ², 4 on Canada's AQHI ³ and the UK's DAQI ⁴, "Moderate" on the European index ⁵. The U.S. and UK levels, about 36 µg/m³ of fine particles ²⁴, are more than twice the World Health Organization's 2021 guideline of 15 µg/m³ over 24 hours ¹.

What does an air quality index actually measure?

An index turns pollutant concentrations into one number, word or colour. Most indices work the same way: each pollutant gets a sub-index, and the worst one becomes the day's value. India's National Air Quality Index covers eight pollutants and "the worst sub-index reflects overall AQI" ⁸. France's ATMO index takes five pollutants and equals "le sous-indice le plus dégradé", the most degraded sub-index ⁶.

Canada is the exception. Its Air Quality Health Index (AQHI), in use since 2007, combines the health risks of ozone, particulate matter and nitrogen dioxide into one number from 1 to 10+, instead of reporting each pollutant against its own limit ³. During wildfire smoke, it is calculated hourly from PM2.5 alone ³.

Every index in this guide includes fine particles (PM2.5) ^2345678, so the comparison below uses PM2.5.

Why does the same air get a different colour in each country?

Each country draws its band edges from its own legal standards and health choices. Take a day with a 24-hour PM2.5 average of 30 µg/m³:

• United States: "Moderate" (9.1 to 35.4 µg/m³) ².
• United Kingdom: index 3, still "Low" (24 to 35 µg/m³) ⁴.
• European Environment Agency: "Moderate" (16 to 50 µg/m³) ⁵.
• France: "Mauvais", shown in red (above 25 up to 50 µg/m³) ⁶.
• India: "Good" (0 to 30 µg/m³) ⁸.
• Mexico: "Mala", shown in orange (above 25 up to 79 µg/m³ since January 2026), although Mexico reads particles as a 12-hour weighted moving average ⁷.
• WHO: twice the 24-hour guideline of 15 µg/m³ ¹.

The same air is "good" in New Delhi and "bad" in Lyon. Neither label is wrong; they answer different questions. When you travel or read a foreign app, look at the concentration, not the colour.

When should sensitive people and everyone else act?

Every index here except France's ATMO separates people at higher risk from the general public. Definitions vary: Canada names children, adults over 65 and people with health conditions ³; Mexico names children under 12, adults over 60, pregnant people and anyone with heart or lung disease such as asthma or COPD ⁷; the UK names adults and children with lung problems, adults with heart problems and older people ⁴.

The pattern is consistent: at the first warning level, the advice is to cut strenuous outdoor exertion, not to stay indoors. Staying inside is reserved for the top bands, such as Mexico's "Extremadamente mala" (above 130 µg/m³), where everyone should remain indoors and reschedule outdoor activities ⁷, or the European "Extremely poor" level (above 140 µg/m³), where sensitive people should avoid outdoor physical activity ⁵.

How do the indices relate to the WHO 2021 guidelines?

The WHO's 2021 guidelines set PM2.5 at 5 µg/m³ as an annual mean and 15 µg/m³ over 24 hours; PM10 at 15 and 45 µg/m³; nitrogen dioxide at 10 and 25 µg/m³ ¹. These are health targets, not alert levels, and no index above treats 15 µg/m³ as an emergency.

Some indices now start their scale at the WHO values. The European index's "Good" band ends at 5 µg/m³ and its "Fair" band at 15 µg/m³ ⁵, matching the WHO figures ¹. Mexico's "Buena" band ends at 15 µg/m³ ⁷, and its norm cites the 2021 WHO update ⁷. The U.S. moved the top of "Good" from 12.0 to 9.0 µg/m³ in 2024, to match its new annual standard of 9.0, and kept the 24-hour standard of 35 µg/m³ ². India's "Good" band runs to 30 µg/m³ ⁸, twice the WHO 24-hour guideline ¹.

Practical reading: an index showing "good" or "low" can still be above the WHO guideline. That matters most for the long-term average of where you live, less for a single day.

How do you use an index day to day?

• Know your group first. Use your own index's definition of at-risk people ³⁴⁷. If you have asthma, heart disease or COPD, ask your doctor for a written plan that says what to do at each level; Mexico's norm tells people with heart or lung disease to keep their condition medically controlled and follow their doctor's instructions ⁷.
• Follow how the air quality evolves, as Mexico's messages ask at every level above "Buena" ⁷.
• Watch symptoms. Most messages are conditional: "if you experience symptoms" ⁴⁵. In the UK, people with asthma are told they may need their reliever inhaler more often at "High" ⁴.
• Lower the effort, then the time outside. Swap a run for a walk, add rest breaks, move exercise indoors to a smoke-free space ⁷.
• For smoke events, use the clean-room, air-cleaner and respirator steps in our wildfire smoke guide rather than repeating them here.

What official agencies recommend

• WHO: PM2.5 of 5 µg/m³ annual and 15 µg/m³ over 24 hours ¹.
• United States (EPA): "Unhealthy for Sensitive Groups" from AQI 101 (35.5 µg/m³); "Unhealthy" from 151 (55.5 µg/m³) ²⁹.
• Canada (ECCC): at-risk people with symptoms consider reducing strenuous outdoor activity from AQHI 4 ³.
• United Kingdom (DAQI): at-risk people with symptoms consider reducing strenuous activity from index 4 (36 µg/m³) ⁴.
• European Union (EEA): sensitive people with symptoms consider reducing intense outdoor activity from "Moderate" (16 µg/m³) ⁵.
• France (ATMO): six bands from "Bon" (blue) to "Extrêmement mauvais" (magenta); "Mauvais" starts above 25 µg/m³ ⁶.
• Mexico (SEMARNAT): heart and lung patients and over-60s reduce vigorous outdoor activity from "Aceptable", above 15 µg/m³ ⁷.
• India (CPCB/MoEFCC): "minor breathing discomfort to sensitive people" from "Satisfactory", 31 µg/m³; lung and heart patients, children and older adults named from "Moderately polluted", 61 µg/m³ ⁸.

Where they differ: the first sensitive-group threshold for PM2.5 ranges from above 15 µg/m³ in Mexico ⁷ and 16 µg/m³ in Europe ⁵ to 31 µg/m³ in India ⁸ and 36 µg/m³ in the UK ⁴. Averaging periods also differ: 24 hours in the UK and India ⁴⁸, a 12-hour weighted average in Mexico ⁷, hourly concentrations for the European index ⁵, and the maximum daily mean in France ⁶. Canada's AQHI is not tied to a single PM2.5 figure at all, except in smoke events ³.

Frequently asked questions

Can I compare a U.S. AQI number with a European or Indian reading?

Not directly. The U.S. AQI runs from 0 to 301 and above ², India's from 0 to 500 ⁸, and the UK and Canada use 1 to 10 scales ³⁴. Compare the PM2.5 concentration in µg/m³ when the app shows it.

Is "Moderate" safe?

For most people, yes: the European index tells the general population to enjoy usual outdoor activities at "Moderate" ⁵. Sensitive people with symptoms are told to consider reducing intense outdoor activity at that level ⁵.

Why is my app's colour different from the government's?

An app may show one country's scale everywhere. A U.S. "Moderate" reading of 30 µg/m³ ² would be "Mauvais" under France's official index ⁶. Check which scale the app uses.

Does "good" air mean it meets WHO guidelines?

Not always. India's "Good" goes up to 30 µg/m³ ⁸, and the U.S. "Good" up to 9.0 µg/m³ ², both above the WHO annual guideline of 5 µg/m³ ¹.

Should children stay indoors on a bad air day?

The indices mostly ask for less strenuous activity, not confinement. Mexico tells everyone, children included, to stay indoors at "Extremadamente mala", the top band ⁷. For a child with asthma, follow the plan agreed with your doctor.

Next step

For the air and water risks that apply where you live, see the air quality pages, country by country.

The Phaenyx plan's "Water and air" section lists the water and air risks for your postal code, including which official index applies to you. The Phaenyx Safe app keeps the reflex sheets offline, so the thresholds are at hand when the network is not. Build your Phaenyx plan.

Sources

1. World Health Organization (WHO) — Air quality, energy and health: types of pollutants — https://www.who.int/teams/environment-climate-change-and-health/air-quality-and-health/health-impacts/types-of-pollutants (accessed 2026-10-27)
2. U.S. Environmental Protection Agency (EPA) — Final Updates to the Air Quality Index (AQI) for Particulate Matter: Fact Sheet and Common Questions — https://www.epa.gov/system/files/documents/2024-02/pm-naaqs-air-quality-index-fact-sheet.pdf (accessed 2026-10-27)
3. Environment and Climate Change Canada — About the Air Quality Health Index — https://www.canada.ca/en/environment-climate-change/services/air-quality-health-index/about.html (accessed 2026-10-27)
4. Welsh Government, Air Quality in Wales — Daily Air Quality Index — https://airquality.gov.wales/about-air-quality/daily-air-quality-index (accessed 2026-10-27)
5. European Environment Agency (EEA) — European Air Quality Index — https://airindex.eea.europa.eu/AQI/index.html (accessed 2026-10-27)
6. French Ministry for the Ecological Transition — Arrêté du 10 juillet 2020 relatif à l'indice de la qualité de l'air ambiant (Journal officiel, 29 July 2020; copy hosted by Air Pays de la Loire) — https://airpl.org/sites/default/files/reports/Arr%C3%AAt%C3%A9%2010072020%20-%20indice%20qualit%C3%A9%20de%20l%25u2019air%20ambiant.pdf (accessed 2026-10-27)
7. SEMARNAT, Mexico (Diario Oficial de la Federación) — NOM-172-SEMARNAT-2023, Lineamientos para la obtención y comunicación del índice de calidad del aire y riesgos a la salud (DOF, 25 January 2024) — https://dof.gob.mx/nota_detalle.php?codigo=5715154&fecha=25/01/2024 (accessed 2026-10-27)
8. Press Information Bureau, Government of India — National Air Quality Index (AQI) launched by the Environment Minister (17 October 2014) — https://www.pib.gov.in/newsite/printrelease.aspx?relid=110654&reg=48&lang=2 (accessed 2026-10-27)
9. AirNow (U.S. Environmental Protection Agency and partners) — AQI Basics — https://www.airnow.gov/aqi/aqi-basics/ (accessed 2026-10-27)