How to Read Air Quality: European and International Indices Explained

Air-quality numbers can look deceptively simple. A map may show a green dot, a phone may report “AQI 85”, and an indoor monitor may display “PM2.5: 18 µg/m³”. These are not three ways of saying the same thing. An index is a health-oriented classification derived from pollutant measurements; a concentration is a physical measurement of a particular pollutant; and different countries use different index scales and calculations.

This guide explains how to read the European Air Quality Index, compare it with the US Air Quality Index, UK Daily Air Quality Index, Canadian Air Quality Health Index and India’s National Air Quality Index, and assess the health significance of the pollutants underneath the headline number.

Key principle: Always identify the index system, dominant pollutant, measurement units, averaging period, time of observation, and location before interpreting an air-quality reading. “Good” on an index does not mean zero pollution or zero health risk.

Quick guide: what should you look at?

1. Which scale? “European AQI”, “US AQI”, “UK DAQI” and “Canadian AQHI” are not interchangeable.
2. How bad is the category? Read the label and its official health advice, not only the colour or number.
3. What is driving it? PM2.5, PM10, ozone, NO₂ and SO₂ have different sources and health effects.
4. Is the reading current or forecast? An hourly station reading, a daily average, a modelled value and a forecast may differ.
5. Who is exposed and for how long? A brief walk is different from strenuous exercise outdoors or years of sustained exposure.

1. What does “air quality” measure?

Air quality describes the presence of substances in the air that can affect health or the environment. Public outdoor-air monitoring generally reports concentrations of particulate matter and specific gases. An air-quality index (AQI) converts selected concentrations to categories or index values designed to communicate likely short-term health concern.

Concentration is not an index

• µg/m³ (micrograms per cubic metre): mass of a pollutant in one cubic metre of air; commonly used for PM2.5, PM10, ozone, NO₂ and SO₂ in Europe.
• mg/m³ (milligrams per cubic metre): a larger mass unit; 1 mg/m³ = 1,000 µg/m³. WHO uses mg/m³ for its CO guideline.
• ppm (parts per million) / ppb (parts per billion): mixing-ratio units commonly used for gases. Converting gas concentrations between ppm/ppb and µg/m³ depends on molecular mass, temperature and pressure.
• AQI or AQHI score: a calculated classification, not a pollutant concentration. A score of 100 has no universal µg/m³ equivalent.

Averaging time matters. A 1-hour PM2.5 concentration of 30 µg/m³ is not equivalent to a 24-hour mean of 30 µg/m³ or an annual mean of 30 µg/m³. Compare a reading with a guideline only when the pollutant and averaging period match.

2. European Air Quality Index (EAQI): how to read it

The European Environment Agency (EEA) and European Commission provide the European Air Quality Index. It displays air-quality conditions across Europe through monitoring-station and modelled map layers. The current EEA scheme has six qualitative bands: Good, Fair, Moderate, Poor, Very poor and Extremely poor.

The index considers up to five pollutants: PM2.5, PM10, ozone (O₃), nitrogen dioxide (NO₂) and sulphur dioxide (SO₂). For a particular station or model grid, the worst category among the qualifying pollutants determines the headline index. Its displayed station/model index uses hourly concentrations; it should not be interpreted as a 24-hour or annual guideline assessment.

Official European bands by pollutant

All values below are EEA hourly concentration categories, in µg/m³, as published by the European Air Quality Index methodology. The EEA displays integer bands; consult its documentation when implementing borderline or unrounded values programmatically.

How to interpret the categories (adapted from the EEA’s published health messages):

Sensitive groups include people with respiratory conditions and people with cardiovascular conditions; age, pregnancy, individual susceptibility and medical advice may also influence precautions.

Example: An hourly PM2.5 value of 20 µg/m³ is in the EEA’s Moderate PM2.5 band. If ozone at the same location is 170 µg/m³, ozone is Very poor, so the overall European index should reflect the worse Very poor level (provided the data qualify under EEA methodology). This is why identifying the dominant pollutant matters.

Data caveat: The EEA station map may include recent, not-yet-fully-validated observations and modelled gap filling; its model layer is an estimate rather than a direct measurement at your doorstep. Missing pollutants and the station’s location can affect what the displayed index represents. An hourly category also does not establish whether annual legal limits have been met.

3. US Air Quality Index (US AQI)

The US Environmental Protection Agency (EPA) / AirNow index is a numerical scale with six coloured health categories. It evaluates ground-level ozone, particulate matter (PM2.5 and PM10), CO, SO₂ and NO₂ under pollutant-specific calculations; the reported overall AQI is generally determined by the highest relevant pollutant sub-index.

The US EPA revised PM2.5 AQI breakpoints in 2024, so older charts and third-party calculators may disagree with current US AQI values. For illustration, under the revised US 24-hour PM2.5 breakpoints, 0.0–9.0 µg/m³ corresponds to AQI 0–50 and 9.1–35.4 µg/m³ to AQI 51–100. These are 24-hour PM2.5-specific breakpoints, not universal AQI conversion rules. Source: EPA 2024 rule.

Why AirNow’s “NowCast” can differ from a daily AQI

AirNow also uses a NowCast AQI for ozone and particles to provide a more timely indication of conditions, using multiple recent hourly measurements and pollutant-specific algorithms. It is not simply the last one-hour reading or necessarily the final daily AQI. See AirNow: Using the AQI.

4. UK Daily Air Quality Index (DAQI)

The United Kingdom uses the Daily Air Quality Index, which runs from 1 to 10, with four health bands. It is not the European EEA’s six-category system, even though both cover several of the same pollutants.

The UK DAQI is determined by the highest band across PM2.5, PM10, NO₂, O₃ and SO₂. The pollutant-specific measurement and averaging periods vary: for example, PM2.5 uses a daily mean or a 24-hour running mean for current observations, whereas NO₂ uses an hourly mean. Read the government’s pollutant-specific concentration tables and its health advice if implementing thresholds or giving tailored guidance.

5. Canada’s Air Quality Health Index (AQHI)

Canada commonly reports the Air Quality Health Index, which intentionally expresses health risk on a 1–10+ scale, rather than using the US AQI conversion. The conventional AQHI considers the combined effects of PM2.5, NO₂ and ozone; local health authorities may also issue smoke-related adjustments or advisories.

See Environment and Climate Change Canada’s official AQHI guide. Canadian AQHI 8 does not mean US AQI 80 or European AQI 8.

6. India’s National Air Quality Index (NAQI)

India’s Central Pollution Control Board (CPCB) uses six numerical categories, and its framework includes pollutant sub-indices for PM10, PM2.5, NO₂, SO₂, CO, O₃, ammonia (NH₃) and lead (Pb), subject to data-availability requirements.

The bands share some numerical endpoints with the US index but have different pollutant breakpoints and different category meanings. Do not apply US AQI health labels to an Indian AQI score. See the CPCB technical overview and CPCB AQI portal.

7. Other variants and why a number alone is not enough

Air-quality systems are also published by other national, regional and commercial providers. Some apps show US AQI globally even when the location is in Europe; others show an EEA-derived index, local government classifications or a proprietary score. Some cities and states additionally have their own advisory schemes. A country name in an app’s location field does not establish which index it uses.

In particular:

• China: national AQI frameworks use pollutant-based sub-indices and numerical categories. Consult the Ministry of Ecology and Environment and the current national standard before reproducing thresholds; revisions can affect comparability over time.
• Australia: air-quality presentation and health-advice categories can vary among states and territories. Start with your state or territory environment authority, and verify the local averaging period and reporting method.
• Third-party apps: look for an explicit label such as “US EPA AQI”, “European AQI”, or “AQHI”, the named measurement source, timestamp, units, and pollutant list. Without these details, numerical comparisons may be misleading.

Bottom line: For cross-border comparison, compare the same pollutant concentration, measurement method and averaging window rather than ranking places solely by their headline AQI scores.

8. What does each pollutant mean for health?

PM2.5 — fine particulate matter

Definition: Airborne particles with aerodynamic diameter 2.5 micrometres or smaller. Sources include combustion (vehicles, industry, wood burning, wildfires), secondary particles formed from gaseous emissions, and other processes.

Why it matters: Fine particles can penetrate deep into the lungs. Research links PM2.5 exposure to airway inflammation, asthma exacerbations, reduced lung function, heart attacks, strokes, cardiovascular and respiratory hospitalisation, and increased risk of premature death. Risks are associated with both hours-to-days exposure and years of exposure. These are population-level risk associations, not predictions that a particular reading will cause an individual illness.

How to read it: Check µg/m³, the averaging period, and whether the value is measured or estimated. PM2.5 is one of the most useful individual indicators to track alongside the overall index, especially during wildfire-smoke episodes. EPA health evidence; EPA cardiovascular assessment.

PM10 — inhalable particulate matter

Definition: Inhalable particles with aerodynamic diameter 10 micrometres or smaller, including PM2.5. Sources can include road and construction dust, windblown soil, sea salt and combustion particles.

Health implications: Irritation of the respiratory tract, cough, aggravated asthma and other respiratory effects; smaller constituents may also reach deep into the lungs. PM10 is not the same as “coarse PM” alone, because its reported mass includes PM2.5. EPA particulate matter overview.

PM1 and ultrafine particles — useful supplementary metrics

PM1 refers to particles around 1 micrometre or smaller. Ultrafine particles are often defined as particles less than 0.1 micrometre (100 nanometres). Their sources can include high-temperature combustion and traffic. Smaller particles may reach deep into the respiratory system, but consumer PM1 measurements are not necessarily standardised or comparable across devices. There is no WHO 2021 numerical ambient-air guideline for PM1 or ultrafine particles comparable to the PM2.5 mass guideline. Treat these readings as supplementary information, not an internationally standard AQI.

Ozone (O₃) — a summer and sunshine concern

Definition: Ground-level ozone is a secondary pollutant formed through atmospheric reactions involving nitrogen oxides and volatile organic compounds in sunlight. It is different from the protective ozone layer high in the atmosphere.

Health implications: It irritates and inflames airways, can worsen asthma, cause coughing or chest discomfort and reduce lung function, particularly during outdoor exertion. Ozone can be elevated away from the busiest road, and PM2.5 may be low while ozone is high. Check ozone separately on warm, sunny days and when doing strenuous outdoor exercise. Source: WHO air-quality guidelines.

Nitrogen dioxide (NO₂) — combustion and traffic

Definition: A reactive gas associated with road transport, combustion and some indoor fuel-burning appliances.

Health implications: Irritates airways and is associated with worsened asthma and respiratory symptoms; traffic-related NO₂ is also an indicator of a broader pollution mixture. Elevated roadside concentrations can differ sharply over short distances. A low PM2.5 measurement does not rule out elevated NO₂. Source: WHO guidelines.

Sulphur dioxide (SO₂) — combustion and industrial emissions

Definition: A gas released by combustion of sulphur-containing fuels and certain industrial operations, including some shipping-related emissions.

Health implications: Can provoke breathing symptoms and bronchoconstriction, especially among people with asthma. Exposure is particularly relevant near some industrial sources and during episodes of elevated emissions. Source: WHO guidelines.

Carbon monoxide (CO) — potentially acute and dangerous

Definition: A colourless, odourless gas produced by incomplete combustion (for example from faulty fuel-burning appliances, engines and fires).

Health implications: CO interferes with the blood’s ability to transport oxygen. Significant exposure may cause headache, dizziness, confusion, collapse and death. An outdoor AQI reading or standard PM sensor is not a substitute for a certified indoor CO alarm. If a CO alarm activates or CO poisoning is suspected, leave the affected area, seek fresh air and contact local emergency services. Source: WHO guidelines.

Carbon dioxide (CO₂) — primarily an indoor ventilation indicator

Definition: A gas exhaled by occupants and produced by combustion. Indoor CO₂ is often displayed in ppm, not µg/m³.

How to interpret it: In normally occupied indoor spaces, elevated CO₂ often indicates ventilation is insufficient relative to occupancy. It is not itself an outdoor AQI pollutant and is not a reliable direct measure of infectious particles, PM2.5, or all indoor contaminants. The US CDC notes that around 800 ppm may be a useful context-dependent ventilation target, not a universal safety cutoff. Use CO₂ trends together with occupancy, outdoor baseline and ventilation information. Source: CDC/NIOSH ventilation FAQ.

VOCs, formaldehyde, ammonia and lead

Volatile organic compounds (VOCs) come from many sources, including solvents, coatings, cleaning products, fuels and furnishings. The term describes a very broad mixture: health effects range from irritation and headache to longer-term risks associated with specific compounds. A consumer TVOC score cannot diagnose which chemical is present or establish safety against chemical-specific limits.

Formaldehyde is a specific indoor pollutant associated with irritation and longer-term cancer risk at relevant exposures. Ammonia (NH₃) is irritating at higher exposures and contributes to the formation of secondary PM2.5 outdoors. Lead (Pb) is a toxic metal; exposure can harm neurological development and other body systems, especially in children. These substances are not all included in the standard European headline AQI. For suspected specific exposures, rely on targeted sampling and relevant public-health standards rather than a generic consumer sensor score.

9. WHO guidelines: what “safe” air actually means

The World Health Organization’s 2021 Global Air Quality Guidelines are health-based reference levels for selected outdoor air pollutants. They are not an AQI scale, not an assurance of no risk below each value, and not automatically the legal limit in every country. They use different averaging periods because both short-term and long-term exposure matter.

¹ The 2021 short-term WHO guideline levels have a 99th-percentile convention, approximately 3–4 exceedance days per year; they are not described correctly as limits that must never be exceeded on any day. ² The ozone peak-season measure averages daily maximum 8-hour concentrations in the six consecutive months with the highest running six-month mean.

Example of why averaging matters: A European map may classify an hourly PM2.5 concentration of 12 µg/m³ as Fair. The WHO 15 µg/m³ PM2.5 reference is a 24-hour guideline and should not be used to declare the same hourly value “within” or “outside” that daily guideline. Similarly, an annual mean of 12 µg/m³ would exceed the WHO annual PM2.5 guideline of 5 µg/m³ even if the hourly index is frequently labelled Good or Fair.

Source: WHO 2021 Global Air Quality Guidelines and WHO guideline table reproduced by NCBI Bookshelf.

10. Short-term alerts, long-term exposure and EU legal limits

An AQI primarily supports day-to-day decisions. Long-term exposure is assessed through representative months and years of monitoring, with annual means and legally specified assessment methods. Regularly inhaling moderately elevated pollution can still contribute to lifetime risk even if no single day looks alarming.

The EU’s revised Ambient Air Quality Directive (EU) 2024/2881 sets legal air-quality requirements and timelines, which are distinct from the EEA’s public-facing six-band index and the WHO’s health-based recommendations. The Directive’s 2030 annual limit values include 10 µg/m³ for PM2.5 and 20 µg/m³ for NO₂ (subject to the Directive’s full legal provisions and implementation). Neither value should be treated as an hourly EAQI category boundary or as identical to WHO’s 5 µg/m³ PM2.5 and 10 µg/m³ NO₂ annual guideline levels. Read the Directive text on EUR-Lex.

For a place-based long-term assessment, favour validated official annual monitoring data and the appropriate measurement network, not the colour of a single map dot at one time.

11. Who is most vulnerable?

Some people may experience adverse effects at lower concentrations, or experience stronger effects from the same exposure:

• People with asthma, COPD or other lung conditions may develop coughing, wheezing, chest tightness or worsening symptoms.
• People with cardiovascular disease may be vulnerable to the effects of fine particles, including exacerbation of existing heart conditions.
• Children and teenagers have developing lungs and may spend more time being active outside.
• Older adults are more likely to have chronic conditions that increase susceptibility.
• Pregnant people may benefit from reducing avoidable pollution exposure because research links air pollution with adverse pregnancy and birth outcomes.
• Outdoor workers and athletes may inhale more pollution because of prolonged exposure and heavier breathing.

These groupings describe higher average vulnerability, not a universal medical threshold. Follow personal medical advice where relevant, and seek urgent medical help for severe breathlessness, chest pain, confusion or fainting rather than relying on an AQI app.

12. Practical decisions: walking, exercise, home and travel

For routine outdoor activity: Use the local official index and its health message, identify the dominant pollutant, and adjust how long or how vigorously you exercise if readings worsen. Higher breathing rates during hard exercise increase the amount of pollution inhaled.

For sensitive people: When advice recommends reducing activity, consider shorter, lower-intensity sessions; choose routes away from heavy traffic or industrial emissions where feasible; and follow an existing asthma or other medical action plan.

During wildfire smoke or severe particle episodes: Reduce outdoor exposure, keep windows closed when outdoor air is worse, limit indoor particle sources, and consider appropriately sized HEPA air cleaning. A well-fitting, approved particulate respirator (such as N95 or equivalent) may reduce inhalation of fine particles when worn correctly; it does not remove gaseous pollutants such as CO. Follow public-health warnings and any evacuation instructions. US EPA smoke guidance.

For indoor air: Track PM2.5 separately from CO₂ and VOCs. A purifier can reduce particles without necessarily fixing poor ventilation or gaseous pollutants; opening windows may improve ventilation but may also import smoke, traffic pollution or ozone. Match the intervention to the pollutant and outdoor conditions.

When travelling: Do not compare an EEA category with a US AQI number as though they share a common scale. Compare station pollutant concentrations with matched averaging periods where available, and check local forecasts for the day and activity planned.

13. Why two apps can disagree

Differences do not automatically mean that one source is defective. Check whether the services use:

• Different index methodologies (EEA vs EPA vs national vs proprietary).
• Different averaging times (hourly, NowCast, rolling 24-hour, daily or annual).
• Different observation locations (roadside, urban-background, suburban, rural or indoor).
• Different data sources (regulatory stations, lower-cost sensors, satellite/modelled estimates, forecasts).
• Different pollutant coverage (one app may omit ozone or NO₂).
• Different timestamps and validation procedures, or calculations based on incomplete data.

Lower-cost optical particle sensors can be affected by humidity, aerosol composition, calibration and placement. Sensor readings are useful for patterns and local awareness but may not match certified regulatory-grade measurements. A single indoor sensor also cannot represent an entire city.

14. Worked examples

Example A — European index: PM2.5 8 µg/m³ (Fair), NO₂ 35 µg/m³ (Moderate), ozone 130 µg/m³ (Poor). Assuming qualifying measurements for the same hour, the overall European category is Poor, driven by ozone. An individual who is sensitive to ozone may modify strenuous outdoor activity.

Example B — US AQI: A US app reports AQI 135, explicitly using US EPA AQI. This means Unhealthy for Sensitive Groups. Check which pollutant is responsible and follow the specific advice; the number 135 is not 135 µg/m³.

Example C — Canada: The forecast says AQHI 8. That is High Risk on Canada’s scale. People at risk should reduce or reschedule strenuous outdoor activities; this is not equivalent to US AQI 8.

Example D — indoor readings: A monitor shows PM2.5 7 µg/m³ and CO₂ 1,400 ppm. These values address different problems: fine-particle concentration and a possible ventilation/occupancy issue. You cannot calculate a valid “overall AQI” simply by averaging them. Investigate ventilation in context, while keeping outdoor pollution in mind.

15. Frequently asked questions

Is AQI 100 healthy or unhealthy?

It depends on the index. In the US, AQI 100 is the upper end of Moderate. In India, 100 is the upper end of Satisfactory. The UK and Canada use completely different index ranges. In the EEA scheme, public-facing categories are qualitative rather than a universal 0–500 numeric scale.

Is PM2.5 more important than PM10?

PM2.5 is particularly important for long-term cardiovascular and respiratory risk, but both are relevant. Do not dismiss PM10 during dust events or treat a low PM2.5 result as proof that all pollution is low.

Does “Good” air quality mean pollution-free?

No. It means that measured or estimated concentrations fall within the lowest category of that particular index, for the pollutants and times considered. There is no guarantee of zero risk or absence of unmeasured substances.

Which is better: US AQI or European AQI?

Neither scale is universally “more accurate” in every context. They apply different breakpoints, averaging conventions and reporting methods to related pollutants. Use the authority’s locally relevant index for action, and concentrations plus averaging periods for international comparisons.

Should I use an hourly reading or the forecast?

Use recent measured or modelled conditions to understand what is happening now and the forecast to plan activities later. Recheck if weather, wildfire smoke, traffic or other conditions are changing quickly.

Can indoor CO₂ tell me whether outdoor air pollution is safe?

No. Indoor CO₂ can help assess ventilation relative to occupancy, while outdoor pollution requires particulate and gas measurements. Neither substitutes for a functioning CO alarm where combustion hazards exist.

Authoritative sources and further reading

The sources below are primary governmental, intergovernmental or official reference materials. Publication methods, thresholds and legal requirements can change; verify the latest methodology before deploying an automated AQI calculator.

1. European Environment Agency (EEA) — European Air Quality Index: bands, methodology and health messages (airindex.eea.europa.eu).
2. EEA — Air pollution topic and EU monitoring resources (eea.europa.eu).
3. World Health Organization (WHO) — 2021 Global Air Quality Guidelines (who.int).
4. WHO / US National Library of Medicine — Full table of 2021 guideline levels and averaging periods (ncbi.nlm.nih.gov).
5. US EPA / AirNow — US AQI categories and NowCast explanation (airnow.gov).
6. US EPA — 2024 PM2.5 AQI breakpoint revision and particle health effects (epa.gov).
7. UK Government / Defra — How to use the DAQI, pollutant breakpoints, and health advice (gov.uk).
8. Environment and Climate Change Canada — Air Quality Health Index (canada.ca).
9. India Central Pollution Control Board — National AQI technical document and live AQI portal (cpcb.gov.in).
10. European Union / EUR-Lex — Directive (EU) 2024/2881 on ambient air quality (eur-lex.europa.eu).
11. US CDC / NIOSH — Ventilation and CO₂ monitoring FAQ (cdc.gov).

Medical-information notice: This article provides general education, not individual diagnosis or medical advice. Follow local public-health alerts, emergency instructions and guidance from a qualified clinician for personal health decisions. Source review date: 14 October 2026.

• air quality
• pollution
• health
• European Air Quality Index
• AQI
• environment