Air Quality Standards and Measurement (AQS, AQI, EAQI, DAQI, AQHI, NAQI)
Published on:
May 28, 2026
Updated on:
May 28, 2026
Published on:
Updated on:
Air quality standards matter because thesame pollution reading can be interpreted very differently depending on whichindex a country or app uses. This affects public warnings, health guidance, andhow people compare air quality across borders. The World Health Organizationsays the combined effects of ambient and household air pollution are associatedwith about 7 million premature deaths a year, which is why clear, trustworthyair-quality communication matters so much.
Air quality standards are frameworks thattranslate pollutant concentrations into something the public can act on. Theyvary because countries do not use identical legal limits, pollutant mixes,time-averaging rules, or health-communication models.
In practice, one index may be builtaround regulatory standards, another around short-term health risk, and anotheraround product usability inside an app. That is why a reading considered“moderate” in one system may not align neatly with a “moderate” readingelsewhere.
● Pollutants included, such asPM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide, carbon monoxide, orindoor metrics like CO₂ and TVOC.
● Averaging periods, such as 1-hour,8-hour, 24-hour, or rolling means.
● Breakpoints and category names.
● Scale shape, for example 0–100,0–500, 1–10, or a letter-grade style.
● Decision rule: many systems reportthe highest pollutant sub-index, but some add health-risk weighting or extrarules.
The table below provides a quickcomparison. It shows what each standard is for, what it measures, and the mainaveraging window used in public reporting. Important note: many AQI systems donot use one universal time interval; they use pollutant-specific windows suchas 1-hour, 8-hour, 24-hour, or rolling means.
Atmotube’s AQS is a product-specificpersonal air score, not a national regulatory index. It can be used indoors andoutdoors because it is based on portable sensor readings, and it is designed toturn multiple real-time sensor streams into a simple 0–100 cleanliness scorefor everyday decision-making.
Atmotube says AQS uses an internalalgorithm based on real-time readings for CO₂, TVOC, NOx, PM1, PM2.5, and PM10,and that the score reflects the most critical factor at a given moment. Thepublished AQS bands are Good (81–100), Moderate (61–80), Polluted (41–60), VeryPolluted (21–40), and Severely Polluted (0–20).
Unlike government AQIs, AQS mixes indoorcomfort and personal-exposure indicators such as CO₂ and TVOC with particlepollution. That makes it useful for room-level and on-the-go checks, especiallywhen an instant reading matters, but it is not appropriate for directcomparison with city or country AQI values.
The US AQI is one of the world’sbest-known public air-pollution scales. It converts pollutant concentrationsinto a 0–500 scale and reports the highest pollutant sub-index as thelocation’s AQI. Its averaging window depends on the pollutant, so it should notbe described as a single 24-hour index.
The US Environmental Protection Agencycalculates AQI from pollutant concentrations using pollutant-specificbreakpoints and equations. For real-time reporting, EPA also uses NowCastmethods for PM and ozone so the index better reflects current conditions ratherthan waiting for a full historical averaging window.
The US AQI covers PM2.5, PM10, ozone,carbon monoxide, sulphur dioxide, and nitrogen dioxide, with category labelsranging from Good to Hazardous.
China’s AQI is the national ambient-airindex used for public reporting and health advice. Like the US AQI, it usespollutant sub-indexes and reports the worst one.
China’s technical regulation HJ 633-2012says the AQI standard defines classification and calculation methods forambient-air reporting. It uses pollutant sub-indexes and publishes daily,real-time, and forecast values for public communication.
China’s AQI is deeply tied to thecountry’s official technical reporting framework and is widely used in domesticdashboards and data services.
CAQI, the Common Air Quality Index, wascreated to help people compare city air quality across borders in Europe. It isa comparative urban index rather than a universal replacement for localsystems.
The CITEAIR method calculates CAQIthrough linear interpolation between class borders and then reports the highestpollutant sub-index. It has separate versions for traffic sites andcity-background sites, which is important because roadside air can be muchworse than city averages.
CAQI was designed for cross-citycomparability. That is useful for travelers and dashboards, but it also meansit is a compromise system rather than a local health standard tailored to onenational context.
The EAQI is the European EnvironmentAgency’s real-time Europe-wide air-quality index. It is built to help userscompare current air conditions across countries, cities, and stations on onecommon map. It reports an hourly index, but particulate matter is assessedusing 24-hour running means.
The EEA says the index is based on fivepollutants: PM10, PM2.5, NO₂, O₃, and SO₂. The hourly overall index isdetermined by the pollutant with the poorest air quality for that hour.
EAQI focuses on timely, Europe-wideharmonized communication. It is especially useful when consistent cross-bordercomparisons are needed, even though countries may still maintain their owndomestic reporting systems.
The UK DAQI is a short-term health-riskcommunication tool that uses a simple 1–10 scale. It translates measuredpollutant concentrations into bands that are easier for the public to act on.Its averaging windows are pollutant-specific rather than one universal dailyvalue.
The UK government publishes pollutantconcentration bands for each DAQI level. PM2.5 and PM10 are based on daily meanor latest 24-hour running means, NO₂ on hourly mean, ozone on running 8-hourmean, and SO₂ on 15-minute mean.
Its main strength is clarity: a 1–10score with linked public-health advice. That makes it especially readable forweather services and everyday planning.
Australia’s AQI is commonly expressed asa pollutant concentration relative to the National Environment ProtectionMeasure standard. In practical terms, a value of 100 means a pollutant hasreached the relevant standard.
Australia’s State of the Environmentreport explains that AQI is calculated by dividing pollutant concentration bythe relevant NEPM standard and multiplying by 100. A value of 50 thereforemeans half the standard, while 100 means the standard has been reached.
Australia’s system is unusually intuitivefor regulatory comparison because it is percentage-like. But the exactcategories and presentation can still vary by state or territory, so dashboardsare not always identical nationwide.
Canada’s AQHI is a health-risk indexfirst and a pollution index second. It is designed to tell people how currentair conditions may affect their health, not only whether a standard has beenexceeded.
Environment and Climate Change Canadasays AQHI is based on the relative risks of a mixture of common pollutants:ground-level ozone, nitrogen dioxide, and fine particulate matter. The scaleruns from 1–10+, with categories of Low, Moderate, High, and Very High healthrisk.
This is one of the clearest examples ofan index designed around health communication rather than regulatory compliancealone. For sensitive groups, that framing is often more actionable.
Hong Kong’s AQHI is anotherhealth-risk-based system, but its calculation design differs from Canada’s. Itis built from local health studies and short moving averages.
Hong Kong’s Environmental ProtectionDepartment says AQHI is calculated from the cumulative health risk attributableto 3-hour moving averages of O₃, NO₂, SO₂, and PM2.5/PM10. The risk factors arederived from local health studies.
Hong Kong’s use of local epidemiologicalevidence and moving 3-hour risk windows makes it especially tuned to short-termurban exposure patterns.
India’s National Air Quality Index isbuilt for broad public communication across a large and highly variedmonitoring network. It turns multiple pollutant readings into one overallnumber and category.
CPCB explains that India’s AQI usespollutant sub-indexes based on 24-hour averages, except for CO and O₃, whichuse 8-hour averages. The worst sub-index becomes the AQI. An overall AQI iscalculated only when at least three pollutants are available and one of them isPM2.5 or PM10.
India explicitly defines minimum datasufficiency rules before it will publish an overall AQI. That is a usefulquality-control feature for a country with uneven monitoring coverage acrosslocations.
Korea’s Comprehensive Air-quality Indexis a 0–500 public index based on health risk. It uses multiple pollutants andcan apply extra penalties when several pollutants are elevated at once.
AirKorea says CAI calculates index valuesfor six pollutants and uses the highest value as the base CAI. If two or threepollutants fall into worse categories at the same time, additional points areadded to the responsible pollutant’s value.
The additive penalty mechanism is thedistinguishing feature. It recognizes that a ‘multiple pollutants are badtoday’ situation can be more concerning than a single-pollutant event with thesame headline number.
Singapore’s PSI is the country’s officialpublic air-quality index. It uses a rolling 24-hour pollutant picture andreports the highest sub-index as the overall PSI value. Singapore also reports1-hour PM2.5 separately, but that is not the same as the 24-hour PSI headlinevalue.
Singapore’s National Environment Agencysays the concentrations of six criteria pollutants are used to compute the PSIover a rolling 24-hour period, and the highest sub-index becomes the PSI. Theofficial pollutant set includes PM10, PM2.5, SO₂, NO₂, O₃, and CO.
Singapore’s PSI is especially importantduring haze episodes. The 24-hour framing gives a conservative public-healthview, but it can also feel less responsive to sudden short-term changes thanhourly particle-only displays.
GO IAQS is an open indoor-air frameworkrather than a government outdoor-air index. It aims to harmonize indoor airthresholds and public communication across buildings, products, andgeographies.
GO AQS describes GO IAQS as a two-tiersystem: Starter focuses on PM2.5 and CO₂, while Ultimate adds O₃, CO,formaldehyde (CH₂O), NO₂, and radon. The framework also includes a GO IAQSScore for public-facing communication.
GO IAQS is built as an open, cross-borderindoor standard with an accessibility focus. This makes it closer to a sharedindoor-air framework than to a traditional national AQI.
Because the number shown in an app isnever just the raw pollution concentration. It is the concentration after aplatform-specific translation step: pollutant selection, averaging time,breakpoints, scaling, and category logic.
● One app may emphasize PM2.5because it dominates health risk.
● Another may switch to ozone in theafternoon because ozone peaks with sunlight.
● An indoor product may include CO₂and VOCs, which outdoor AQIs do not.
● A system based on 24-hour averagesmay react more slowly than a near-real-time app score.
No single index is best for everysituation. The strongest choice depends on what you need to know.
AQI usually means Air Quality Index, butthe scale behind it depends on the country or platform using it.3
No. AQI systems differ in pollutants,averaging times, breakpoints, scales, and health-advice logic.
Europe commonly uses the European AirQuality Index (EAQI) from the EEA, while CAQI still appears in historical orlegacy city comparisons.
AQI usually communicates pollutantseverity against breakpoints, while AQHI is more explicitly framed as ahealth-risk index.
Apps may use different standards,different stations, different timing windows, or even different pollutant sets.
PM2.5 is often the dominant health-riskpollutant, but ozone, NO₂, SO₂, CO, and PM10 can also drive the index.
Usually not. Indoor frameworks ofteninclude CO₂, VOCs, formaldehyde, and room-specific thresholds that outdoor AQIsignore.
There is no universal answer, and lowervalues generally indicate cleaner air. For pollutants such as PM2.5, healthrisk can still exist at low concentrations, so sensitive groups should followthe health advice attached to the local scale, not just the number.
Longer averaging windows smooth shortspikes and align with public-health or regulatory reporting, but they may reactmore slowly to sudden pollution changes.
Not directly. AQS is a product-specificindoor score, while US AQI and EAQI are public outdoor indexes built ondifferent pollutant logic.
