Ways to Measure Air Pollution

Ways to measure Air Pollution

Air quality index (AQI) is a measure used for reporting the daily air quality, by factoring the level of Pollution in the air. Different countries use different indices for measuring air quality by monitoring some or all of the following Pollution: carbon monoxide, PM2.5, PM10, Nitrogen dioxide, ground Ozone (O3), sulphur dioxide among others.

Air quality indices have two main purposes:

  1. To relay necessary air quality information to the public so that people can modify their behaviour and stay healthy.
  2. By monitoring pollution levels, countries can assess the effectiveness of their policies and adjust as needed to achieve better air quality in the future.

When air quality indices record high levels, government agencies deal with such instances through any of the following ways:

  • Sensitive groups, including children, the elderly and people with respiratory and cardiovascular problems are advised not to take part in any outdoor activities
  • Factories that emit large amounts of pollutants are ordered to shut down or cut production. People are also encouraged to use public transport to reduce vehicle emissions. This has happened severally in China where the authorities have increased their efforts to bring China pollution to manageable levels.
  • People are asked to wear protective masks to avoid breathing in pollutants, particularly PM2.5 and ground ozone.

How pollutants are measured

Different instruments are used to collect samples from different locations and to physically measure them. The list of pollutants tracked can vary depending on a country’s priorities.

Concentration is either expressed in unitless proportions such as parts/million (ppm) or parts per billion (ppb) or in mass per volume (micrograms per cubic meter (mg/u3)).

These concentrations are then converted and represented on a pollution index scale. For example, in the U.S., this is a number between 0 to 500, for the new China air quality index this is also a number between 0 to 500. Cities in China that have not adopted the AQI, still use API to measure china air quality on a scale of 0 to 300.

The values in the pollution index are known as ‘breakpoints’. Every breakpoint corresponds to a defined pollution concentration.

The pollutant that reports the highest index determines the AQI (or AQHI, PSI, API, etcetera). For instance, if measurements for Beijing air quality show that PM2.5 has an AQI of 150 and all other pollutants report lesser than 150, then the AQI will be based on the PM2.5 levels.

Though most countries in creating their pollution indices adhere to WHO guidelines, a lot of them base their pollution index decisions on local needs. The WHO Air Quality Guidelines (released in 1987, with subsequent updates in 1997 & 2005), are not significant from a legislative point of view, but are instead meant to advice policy.

The history of Air Quality indices

U.S. Air Quality Index (AQI)

The US Environmental Protection Agency (EPA) formerly used PSI (Pollutant Standards Index) to measure air pollution in the US. In June 2000, a review of PSI led to the inclusion of a new interval (unhealthy for sensitive groups) and addition of ozone and PM2.5 to make a total of five pollutants.

The updated index was named AQI. AQI converts the concentration in the air of these 5 pollutants and represents them in 6 categories on a scale of 0 to 500.

BlogGraphic_Pollution Measurting

China’s Outgoing and Incoming Pollution Indices

Current and outgoing index - API or Air Pollution Index

API (Air pollution index) is the local index currently used for measuring air pollution in China. The index represents 5 pollutants on a scale between 0 to 300.

New and incoming index - AQI or Air Quality Index

PM2.5 is a major threat in China and this is one of the main reasons for the creation and adoption of a new index - China’s Air Quality index - as it measures PM2.5. Beijing air quality is now measured on the new AQI; most other Chinese cities have also adopted the new index.

Though China’s AQI is based on the US AQI, it differs from U.S. AQI in that it uses different concentration thresholds. While the U.S. uses a threshold of 15, China uses 35.

Table from WHO Air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide. Global update 2005. More detailed information about the AQG and interim targets in the table above are contained in the full report here.


Singapore’s PSI

The national environmental agency of Singapore (NEA) is responsible for monitoring concentration of pollutants in the air. According to NEA, the PSI used in Singapore maintains the same national ambient air quality standards used by U.S EPA.

Earlier PSI used by Singapore was previously based on PM10 readings but as from April 2014, PM2.5 was included in the list of pollutants, in addition to sulphur dioxide, ozone, PM10 and nitrogen dioxide.

Singapore arrived at the new PSI by adopting the practices of countries that were already measuring PM2.5 levels in their indices, such as the USA, Hong Kong, China pollution index, and others. The current PSI is thus based on PM2.5 reporting, which is more stringent than reporting for PM10.

Singapore’s PSI is expressed on a scale of 0 to 300 but it also considers that any reading above 400 is life threatening especially to sick people and elderly persons.

Canadian Air quality health index (AQHI)

In 2001, Canadian departments of health and environment were concerned that the AQI which was in use at the time:

  • Generated alerts based on threshold levels that were not very scientific
  • Did not standardise thresholds used across the county and this led to data inconsistencies
  • Lacked consistency in the advisory messages communicated to the public about the impact of air quality.

Therefore, Canada’s environmental and health agencies (known as Environment Canada and Health Canada respectively), created the Air Quality Health Index (AQHI) that would integrate health knowledge and environmental monitoring, report the results to the public in an easily understandable index, and do so in a consistent way across the country (2011, HSE report, pg.16).

Testing and implementation of the model started in 2002, and by 2009, most provinces in Canada adopted AQHI. The AQHI has 4 categories, represented on a scale of 1 to 10. All ten levels bear a different color code and each category bears elaborate health advisory messages.


The UK decided to review its AQI in 2009 due to the following reasons:

  • The AQI in use at the time had existed for 12 years, during which time changes in European air quality legislation and WHO guidelines had occurred. For instance, the World Health Organization (WHO) released updated guidelines in 2005, including providing new information about effects of pollutants.
  • Carbon monoxide and sulphur dioxide levels had reduced in the UK in line with EU legislation.
  • There was increased concern about particulate matter as a major pollutant.
  • Surveys conducted among end users also revealed that people wanted health advisory messages that were clear and didn’t contain any jargon.

The revised AQI used the Canadian system as its basis (HSE report, pg.26,) and has the following changes:

  • It has 4 bands (low, medium, high and very high), which are accompanied by health advice for the population, including advisory for susceptible individuals.
  • It no longer contains carbon monoxide
  • PM2.5 was added to the index
  • Conditions for measuring ozone (O3), Nitrogen dioxide (NO2) and (PM10) are more stringent
  • For most pollutants, breakpoints for the bands are based on WHO values.

Converting pollution levels to a universal Index

There’s a lot of confusion around pollution/air quality index conversions. It’s difficult to make a direct conversion say from AQI to PSI or from PSI to AQHI etcetera. This is because air quality indices are local, hence a classification on an index in one country might mean something very different in another.

The most reliable way of understanding pollution data, no matter the index it’s represented in, is by looking at actual measurements in micrograms per m3, or to convert the index value to concentration. This way, you’ll be looking at actual measurements.

According to a report by WHO, if pollution data from different countries is to be comparable, it’s to be expressed in standard units (1969 WHO report, pg. 16) The report further states that the recommended unit for pollution measurement is micrograms per cubic meter (ug/m3). Therefore, as mg/m3 is a standard measurement, looking at actual measurements of pollutants allows you to compare apples to apples.

This simple calculator from Clean Air Initiative (CAI) makes conversion from PSI to concentration values a lot easier from. You can also use these calculators developed by the U.S EPA to:

Convert concentration to AQI here.

Convert AQI to concentration here.

This means that with the second calculator, once you have concentration levels say for air pollution in China, you can represent your pollution levels on an index of your preference.

Stay healthy