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Air pollution: Various acronyms, numbers and units

Various acronyms, numbers, and units make any discussion of air pollution confusing. Here is a ready reckoner.

Air pollution is fast becoming a new health hazard. To get an overview of the problem, read this article on this website: Indoor air pollution: An overview.

Any discussion of air pollution involves different acronyms, numbers, and units. While they are simple, without knowing them, any discussion becomes difficult to understand.

Air pollutants

Air has physical pollutants, such as dust particles. It also has chemical pollutants, such as carbon monoxide and nitrogen dioxide gases. Finally, in some air pockets, such as hospitals, there are biological pollutants, viz., viruses and bacteria.

The outdoor pollution is mainly physical and chemical in nature.

Physical pollutants

Outdoor air contains particles of various sizes. While we assume that dust is a part of nature, prolonged exposure to dust — especially superfine dust — is harmful to the body.


PM10 refers to particulate matter smaller than 10 micrometers in diameter. Micro is written as µ. So, PM10 is the total weight of all the particles in the air, which are smaller than 10 µm in their largest dimension. The unit of PM10 is µg/m3 of air.

What if there is a superfine strand of something, which is much smaller than 10 µm in thickness, but is larger than 10 m in length? Exclude it.

This is because PM10 are the particles that can pass through your nasal hair and nasal mucus to go into your lungs. That is where they cause harm. If we get such a strand, it will be stopped by the nose and won’t reach the lungs.


PM2.5 refers to particulate matter smaller than 2.5 µm in diameter. PM2.5 is the total weight of all the particles in the air, which are smaller than 2.5 µm in their largest dimension. It is also measured in µg/m3 of air.

PM2.5 is relevant because these are the particles that cannot be stopped by nasal hair, nasal mucus, or lungs. They pass through the small alveoli that help exchange oxygen and carbon dioxide between the air and the blood. Thus, PM2.5 can enter your bloodstream easily and spread all over your body.

PM2.5 and PM10 are both counted as measures of air pollution: PM10 indicates coarse pollution, while PM2.5 indicates fine pollution.

PM10 includes PM2.5

You may point out that PM10 includes PM2.5. So, are we double–counting the PM2.5 pollution?

Yes, we are. But, 10 µm is 4 times bigger than 2.5 µm. This is for one dimension.

Typically, a PM10 particle will be roughly equally big in its length, breadth, and depth — in all three dimensions. So, in general, a typical PM10 particle will be 4 x 4 x 4 = 64 times bigger than a typical PM2.5 particle. Thus, it may weigh 64 times more than a typical PM2.5 particle.

Of course, the number of PM2.5 particles will be many times more than PM10 particles, which are larger than 2.5 µm, given that they are very small. Thus, PM2.5 may be a small contributor to PM10 and its percentage changes based on the type of pollution.

PM10 pollution versus PM2.5 pollution

Harmful effects of PM2.5 are seen at much lower levels of pollutant weights than those of PM10 because PM2.5 particles are more penetrative. For example, as the US Environmental Protection Agency (US EPA), safe 24–hour levels of PM10 are 150 µg/m3, while those for PM2.5 are 35 µg/m3.

Since these numbers are derived from epidemiological studies, logic tells me that 150 µg/m3 of PM10 must be containing less than 35 µg/m3 of PM2.5.

PM2.5 is quite a high fraction of PM10, when it comes to combustion-based pollution, such as that from vehicles. However, non–combustion sources, such as dust storms or road dust, throw up bigger particles. So, they cause coarser pollution, which is captured in the PM10 number but not in the PM2.5 number.

Chemical pollutants

As you can guess, PM2.5 and PM10 cannot account for any gases in the air. Gases are mixed in the air at the molecular level. So you cannot identify as separate particles in the air.

There are many air pollutants in the gaseous form, such as sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and ground-level ozone (O3). These are important parts of outdoor pollution, as they arise from vehicular and industrial emissions.

While there are many more pollutants, outdoor air quality is broadly defined based on 2 physical pollutants — PM2.5 and PM10 — and 4 chemical pollutants — SO2, NO2, CO, and O3.

Air quality index or AQI

The air quality index is an index calculated using the levels of the air pollutants discussed above. It gives a rough idea about the severity of air pollution.

First, AQI is calculated for individual pollutants, using a formula derived from epidemiological data.

For example, hundreds of studies on a pollutant may indicate what levels are safe, and to what extent. So, governmental agencies develop a formula, or a table, to calculate AQIPM2.5 from measured values of PM2.5. As an example, PM2.5 of 6 µg/m3 corresponds to AQIPM2.5 = 25. This is an individual AQIPollutant.

Since we don’t want to become pollution scientists, let us leave out these complex conversion formulas and just stay with the basics.

Historical AQI

In earlier days, AQI number was calculated using different weightages for AQIs of multiples pollutants.

However, that started becoming a problem. In many places, one or more of the pollution constituents were not measured. Hence, the formula could not calculate air quality index, due to incomplete data.


So, later on, government agencies decided a new approach. AQI was calculated for various pollutants, for which the data was available at any place. And, the highest, or worst, AQI value among the different pollutants was taken as the air quality index.

Calculating new AQI

For example, below is a partial table of values EPA uses for calculating AQIUS, or AQI as per the US definition. It shows AQIPM2.5 and AQIPM10 values for various PM2.5 and PM10 values, respectively.

PM2.5 (µg/m3)PM10 (µg/m3) AQIPM2.5 or AQIPM10AQI Category
0.0-12.0 (24-hr)0-54 (24-hr)0-50Good
12.1-35.4 (24-hr)55-154 (24-hr)51-100Moderate
35.5-55.4 (24-hr)155-254 (24-hr)101-150Unhealthy for sensitive groups
55.5-150.4 (24-hr)255-354 (24-hr)151-200Unhealthy
150.5-250.4 (24-hr)355-424 (24-hr)201-300Very unhealthy
250.5-350.4 (24-hr)425-504 (24-hr)301-400Hazardous
350.5-500.4 (24-hr)505-604 (24-hr)401-500Hazardous

Thus, as an example, if you have PM2.5 = 45.5 µg/m3, we would say AQIPM2.5 ≈ 125

And, if PM10 = 175 µg/m3, AQIPM10 ≈ 110.

Since AQIPM2.5 > AQIPM10, we will say that AQI = AQIPM2.5 ≈ 125.

On the other hand, if PM10 = 215 µg/m3, AQIPM10 ≈ 130.

In that case, since AQIPM2.5 < AQIPM10, we will say that AQI = AQIPM10 ≈ 130.

New AQI advantages and limitations

This AQI calculation method allowed missing measurements for some pollutants. This also allowed the possibility of adding a new pollutant in the future and taking its effect into consideration.

There is obviously a slight problem. Consider the two situations:

Case 1: AQIPM10 = 120 and AQIPM2.5 = 118

Case 2: AQIPM10 = 120 and AQIPM2.5 = 18

In both these cases, overall AQI = 120 (assuming gaseous pollutants are negligible in quantity). That might suggest that the air quality is equally good, or bad, in both cases. However, in the first case, the fine particle pollution is quite high compared to that in case 2. Thus, the air in the first case is more polluted, but its AQI does not reflect that.

AQI in different countries

Various countries use different sets of pollutants for calculating their AQIs. They also use different formulas to get individual pollutant AQI values. Finally, different countries have varied interpretations of the values of their AQIs. If you want to become an international expert on air pollution, read here: Air quality index. 🙂

The USA uses main 6 pollutants in calculating its AQI: PM10, PM2.5, NO2, SO2, CO, and O3.

China also uses those 6 pollutants in calculating its AQI: PM10, PM2.5, NO2, SO2, CO, and O3.

India uses 8 pollutants in calculating its AQI: PM10, PM2.5, NO2, SO2, CO, O3, NH3 (ammonia), and Pb (lead).

The UK uses 5 pollutants in calculating its AQI: PM10, PM2.5, NO2, SO2, and O3

Volatile organic compounds (VOCs)

There is another category of pollutants called volatile organic compounds or VOCs. For example, paints and coatings, benzene, formaldehyde, and fuels. They are mainly found indoors, especially in enclosed spaces. They are also quite harmful to humans.

If they are released in an outdoor environment, they mix with the air. Their concentrations drop so low that they cannot even be detected. Hence, they are not a part of any air quality index, which measures outdoor air quality. However, for getting a good perspective about indoor pollution, this number must also be noted.

Many handheld machines used for measuring indoor pollution can measure total VOC (TVOC) amount in the air. The units of TVOC are again µg/m3. They can also be measured in ppm (parts per million) or ppb (parts per billion). However, some of those chemicals are very difficult to measure accurately in real life. Hence, there are no standards developed yet for their measurement.

First published on: 30th July 2019
Image credit: alvpics from Pixabay
Last updated on: 3rd May 2022


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