How is Air Quality Reported?
There are various ways to report air quality measurements. Sometimes you will see air quality reports as disaggregated by individual pollutant, where each pollutant is measured separately and you see a separate number assigned to each component of pollution, sometimes with the highest number of concern highlighted.  Air quality measurements are reported in terms of concentrations per cubic meter of air volume (m3). These concentrations are described in terms of micrograms per cubic meter (μg/m3). Below is an example of this type of air quality report. Notice the separate pollutants are listed on the left hand side (PM10; PM2.5 and O3.)

ex from Indian institute of tropican meteoroloty

The Air Quality Index (“AQI”) is another way to report air quality measurements. An AQI number is calculated using a formula that mixes together the levels of several of the major air pollutants to derive a single number denoting the air quality measure. There is no singular, international standard AQI calculation so governments come up with their own way to weigh the different pollutants measured to achieve a singular number. India has an AQI and will report this number as  “Overall AQI” as well as reporting the individual measures of the pollutants that make up the AQI number.  The measurement you get from the U.S. Embassy in Delhi, is only the overall AQI.   (more about the AQI here). Below is an example of an air quality report using the AQI. Notice that you don’t see the different names and measure of each pollutant, as these are combined to form a single number.

example- AQI reading from US embassy

The  AQI is often presented with a color-coded chart with the corresponding level of health concern or activity guidelines.   These charts are created based on the AQI formula. So, the US Embassy AQI will work with the US color-coded chart created by the Environmental Protection Agency.   The AQI numbers you get from the Safar and other Indian Government websites, will work with the Indian color coded AQI chart. These will be slightly different because the countries weigh each pollutant differently in their calculations of AQI.  You can’t cannot compare the measure of a single pollutant to a measure of AQI (so, PM 2.5 reading of 153, is only a part of an AQI number, taken from the same time at the same place.) If you want to use the color-coded health and activity charts with a specific measure of a pollutant like ozone or PM 2.5, you first need to convert that raw data into an AQI, and use an AQI converter. The one here is provided by the US EPA.

To further understand the color-coded charts with activity guidelines and health alerts, see the page on activity guidelines.

Some air quality measures are given in “real time” and you can see changes in pollution concentrations on or shortly after they occur—reflecting everything from traffic patterns to the effects of local weather. Other measures are reported over a longer time span, using a daily or annual mean concentration.  Specific location where measurements are taken also factors in, so measurements taken in different parts of town will reflect the different levels of air pollution in those areas. These factors make comparing numbers reported from different sources very difficult.

Websites that publish Delhi air pollution data:

What do these numbers mean?
Now that you see that air pollution data can be measured and reported in different ways, the next question is what these numbers mean to your health. See Activity guidelines to get advice from the U.S. Environmental Protection Agency on levels of concern related to levels of air pollution. The Health Effects of Air Pollution for general information on health. Or check out International Benchmarks and Standards to compare the numbers we see in Delhi to the levels considered “safe” to our health.