Air Quality in Indian Cities - Part 2 - AQI and what to measure

Air quality Index (AQI) and what to measure - Particles and Gases

This is second article in the series of articles Yuktix is publishing on air quality. First part of this series looked at the requirements of measuring air quality in Indian cities. We tried to answer why we should measure air quality and how we can develop a plan to do so on a budget. Here in this article, we look at what variables to measure and what comprises air quality index and how we should report and interpret it.

The best place to look for information related to air quality is US environment protection agency (referred to as EPA in this article).   There are many gases and different types of particles present in the environment around us. Most can have harmful effect on humans in higher/lower concentrations so it is important to know what we want to measure under ordinary circumstances, e.g. when we are walking on the road or sitting on porch sipping coffee.

The Air quality index (AQI) of EPA, USA comprises of following six pollutants

  1. PM 2.5 (particles less than 10 microns) 
  2. PM 10 (particles less than 2.5 microns) 
  3. NO2 (Nitrogen Dioxide) 
  4. SO2 (Sulphur Dioxide) 
  5. CO (Carbon Monoxide) 
  6. O3 (Ozone) 

The EPA site has a wealth of information and there is no point in repeating that here. EPA site also has AQI calculators, sources of pollution and their effect on humans. 

Government of India Air Quality index comprises of above six and also 

  1. Ammonia (NH3) 
  2. Lead (Pb) 

The ministry of Environment, Forest and climate change links to an IIT Kanpur website that of course, does not work so we are trusting the Wikipedia link.

We first quickly summarize the risk of individual pollutant on humans, their possible sources and then move on to AQI.

Carbon Monoxide (CO)

Carbon monoxide (CO) is a colorless, odorless gas emitted from combustion processes. Nationally and, particularly in urban areas, the majority of CO emissions to ambient air come from mobile sources. It forms when carbon in the fuel does not completely burn. CO can cause harmful health effects by reducing oxygen delivery to the body's organs (like the heart and brain) and tissues. CO can cause issues with mental alertness and vision. At extremely high levels, CO can cause death. Indian cities can have CO from vehicle exhausts, burning coal and wood, furnaces and Diesel generators.

Ozone (O3)

It is hard to believe that ozone can be bad when we all know about the role it plays in shielding us from harmful UV rays. However Ground level or "bad" ozone is not emitted directly into the air. Bad ozone forms when pollutants emitted by sources such as cars, power plants, industrial boilers, refineries, and chemical plants react chemically in sunlight. Breathing ozone can trigger variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma.

Ozone can aggravate asthma and even cause permanent lung damage.

PM 2.5 and PM 10

  • Typical human hair width 17-180 micrometers
  • Average human hair width 70 micrometers
  • Resolution of Human eye - 90 micrometers
  • PM2.5 - particles less than 2.5 micrometers
  • PM10 - particles less than 10 micrometers

PM2.5 and PM2.5 refer to particles that are very small and you cannot see them with naked eye. Particles smaller than 10 micrometers in diameter can cause or aggravate a number of health problems and have been linked with illnesses and deaths from heart or lung disease.

The reporting for particles is usually done in two ways.

Better or more sophisticated instruments actually catch and weigh the particles of a particular size. The instrument will catch particles over a sampling period in a volume, weigh them and report the concentration as micrograms/cubic meter (Ug/m^3).

Simple instruments report number of particles found in a particular volume. They don’t have the mechanism to actually weight the particles. That volume is usually reported in 0.01cf (0.01 cubic feet or 283 ml). So you will see reporting done as 2000/0.01cf or 283 ml. That means the instrument has detected 2000 particles below this particular size in a sampling period in 283ml volume.

PM10, PM2.5, PM1 are not different things but 3 different metrics of the same aerosol population. The reasons for using one over the others are largely historic as there is loads more info on PM10 than there is on PM2.5 and on PM1 but these are all indicators and not in themselves the goal of any measurement.

Fine particles (PM 2.5)

The smallest particles (those 2.5 micrometers or less in diameter) are called “fine” particles. These particles are so small they can be detected only with an electron microscope. Major sources of fine particles include motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, some industrial processes, and other combustion processes.

Coarse particles (PM10)

Particles between 2.5 and 10 micrometers in diameter are referred to as “coarse.” Sources of coarse particles include crushing or grinding operations, and dust stirred up by vehicles traveling on roads.

Sources for PM2.5 are

  •     Acids
  •     Organic chemicals,
  •     Metal,
  •     Dust particles
  •     Gases from automobiles
  •     Industrial pollution

   Sources for PM10

  •  Dust
  •  Pollen
  •  Mold

Nitrogen dioxide (NO2)

NO2 is one of a group of highly reactive gasses known as "oxides of nitrogen," or "nitrogen oxides (NOx)." NO2 forms quickly from emissions from cars, trucks and buses, power plants, and off-road equipment In addition to contributing to the formation of ground- level ozone, and fine particle pollution. NO2 is linked with a number of adverse effects on the respiratory system.

Sulphur dioxide

SO2 is one of a group of highly reactive gasses known as “oxides of sulphur.” The largest sources of SO2 emissions are from fossil fuel combustion at power plants (73%) and other industrial facilities (20%). Smaller sources of SO2 emissions include industrial processes such as extracting metal from ore, and the burning of high sulphur containing fuels by locomotives, large ships, and non-road equipment. SO2 is linked with a number of adverse effects on the respiratory system.


Ammonia is a colorless gas with a pungent odor that is noticeable at concentrations above 50 ppm. Most of the NH3 emitted is generated from livestock waste management and fertilizer production.

Ammonia is poisonous if inhaled in great quantities and is irritating to the eyes, nose, and throat in lesser amounts. It combines in the atmosphere with sulphates and nitrates to form secondary fine particulate matter (PM2.5). PM2.5 is known to have harmful effects on human health and the environment. NH3 can also contribute to the nitrification and eutrophication of aquatic systems.

Lead (Pb)

Lead (Pb) is a metal found naturally in the environment as well as in manufactured products.  The major sources of lead emissions have historically been from fuels in on-road motor vehicles (such as cars and trucks) and industrial sources

Lead exposure can happen from air pollution (vehicle exhausts), lead in drinking water supply and lead contaminated food.  Once taken into the body, lead distributes throughout the body in the blood and is accumulated in the bones.  Depending on the level of exposure, lead can adversely affect the nervous system, kidney function, immune system, reproductive and developmental systems and the cardiovascular system.  Lead exposure also affects the oxygen carrying capacity of the blood


Air quality index (AQI) is an effort to map the different units of measurement and ranges into a friendly number scale. To illustrate, we could have reported air quality information as

  • CO – 35 PPM
  • O3 – 75 PPB
  • PM2.5 -- 10 ug/m^3 to 100ug/m^3
  • PM10 – 10ug/m^3 to 100 ug/m^3
  • NO2 – 100 PPB
  • SO2 – 100 PPB
  • Lead  - 0.15 ug/m^3

Here gases are reported in parts per million (PPM) or parts per billion (PPB). There are different acceptable ranges for different gases, e.g. CO at 10 PPM is not hazardous but NO2 at 10 PPM is hazardous.  Also, the numbers do not indicate whether the concentrations are hazardous or at moderate levels.  

For quick reading, we need a system where we can map full concentration range onto a number scale so we can say something like

CO – 0-10 PPM – OK   1 on Danger scale
CO – 10–35 PPM – Not OK but can tolerate – 2 on Danger scale
CO – 35–70 PPM – NOT OK, approaching hazardous levels – 3 on Danger scale

Then we can render the information as

1      Green  - 1 
    Yellow - 2   
    Red - 3 

    Dead - 4 

There is nothing magical about EPA 0-300 scale.  They might as well have had 0-10 or 1-5 scale. There in the report, you have a number and that number means a concentration of particle or gas. You can convert from AQI number to concentration using a calculator

Most people are likely to follow EPA reporting standards

  • Good – 0-50
  • Moderate 50-100
  • Unhealthy 101-150
  • Unhealthy for everyone 151-200
  • Very unhealthy 201-300

AQI is not like stock market index

AQI is concentration to number translation for a pollutant in the index. This is not a weighted or average index like a stock market index. That is why you have separate AQI number for every pollutant included in the index. AQI is “index” in the sense that you have an index of “pollutants”.

So at least in theory you can have safe AQI numbers for pollutant A and hazardous levels for pollutant B. Tomorrow it may as well happen that Government of India decides to create their own AQI index.  They may as well say that in India, CO till 35 PPM is Level 1 and after 70 PPM is level 2 etc.

Third part of this series will look at how to measure particles and gases. We will try to demystify the terms and jargons. We will look at typical sensors deployed to measure particles and gases and what can be done to make a sensor cocktail on budget.  


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