Blowing in the Wind

Sections on India's sources, on winter inversion and on the Air Quality Index were written by Swechha in September 2026 and are not the manual's. The manual's own text taught the United Kingdom — the 1952 London smog, British vehicle counts — and explained a foreign air quality index whose band names do not match CPCB's. The pollutant chemistry and the note on sensitive groups are the manual's, kept as written. Standards are CPCB's; source-apportionment figures are quoted from the two government-commissioned studies named in the text.

AIR POLLUTION AND ITS IMPACT

Air pollution is the biggest environmental threat to our health, wherever we live. It can harm every organ in the body, it shortens lives, and it makes chronic illness more likely. In India it is also one of the few environmental problems that almost every student in the room has already experienced directly — a cancelled games period, a grandparent's cough, a morning when the far end of the street was not visible.

This chapter is about what is in the air, where it comes from, how India measures it, and what those measurements can and cannot tell you.

What is air pollution?

Air pollution is anything that causes the air to become contaminated with pollutants at levels harmful to our health or the environment.

What makes the air dirty in India

Most of it is burning. Petrol and diesel burned in vehicles; coal burned in power stations, brick kilns and industry; wood, dung and crop waste burned in kitchens that have no gas connection; rubbish burned at the roadside because nobody collects it. To that, add the dust that does not burn at all — from construction sites, from unpaved roads and from the surface of paved ones.

Two of these are seasonal and northern rather than year-round. In the weeks after the paddy harvest, fields in Punjab and Haryana are cleared by burning the stubble left behind, and the smoke travels. In winter, more fuel is burned for warmth.

How much each of these contributes is genuinely disputed, and a teacher should say so rather than hand out a pie chart. Two studies commissioned by different arms of government looked at the same city and did not agree. IIT Kanpur's 2016 study for the Delhi government put vehicles at 6 to 29 per cent of PM2.5 depending on the site and the season. The TERI-ARAI study commissioned by the Ministry of Heavy Industries put transport at 17 to 28 per cent in its dispersion model, and 18 to 23 per cent in its receptor model. TERI's figure is higher partly because it counted secondary particles formed in the air as well as what came out of the exhaust — so the two are not quite measuring the same thing.

That gap is not a mistake by either team. A source apportionment is not a measurement: it takes measured concentrations, wind data and an inventory of what is emitted where, and works backwards to a plausible split. The answer depends on the method, the season and which sources the inventory happens to know about. The honest statement is that vehicles, industry, dust and burning all matter a great deal, and that their exact shares are contested.

Why the same air is worse in winter

Across the Indo-Gangetic Plain, the same quantity of smoke produces far worse air in January than in July, and understanding why explains most of what students see.

In winter the ground cools quickly after sunset and the layer of air just above it becomes colder than the air higher up. Warm air normally rises and carries pollution away with it; when the temperature is inverted like this, nothing rises. The pollution stays in a shallow layer near the ground, exactly where people are breathing, and it accumulates night after night while the wind is still. The monsoon does the opposite: rain washes particles out of the air and stronger winds disperse what is left.

So an air quality reading is telling you about two things at once — how much was emitted, and what the weather did with it. A still, cold week can make a city look as though its emissions have doubled when nothing about them has changed.

Air pollution also does not respect a city boundary. What is emitted in one district is breathed in another, and a single city's figure hides large differences between neighbourhoods on the same morning — which is why India measures at stations rather than at cities.

The main pollutants

Any gas could qualify as pollution if it reached a high enough concentration to do harm. Theoretically, that means there are

dozens of different pollution gases. It's important to note that not all the things we think of as pollution are gases: some are aerosols (liquids or solids dispersed through gases). In practice, about ten different substances cause most concern:

2.4.1 Sulfur dioxide: Coal, petroleum, and other fuels are often impure and contain sulfur as well as organic (carbon-based) compounds. When sulfur (spelled "sulphur" in some countries) burns with oxygen from the air, sulfur dioxide (SO2) is produced. Coal-fired power plants are the world's biggest source of sulfur-dioxide air pollution, which contributes to smog, acid rain, and health problems that include lung disease. Large amounts of sulfur dioxide are also produced by ships, which use dirtier diesel fuel than cars and trucks.

2.4.2 Carbon monoxide: This highly dangerous gas forms when fuels have too little oxygen to burn completely. It spews out in car exhausts and it can also build up to dangerous levels inside your home if you have a poorly maintained gas boiler, stove, or fuel-burning appliance. (Always fit a carbon monoxide detector if you burn fuels indoors.)

2.4.3 Carbon dioxide: This gas is central to everyday life and isn't normally considered a pollutant: we all produce it when we breathe out and plants such as crops and trees need to "breathe" it in to grow. However, carbon dioxide is also a greenhouse gas released by engines and power plants. Since the beginning of the Industrial Revolution, it's been building up in Earth's atmosphere and contributing to the problem of global warming and climate change. [8]

2.4.4 Nitrogen oxides: Nitrogen dioxide (NO2) and nitrogen oxide (NO) are pollutants produced as an indirect result of combustion, when nitrogen and oxygen from the air react together. Nitrogen oxide pollution comes from vehicle engines and power plants, and plays an important role in the formation of acid rain, ozone and smog. Nitrogen oxides are also "indirect greenhouse gases" (they contribute to global warming by producing ozone, which is a greenhouse gas).

2.4.5 Volatile organic compounds (VOCs): These carbon-based (organic) chemicals evaporate easily at ordinary temperatures and pressures, so they readily become gases. That's precisely why they're used as solvents in many different household chemicals such as paints, waxes, and varnishes. Unfortunately, they're also a form of air pollution: they're believed to have long-term (chronic) effects on people's health and they play a role in the formation of ozone and smog. VOCs are also released by tobacco smoke and wildfires.

2.4.6 Particulates: There are many different kinds of particulates, from black soot in diesel exhaust to dust and organic matter from the desert. Airborne liquid droplets from farm pollution also count as particulates. Particulates of different sizes are often referred to by the letters PM followed by a number, so PM10 means soot particles of less than 10 microns (10 millionths of a meter or 10µm in diameter, roughly 10 times thinner than a thick human hair). The smaller ("finer") the particulates, the deeper they travel into our lungs and the more dangerous they are. PM2.5 particulates are much more dangerous (they're less than 2.5 millionths of a meter or about 40 times thinner than a typical hair). In cities, most particulates come from traffic fumes.

2.4.7 Ozone: Also called trioxygen, this is a type of oxygen gas whose molecules are made from three oxygen atoms joined together (so it has the chemical formula O3), instead of just the two atoms in conventional oxygen (O2). In the stratosphere (upper atmosphere), a band of ozone ("the ozone layer") protects us by screening out harmful ultraviolet radiation (high-energy blue light) beaming down from the Sun. At ground level, it's a toxic pollutant that can damage health. It forms when sunlight strikes a cocktail of other pollution and is a key ingredient of smog (see box below). [

2.4.8 Chlorofluorocarbons (CFCs): Once thought to be harmless, these gases were widely used in refrigerators and aerosol cans until it was discovered that they damaged Earth's ozone layer. We discuss this in more detail down below.

2.4.9 Unburned hydrocarbons: Petroleum and other fuels are made of organic compounds based on chains of carbon and hydrogen atoms. When they burn properly, they're completely converted into harmless carbon dioxide and water; when they burn incompletely, they can release carbon monoxide or float into the air in their unburned form, contributing to smog.

Lead and heavy metals: Lead and other toxic "heavy metals" can be spread into the air either as toxic compounds or as aerosols (when solids or liquids are dispersed through gases and carried through the air by them) in such things as exhaust fumes and the fly ash (contaminated waste dust) from incinerator smokestacks.

Nothing is more a part of everyday life than the air we breathe. Most of us don’t give it a second thought, but we could not survive without it. And when the quality of the air around us suffers, our quality of life suffers. Over the past few decades, instruction about environmental issues has become more prominent in our classrooms. From as early as preschool, children are learning about topics like recycling and water conservation. However, one environmental topic that sometimes gets less focus is that of air quality - despite it being a growing concern for scientists. Air Pollution and Health: Through regulation and voluntary change, levels of many air pollutants have decreased significantly in recent decades. Still, in many parts of the U.S., the air is often polluted at levels that can affect our health. Millions of people are exposed to unhealthful levels of ground-level ozone or particulate pollution every year. Local Impacts - It’s a bigger problem than you think. In an area where people spend so much time outdoors, it’s hard to imagine that we have an air quality problem – especially since ground-level ozone is odorless and colorless. But there’s increasing evidence that humans are more affected by ozone pollution than previously thought.

2.5 Ozone: What is ozone?

Ozone is an odorless, colorless gas composed of three atoms of oxygen. Ozone occurs naturally in the Earth’s upper atmosphere (the stratosphere) and as a pollutant at ground level. Stratospheric ozone protects us from the sun’s harmful ultraviolet rays.This beneficial ozone is gradually being destroyed by man-made chemicals. There's been some progress in restoring the protective layer of ozone in the stratosphere harmed by man-made chemicals but it could take decades for it to be restored to an adequate level. At ground level, ozone is a harmful pollutant formed when emissions from vehicles, power plants, and industrial sources react in the presence of sunlight and heat.

2.5.1 When and where is ozone a concern? Because it needs heat to form, ozone pollution is a concern in warmer weather, particularly in the afternoon and early evening. Ozone can be transported by winds hundreds of miles from where it formed, so it can be found in both urban and rural environments.

2.5.2 Can we see ozone in the air? By itself, ozone in the air is invisible, so we can be breathing harmful ozone levels even when the air looks clear. When ozone mixes with particles (described below), it forms a brown summertime haze known as “smog.” 2.5.3 Why is ozone pollution bad? Ozone can trigger a variety of health problems, even at relatively low levels. Health effects from ozone include aggravated asthma and increased susceptibility to respiratory illnesses like pneumonia and bronchitis. Symptoms to watch for when ozone is in the air include coughing, pain when taking a deep breath, and breathing difficulties, especially when you are active outdoors. But ozone damage can also occur without any noticeable signs. And, for some people, several months of repeated exposure to ozone can permanently damage the lungs. Ozone is also bad for our environment, damaging plants and trees and reducing crop and forest yields.

2.5.4 Who’s at risk from ozone pollution? People with respiratory problems are most vulnerable, but even healthy people and children who are active outdoors can be affected when ozone levels are unhealthy. This is because during physical activity, ozone penetrates deeper into the parts of our lungs that are most vulnerable to ozone.

The AQI is an index for reporting daily air quality. It uses a simple color-coded scale to tell you how clean or polluted your air is, and how you can protect your health at different levels of pollution. The AQI helps to make daily air quality information as easy to understand as weather forecasts.

India's Air Quality Index

India has its own National Air Quality Index, published by the Central Pollution Control Board. It is not the same index other countries use, so a number from an app built elsewhere may not mean what the Indian band names say it means.

How it is built

CPCB measures up to eight pollutants at a monitoring station — PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. Each measured concentration is converted to its own sub-index on a 0 to 500 scale using breakpoints CPCB publishes per pollutant.

Then comes the part almost everybody gets wrong. The station's AQI is the HIGHEST of those sub-indices, not the average of them. One pollutant at a dangerous level is not cancelled out by seven that are fine — and it should not be, because you breathe the bad one too. So the AQI is better understood as a worst case than as a summary.

A station also needs at least three valid pollutants, one of which must be a particulate, before it may report an AQI at all. This is why stations appear and disappear from the count.

The six bands

• Good — 0 to 50
• Satisfactory — 51 to 100
• Moderately Polluted — 101 to 200
• Poor — 201 to 300
• Very Poor — 301 to 400
• Severe — 401 to 500

The bands are unequal on purpose. Good and Satisfactory are fifty points wide; the four above them are a hundred. An AQI of 180 and an AQI of 290 sit in different bands but are closer to each other than 290 is to 390, even though those two share a boundary. Reading the band alone loses that.

Separately from the index, India sets legal limits on concentration. Under the National Ambient Air Quality Standards of 2009, PM2.5 may not exceed 60 micrograms per cubic metre averaged over 24 hours, or 40 annually; for the larger PM10 the limits are 100 and 60. Those are the numbers a reading is actually being judged against, and they are Indian limits — the World Health Organization's guideline values are considerably stricter.

Standards, band names and breakpoints in this section: Central Pollution Control Board, National Air Quality Index (cpcb.nic.in/National-Air-Quality-Index/) and NAAQS 2009.

What is a sensitive group?

Some people are more sensitive to air pollution than other people. Different people can be sensitive to different air pollutants. For example, ozone might make you cough. Particulate matter may not bother you, but it may make your grandmother cough and need to rest. One sensitive group is people with asthma. Asthma is a disease that can make it hard to breathe and ground-level ozone can aggravate that condition.

What the index will not tell you

Worth saying to a class before anyone quotes a number with confidence.

• A city figure is an average across stations, weighted by population. It can look better than the worst station in that city, and somebody lives next to the worst station.
• An AQI is the worst sub-index, so two identical numbers can describe different air: one may be a particulate problem and the other ozone.
• An index is not a health measurement. It is a scale built from concentrations, and the health effect depends on how long you were out in it, how hard you were breathing and how old you are.
• A monitoring station reports the air where the station is. A neighbourhood with no station is not reporting clean air; it is reporting nothing.
• Today's reading says nothing about the year. A single bad day and a long-term average are different questions, judged against different limits.

Rather than printing a reading in a book that will be out of date by the time it is taught, look up today's: swechha.in publishes the live Delhi figure and the station it came from at /now/air, and explains what each measurement means at /learn.