Particulate Matter

Particulate matter (PM) means tiny bits of solid or liquid floating in the air: dust, soot, smoke, and droplets. They are so small that most cannot be seen one by one. PM10 means particles 10 micrometres wide or smaller, and PM2.5 means particles 2.5 micrometres or smaller. To picture it, a human hair is about 50 to 70 micrometres thick, so a PM2.5 particle is roughly 20 to 30 times thinner than a hair. PM is the most important air pollutant for human health in India.

Why does particle size matter?

The smaller the particle, the deeper it goes into the body.

• PM10 (coarse particles) can enter the nose, throat and upper lungs. It causes coughing and irritation.
• PM2.5 (fine particles) can reach the deepest parts of the lungs, and the smallest ones can pass into the blood. It is linked to heart disease, strokes, lung cancer and asthma.
• Ultrafine particles (smaller than 0.1 micrometre) are even more harmful, which is why modern vehicle norms also count the number of particles, not just their weight.

Note that PM10 includes PM2.5. Every PM2.5 particle is also a PM10 particle, but not the other way round.

Where does PM come from?

• Primary PM is released directly, for example, soot from diesel engines, smoke from burning crop stubble or wood, dust from roads and construction sites.
• Secondary PM forms in the air when gases like sulphur dioxide (SO₂), nitrogen oxides (NOx) and ammonia (NH₃) react with each other. A large part of PM2.5 in Indian winters is secondary.
• Natural sources include desert dust storms, sea salt and volcanic ash.
• Vehicles produce PM in two ways. Exhaust PM comes out of the tailpipe. Non-exhaust PM comes from brake pads wearing down, tyres rubbing on the road and road dust being thrown back into the air.

Non-exhaust emissions: the new frontier

As tailpipes get cleaner, non-exhaust PM becomes a bigger share of traffic pollution.

• A 2020 OECD report warned that wear from brakes, tyres and road surfaces is set to overtake exhaust as the leading source of traffic PM.
• Brake dust forms when brake pads rub against the disc. Regenerative braking in EVs and hybrids (where the motor slows the car and charges the battery) uses the brake pads less, so EVs release less brake dust.
• Tyre wear depends on vehicle weight. EVs are usually heavier because of the battery, so they can wear tyres faster. Tyre particles also contain microplastics that reach soil and water.
• So an EV has zero exhaust but not zero pollution. This is why new norms like the EU's Euro 7 and India's draft BS-VII set limits on brake particles for all vehicles, including EVs.

How does India measure and control PM?

• National Ambient Air Quality Standards (NAAQS): Notified by the Central Pollution Control Board (CPCB) in November 2009 under the Air (Prevention and Control of Pollution) Act, 1981. They cover 12 pollutants and the same limits apply to industrial and residential areas.

• National Air Quality Index (AQI): Launched in October 2014. It converts pollution levels into one number from 0 to 500, using up to 8 pollutants (PM10, PM2.5, NO₂, SO₂, CO, O₃, NH₃ and lead). It has six bands: Good (0–50), Satisfactory (51–100), Moderate (101–200), Poor (201–300), Very Poor (301–400) and Severe (401–500). At least three pollutants are needed, and one must be PM10 or PM2.5.
• National Clean Air Programme (NCAP): Launched in January 2019 by the Ministry of Environment, Forest and Climate Change. It first aimed at a 20–30% cut in PM by 2024 (from 2017 levels), later revised to up to 40% by 2025-26, in about 130 "non-attainment" cities (cities that did not meet NAAQS for five years).
• Graded Response Action Plan (GRAP): Emergency steps in Delhi-NCR (like stopping construction or some vehicles) based on AQI levels, run by the Commission for Air Quality Management (CAQM).

WHO guidelines vs Indian standards

In September 2021, the World Health Organization (WHO) made its air quality guidelines much stricter.

WHO guidelines are advice, not law. Each country sets its own legal standards. India's limits are much looser than WHO's, and many experts say they need an update.

Commonly confused concepts

• PM10 vs PM2.5: PM10 is the larger group (10 micrometres or less) and includes PM2.5. PM2.5 is more dangerous because it goes deeper.
• PM mass vs particle number (PN): PM mass measures the total weight of particles. PN counts them. Very small particles weigh almost nothing but can be huge in number, so PN catches them better.
• AQI vs NAAQS: NAAQS are the legal limits. AQI is a public communication scale that turns readings into colours and categories that people understand easily.
• Exhaust vs non-exhaust PM: Exhaust PM comes from burning fuel. Non-exhaust PM comes from friction and wear (brakes, tyres, road).

Issues, criticism and the way forward

• Monitoring gaps: Many smaller towns have few or no monitoring stations, so the real pollution level is not known.
• Old standards: India's NAAQS date from 2009 and are far above the WHO 2021 guidelines.
• Many sources across state borders: Stubble burning in one state can pollute another. This needs a regional "airshed" approach rather than city-by-city action.
• Slow progress under NCAP: Independent reviews have found that only a small number of cities met their PM10 reduction targets.
• Way forward: Experts suggest revising NAAQS, more monitoring stations, regional airshed plans, cleaner fuels for cooking and industry, controlling road and construction dust, and including non-exhaust vehicle pollution in rules.

Concepts to Know

• Micrometre (µm): One-millionth of a metre, or one-thousandth of a millimetre.
• µg/m³ (microgram per cubic metre): How many millionths of a gram of a pollutant are present in one cubic metre of air (about the air in a big cupboard). A higher number means dirtier air.
• Secondary pollutant: A pollutant that is not released directly but forms in the air when other gases react, often with sunlight or moisture.
• Non-attainment city: A city where air quality did not meet the national standards (NAAQS) for five years in a row.
• Microplastics: Plastic pieces smaller than 5 mm. Tyre rubber is a major source.
• OECD: The Organisation for Economic Co-operation and Development, a Paris-based group of mostly rich countries that studies economic and policy issues.