Ipsa Tripathy
Bhubaneswar: Air pollution is often discussed as though it belongs to a particular city. A polluted road is blamed on traffic, a smoky industrial area on factories, and a winter haze on local emissions. But the atmosphere does not recognise state borders. Smoke released in one state can travel hundreds of kilometres, mix with pollutants from other regions and influence air quality somewhere else. This makes air pollution not only a local problem, but also a regional atmospheric problem.
The movement begins with something as simple as wind. When pollutants such as PM2.5, PM10, nitrogen oxides, sulphur dioxide, carbon monoxide and volatile organic compounds are released into the atmosphere, wind can transport them away from their original source. How far they travel depends on wind speed and direction, atmospheric stability, temperature, rainfall, terrain and the height at which the pollutants are emitted. A tall industrial chimney, for example, can release pollutants into an elevated part of the atmosphere where winds may carry them much farther than pollution released close to the ground.
But transportation is only part of the story. Pollutants also change chemically while they are travelling. Nitrogen oxides and volatile organic compounds can react in sunlight and contribute to ground level ozone formation. Sulphur dioxide and nitrogen oxides can undergo atmospheric oxidation and contribute to secondary particulate matter. Ammonia from agriculture can react with acidic compounds to form fine particles such as ammonium sulphate and ammonium nitrate. In other words, the pollution that arrives in a neighbouring state may not have exactly the same chemical composition as the pollution that left its original source.
One of the clearest examples of interstate pollution transport in India is the Delhi National Capital Region and the Indo Gangetic Plain. During the post monsoon period, winds frequently become northwesterly, while atmospheric mixing can become weaker. This creates favourable conditions for pollutants from surrounding regions to reach and accumulate over Delhi NCR. Research using atmospheric observations, satellite fire data and chemical transport models has demonstrated that pollution can move from Punjab and Haryana towards Delhi. In one intensive 2022 observation campaign, PM2.5 plumes associated with crop residue burning in Punjab were tracked reaching Delhi NCR after approximately one to three days.
However, the science also provides an important correction to the popular narrative that Delhi’s winter pollution is simply Punjab’s stubble burning. A 2025 study using measurements from 30 locations across Punjab, Haryana, Delhi NCR and western Uttar Pradesh estimated that crop residue burning in Punjab contributed about 14 percent of overall PM2.5 in Delhi NCR during October and November 2022. The study found that local emissions and regional atmospheric conditions also played major roles.
This is an important lesson about transboundary pollution. A pollution episode can have many sources at the same time. Delhi’s traffic, construction activity, industries, waste burning and other local sources can combine with transported pollution from surrounding states. When winds are weak, pollutants can accumulate near their sources. When winds strengthen and change direction, the same pollutants can be transported elsewhere. The atmosphere effectively creates a constantly changing regional mixture.
Crop residue burning provides another useful example. A 2026 study examining air quality across 140 monitoring stations in the Indo Gangetic Plain between 2015 and 2024 found substantial regional variation in PM2.5 and documented the importance of pollution transport across the region. It also found that Delhi recorded very high PM2.5 concentrations even when its own fire activity was negligible, illustrating why the geographical location of an emission source and the direction of atmospheric transport both matter.
The same principle applies to industrial pollution. An industrial cluster located close to a state boundary does not necessarily affect only the population living within that state. Depending on wind direction and atmospheric conditions, emissions can cross into neighbouring regions. The same is true for coal mining, thermal power generation, large construction projects, agricultural burning and even large wildfires. Pollution can also cross national boundaries. The Indo Gangetic Plain is part of a larger atmospheric system in which emissions can move across multiple political boundaries.
This creates a serious problem for environmental governance. The state where pollution is measured is not always the state where all of it originated. Conversely, emissions released in one state can contribute to poor air quality somewhere else. If every state attempts to solve the problem independently, a significant part of the pollution pathway may remain outside its jurisdiction.
This is why scientists increasingly use the concept of an airshed. An airshed is broadly an atmospheric region in which emissions, weather and atmospheric chemistry interact. It is more scientifically useful for managing regional air pollution than treating each administrative boundary as an isolated unit.
India therefore needs stronger airshed based pollution management. States sharing an airshed should coordinate emission inventories, monitoring networks, industrial standards, agricultural practices and emergency pollution response. Satellite observations, ground monitoring stations, weather models and chemical transport models can be combined to identify where pollutants are coming from and where they are going.
For citizens, the lesson is equally important. Pollution generated hundreds of kilometres away may eventually affect the air we breathe, but local emissions still matter. Reducing vehicle emissions, preventing waste burning, controlling construction dust and supporting cleaner energy can reduce the pollution that a city contributes to the regional atmosphere.
Air pollution is invisible, but its movement is not random. It follows the physics and chemistry of the atmosphere. Winds carry pollutants, sunlight transforms them, clouds and rain remove some of them, and atmospheric conditions determine whether they disperse or accumulate. State borders may divide administrative maps, but they do not divide the atmosphere. The solution, therefore, cannot stop at the boundary of a state. If the air is shared, responsibility for keeping it clean must also be shared.