Delhi overtook Lahore on Friday to become the world’s most polluted city, as Delhi air pollution reached hazardous levels and smog blanketed the Indian capital.
Delhi air pollution put the city at the top of the global ranking. The air quality index in Delhi was hazardous at 334, making it the most polluted global major city, according to Swiss group IQAir, which measures air quality levels based on the concentration of toxins. An AQI reading up to 50 is considered good, with little or no health risk.
India’s official Central Pollution Control Board pegged the AQI higher, at 403, and rated it severe. The Swiss and Indian readings vary because the two systems use different scales and methods to calculate and classify air pollution levels.
Cars and motorcycles emerged from dense smog, which also appeared to obscure street lights on the city’s main roads, videos from news agency ANI showed. Visibility dropped as smog covered the city of 20 million people.
Two readings of Delhi air pollution
IQAir ranks major cities around the world by the concentration of toxins in the air. Its Delhi air pollution reading placed Delhi above Lahore in Pakistan on Friday. Lahore had held the top place in the ranking before Delhi moved ahead of it.
The Central Pollution Control Board is India’s official monitor, and its reading of 403 placed Delhi in the severe category. The gap between the two figures reflects method, not a dispute over the air itself: each system applies its own scale and its own way of turning pollutant readings into a single number. Both readings described the same morning in the same city, and both placed the air far above the level counted as good.
On the IQAir scale, the reading of 334 fell in the hazardous band, well above the level of 50 counted as good. On the Indian board’s scale, the reading of 403 fell in the severe band.
The two systems exist side by side because IQAir publishes a global comparison of major cities, while the Central Pollution Control Board is the official monitor for India. Both pointed the same way on Friday, even though the numbers differed.
What drives Delhi air pollution in winter
Pollution is a seasonal problem in many parts of northern India and neighbouring Pakistan, and Delhi air pollution follows the same winter pattern. The cold, heavy air of winter traps vehicle emissions, construction dust and smoke from agricultural burning close to the ground, and the mix builds up over days when the air stays still.
Delhi’s traffic adds to the load each day, and building work across the city throws up dust that stays in the air. Smoke from crop burning in the farm states around the capital drifts in with the wind and adds to the haze.
The pattern of Delhi air pollution repeats each year as temperatures fall. The air grows colder and heavier, the pollutants have nowhere to rise to, and the smog thickens until wind or rain clears it.
The smog on Friday cut visibility across the capital and turned the morning commute into a slow run through grey air. Street lights on main roads were hard to pick out through the haze, the ANI videos showed.
Lahore and the regional pattern
Lahore, across the border in Pakistan, faces the same seasonal problem. The same mix of cold air, vehicle exhaust, dust and farm smoke hangs over much of the region in winter, and Delhi air pollution rises and falls with the same weather.
Lahore sits across the border in Pakistan and faces the same winter conditions. The cold air, the traffic and the farm smoke shape its air in the same way, and Delhi’s move ahead of it in the ranking came on a morning when the smog was thick enough to hide street lights.
The Central Pollution Control Board’s reading of 403 and IQAir’s reading of 334 were the two figures that defined the day for Delhi air pollution.