Srinagar: Delhi’s air has inched towards an “emergency scale”, with the Central Pollution Control Board recording the average air quality at 487 with an upper limit of 500. The Indian Medical Association, which has termed it a “medical emergency”, says that this air can be equated to smoking 50 cigarettes a day.
A thick haze has enveloped Delhi and surrounding areas since Monday last. It is smog. Smog occurs in a location that is far away from the actual source of pollution, and is a result of various factors: geography of the place, sunlight, calmness of winds, post-harvest crop burning, firing of brick kilns, pollution emitted by vehicles and industrial activity. The processes that lead to smog usually take place after the hazardous pollutants have drifted away in the wind. In Delhi, there are two winds — one carrying pollutants from stubble burning in Punjab and the other bringing in moisture from Uttar Pradesh — that are colliding above Delhi. This, combined with the near-still wind conditions near the ground level, have effectively trapped the pollutants, leading to the smog.
How is smog different from fog?
Fog in just condensed water vapour close to the ground. When water vapour saturates the air, the vapour starts to condense back into a liquid, as water droplets. These droplets, suspended in the air, appear as the thick haze that is known as fog. This results in low visibility. On the other hand, when pollution is high, nitrogen oxides and dust particles interact with sunlight to form ground-level ozone, leading to the building up of haze. This is smog, a result of a photochemical reaction of sunlight with pollutants that have been released into the atmosphere.
What are these pollutants?
WHO classifies particulate matter — the major components being “sulphate, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water” — into two broad types, PM10 and PM2.5, with the numbers indicating the diameter of the particles in microns. In Delhi, the ground-level ozone and PM 2.5 play the most significant role in formation of smog.
Chronic exposure to both PM10 and PM2.5 can lead to the “risk of developing cardiovascular and respiratory diseases, as well as of lung cancer”, says WHO. Both can penetrate and lodge deep inside the lungs; PM2.5 can “cross into the blood, causing damage in many organ systems,” says WHO. (Courtesy the Indian Express)