(c) Particulate matter (PM2.5)

PM2.5 (fine particulate matter with diameter under 2.5 microns) is widely considered the most dangerous air pollutant for health in Indian cities. It penetrates deep into the lungs and bloodstream, is linked to the highest burden of air-pollution-related deaths and disease in India (respiratory illness, cardiovascular disease, stroke, lung cancer), and Indian cities consistently show PM2.5 levels far exceeding WHO safe limits, making it the dominant driver of air pollution health impact in the country.

According to Park's Textbook of Preventive and Social Medicine, the correct answer is:

Park's describes particulate matter as a complex mixture of organic and inorganic substances, classified by aerodynamic diameter. PM10 (particles <10 μm) includes the finer PM2.5 fraction (<2.5 μm) plus coarse particles; the difference between PM10 and PM2.5 is called "coarse mass PM." Critically, the text notes that "the fine fraction of PM10... is cut-off from coarse fraction at 2.5 μm, a size fraction with a high probability of deposition in the smaller conducting airways and alveoli" - Park's Textbook of Preventive and Social Medicine, p. (Air Pollutants section).

The major components of PM (sulfate, nitrates, black carbon, mineral dust) allow it to penetrate deep into the lung and enter systemic circulation, which is why Park's - along with WHO and Indian CPCB data cited in the "Effects of Air Pollution" and "Air Pollution Monitoring in India" sections of the same chapter - identifies PM2.5 as the pollutant of greatest health concern in urban India. Indian cities (Delhi, Kanpur, and others frequently cited in the textbook's epidemiological discussion) consistently register PM2.5 levels several times above WHO safe limits, driving the disease burden from air pollution.

While SO2, NO2, and CO are all discussed as significant pollutants with their own toxicological profiles in Park's, none is singled out as having the same scale of health impact (respiratory disease, cardiovascular disease, and premature mortality) attributed to PM2.5 in the Indian urban context.