Question

Read the passage and answer the question that follows. Air pollution has quietly become one of the gravest threats to public health in Indian cities. According to global health estimates, prolonged exposure to fine particulate matter, particles smaller than 2.5 micrometres known as PM2.5, penetrates deep into the lungs and bloodstream, raising the risk of asthma, heart disease, and stroke. Children and the elderly are the most vulnerable, yet the danger is not confined to outdoor air: indoor smoke from cooking fuels affects millions of rural households too. The economic cost is staggering, measured in lost workdays and mounting hospital bills. Although emergency measures such as the Graded Response Action Plan are triggered when smog thickens over Delhi, experts argue that episodic bans achieve little without year-round action. Lasting solutions demand cleaner public transport, stricter industrial emission norms, and a rapid shift to clean cooking fuels. Citizens also have a role to play by carpooling, reducing waste burning, and planting trees in their neighbourhoods. Until pollution is treated as a daily health emergency rather than a seasonal headline, the air that millions breathe will remain a slow poison.

According to the passage, PM2.5 is dangerous primarily because it