Which particulate matter is most harmful due to deep penetration in lungs?
PM2.5 comprises particles with aerodynamic diameters at or below 2.5 micrometres. Their small size allows them to evade much of the upper respiratory filtration, penetrate into bronchioles and alveoli, and in some cases contribute components to systemic circulation. PM10 includes larger particles that are more often deposited in the nose and conducting airways, although it also remains harmful. At organismal level, respiratory surfaces are especially exposed because large volumes of air contact thin, moist epithelia designed for rapid gas exchange. Sound classification links the source, atmospheric fate, exposure route, and mechanism of toxicity rather than relying only on the pollutant's name. Effective management combines cleaner fuels, emission standards, monitoring, urban planning, and protection of populations with high exposure or physiological vulnerability. The distinction is ecologically important because emission control must target either the directly released precursor or the product formed after atmospheric transport and reaction. Exposure depends on concentration, duration, ventilation, weather, and individual susceptibility, so a pollutant's presence alone does not fully predict biological harm.
Ref: Campbell Biology, Urry et al., 12th Ed., Unit 8 Ecology