Which statement best describes the difference between PM2.5 and PM10?

• A. PM10 includes particles ≤ 10 µm.
• B. PM2.5 comprises particles ≤ 2.5 µm.
• C. PM2.5 is the subset most associated with health effects.
• D. All of the above.

Answer: (D)

Understanding how PM sizes are defined helps explain their health relevance. PM10 includes all particulate matter with an aerodynamic diameter up to 10 micrometers, while PM2.5 includes particles up to 2.5 micrometers. That means PM2.5 is contained within PM10—a smaller, finer subset. The fine fraction, PM2.5, is more strongly linked to adverse health effects because these tiny particles can travel deeper into the lungs, reaching the alveoli, and can even enter the bloodstream, contributing to respiratory and cardiovascular problems. They also originate mainly from combustion processes and secondary atmospheric reactions, making them a dominant health concern for exposure. Because each of these points is true—the sizing definitions and the greater health relevance of PM2.5—the best description is that all of the above are correct.