Fine particles small enough to get deep into the lungs and sometimes enter the bloodstream.

• A. Ground-level ozone
• B. PM10
• C. Particulate matter
• D. PM2.5

Answer: (D)

Tiny size determines how far particles travel in the airways. PM2.5 are fine particles with diameters 2.5 micrometers or smaller. Their small size lets them bypass many natural defenses and reach the deepest parts of the lungs—the alveoli—and, in some cases, even cross into the bloodstream. That translocation is why PM2.5 is strongly linked to cardiovascular and respiratory health effects. Ground-level ozone is a gas, not a particle, so it doesn’t fit the description. PM10 includes larger particles up to 10 micrometers and isn’t as capable of reaching the deepest lung regions or entering the bloodstream as PM2.5, which is the specific trait described.