Which pair of particle sizes makes up a large proportion of indoor dust capable of reaching deep into the lungs?

• A. PM2.5 and PM1
• B. PM10 and PM4
• C. PM10 and PM2.5
• D. PM1 and PM4

Answer: (C)

Deep-lung deposition comes from fine and ultrafine particles. Indoor dust contains a mix, but the fraction that can penetrate deeply into the lungs is the fine portion around 2.5 micrometers and smaller. Including PM2.5 captures that deep-lung–reaching part, while PM10 represents the broader inhalable fraction that is commonly present indoors and includes those fine particles. So pairing the sizes that define the inhalable range (PM10) with the fine, deep-lung–penetrating size (PM2.5) best describes the dust that can reach deep into the lungs. The other pairings either emphasize sizes that don’t consistently reach deep lung regions or miss the common indoor fine-particle component.