RAS question
PM2.5 refers to particulate matter with a diameter of:
Correct answer: (A) Less than 2.5 micrometers.
PM2.5 refers to fine particulate matter with a diameter less than, or equal to, 2.5 micrometres.
Explanation
PM2.5 is fine particulate matter defined by particle size: it consists of particles with a diameter less than 2.5 micrometres. The WHO air quality database uses the same size band, covering annual mean concentrations of particulate matter with diameter equal to or smaller than 2.5 micrometres for PM2.5. This distinction matters because PM2.5 is not just a smaller version of ordinary dust in exam terminology; its fine size is why it is treated as more dangerous than PM10. PM2.5 can penetrate deep into the lungs and may even enter the bloodstream, so the defining threshold is 2.5 micrometres, not 10 micrometres, millimetres, or 0.25 micrometres.
Why the other options are wrong
- (B) Less than 10 micrometres describes PM10, not the finer PM2.5 category.
- (C) Less than 2.5 millimetres is far too large for PM2.5, whose defining unit is micrometres.
- (D) Less than 0.25 micrometres is a much smaller ultrafine-particle range, not the standard PM2.5 threshold.
Concept
This tests the Environment and Ecology concept of air pollutants and particulate-matter classification. It recurs in RAS because PM2.5 and PM10 are core terms in air-quality, health-risk and pollution-control questions.