8.3.1—Urban air pollutants
- Syllabus
- First assessment 2026
- Objective
- 8.3.1
- Level
- HL
Particle Size Tells You Which Fraction You Are Measuring
Urban air pollution includes NOx, SO₂, CO and particles; PM2.5 and PM10 are size fractions, so a monitor label tells you which particles it counts.
PM2.5 is no larger than 2.5 µm and PM10 no larger than 10 µm. The finer fraction can penetrate more deeply, but the syllabus labels describe aerodynamic size, not a complete toxicity ranking.
If PM10 is 40 µg/m³ and PM2.5 is 18 µg/m³, the finer fraction is nested inside the PM10 reading; the remainder is larger than 2.5 µm but no larger than 10 µm.
No. PM2.5 is nested within PM10, but each fraction can change by a different amount.
PM10 does not mean ‘ten times worse’; it is a particle-size threshold, not a gas or toxicity score.