Table 1.

Comparison of PM₁₀, PM_2.5 and UFP

Characteristics	PM10	PM2.5	UFP
Aerodynamic diameter (µm)	2.5 – 10	2.5 – 0.1	< 0.1
Deposition in alveolar space	No	No	Yes
Surface area/mass ratio	+	++	+++
Organic carbon content	+	++	+++
Elemental carbon content	+++	++	+
Metal content	+++	++	+
Exposure metrics a, b	Mass	Mass	Particle number or surface area
Central monitoring sites a, b	Yes	Yes	None
National Ambient Air Quality Standards (NAAQS) set by the US EPA	150 mg/m3 (24 hrs) (Not to be exceeded more than once per year on average over a 3-year period)	35 mg/m3 (24 hrs) (98th percentile, averaged over 3 years)	None

^aSub-micron particles have relatively little mass and are affected to a greater degree by forces other than gravity (e.g. thermal, radiation, and electrical forces, and particle concentration) so they are not efficiently collected by traditional particulate samplers that rely on gravitational or inertial forces for particle collection.

^bInstruments to measure airborne ultrafine particles operate on the principles of thermo-phoretics, diffusion charging or condensation, with results reported in units of particle number concentration, particle volume concentration, or particle surface area per volume of air sampled rather than by mass concentration as in the case of PM₁₀ and PM_2.5.