Suspended Particulate Matter: SPM and RSPM

Airborne solid and liquid particles, taken together, are called particulate matter, and they

are classified by size because size, more than composition, determines how deeply a particle can

travel into the human respiratory system and how long it stays airborne.

Suspended Particulate Matter (SPM) is the broad category of all particles suspended in air,

ranging from visible dust and soot up to about $75~\mu\text{m}$ in diameter. Particles at the

coarser end of this range are heavy enough to settle out of the air fairly quickly and, when

inhaled, are mostly trapped in the nose and upper airway, where the body's natural filtering (nasal

hair, mucus) can deal with a good deal of them.

Respirable Suspended Particulate Matter (RSPM), also written $\text{PM}_{10}$, is the finer

fraction of SPM with diameter below about $10~\mu\text{m}$. This size cutoff is what defines

"respirable": particles this small do not settle out readily, stay suspended in air far longer, and

-- crucially -- are small enough to bypass the upper airway's natural filtering and penetrate deep

into the bronchioles and alveoli of the lungs, where they can lodge and cause long-term damage.

Because of this, RSPM is considered far more dangerous to health than an equal mass of coarser SPM,

even though it typically makes up a smaller fraction of total particulate mass.

Sources. Natural sources of particulate matter include windblown dust, volcanic ash and sea

salt spray; the anthropogenic (human-caused) sources that dominate in and around cities are vehicle

exhaust (especially from diesel engines), construction and demolition dust, and emissions from

industrial stacks and thermal power plants.

Control. Industries reduce particulate emissions from stacks using devices such as

electrostatic precipitators, which charge particles electrically so they are attracted to and …