How EPA measures smoke

When smoke is in the air it is monitored with three different instruments.

  • ‘Coarse’ particles are measured as PM10, as this is a standard and widespread measure.
  • ‘Fine’ particles are measured as PM2.5, as this is now the main indicator of human health effects.
  • Also measured is ‘visibility reduction’, which is an indicator of how much distant objects are obscured by smoke.

PM10 and PM2.5 are measured as concentrations in micrograms per cubic meter, whereas visibility reduction is measured as inverse distance, which is directly converted to kilometres, indicating how far a person can see through the smoke, using a distant landmark.

Visibility and PM2.5 are very closely correlated, since it is the fine particles in the smoke causing the visibility reduction.

EPA uses visibility here as the primary indicator because there are more stations measuring this at present (until a planned upgrade to PM2.5 monitors is implemented for all stations). Visibility is also preferred in some circumstances as it directly relates to what people see (or cannot see) due to the smoke, and can judge for themselves once suitable landmarks are identified.

Page last updated on 18 Dec 2014