Particulate formation 

Recent studies (since 1990) have demonstrated adverse effects on cardiorespiratory health, including both increased hospitalizations and increased premature mortality, associated with relatively low particulate levels. It is now widely accepted that tiny particles may have increased impact because of their ability to penetrate deep into the lung. Primary particles are emitted directly into the atmosphere by combustion processes, and are generally very small (less than 2.5um diameter and often less than 1um). Secondary particles are those which are formed in the atmosphere from chemical reaction and include sulphates and nitrates formed from the reactions of emissions of SO₂ and NO_x. These secondary particles are generally less than 2.5um diameter, but the size could vary depending on humidity. The third category is the so-called coarse fraction, which are formed from mostly non-combustion sources.

Related links

Defra - EU air quality policies