Particulates

The composition, size, production, distribution, behaviour and specific effect of particulate matter in the air is a highly complex subject. Particles are produced both directly from such human activities as combustion and other processes (e.g. from brakes and tyres of vehicles) and from such natural activities as the weathering of soils. They are also produced as 'secondary particles' by chemical reactions in the air.

Particles arising by different processes have different properties. In health terms, one important property is the extent to which particles become deposited in the lungs. This in turn depends on a variety of factors, one of which is the size of the particle. Generally, small particles have more chance of reaching the deeper parts of the lungs. These are likely to pass through the nose and larynx and enter the chest. As a rule, particles produced from combustion and condensation tend to be 'fine' while those from mechanical processes tend to be 'coarse'

Associations between particle levels and a range of health outcomes have been identified. These include decreases in lung function, increases in respiratory symptoms and exacerbation of asthma. Because of the variation of individual thresholds within the population and the variability in personal exposure at a measured concentration, it may well not be possible to detect a measured concentration below which no one in the population will be affected.