Particulate matter in the atmosphere can be from a whole range of sources, both natural, such as sand or sea spray, and man-made, such as construction dust or soot.

The amount of particulate matter (measured as black smoke) in the air in urban areas has decreased rapidly over the last 30 years. This is due to a decrease in coal burning, heavy industry and improved industrial pollution control measures; we no longer experience the infamous Smogs of the 1950s.

Attention is now focused on finer particles known as PM10. These fine particles can be breathed more deeply into the lungs and are more likely to have a toxic effect than larger particles.

Measurements of PM10 have only been carried out in this country for the last few years - too short a period to be able to identify any significant trends. Again, it is likely that improvements brought about by a decrease in coal burning and improved technology are at least partly offset by increased numbers of vehicles on the road. The increased market share of diesel vehicles, which typically emit more PM10 particles than petrol vehicles, exaggerates this.

The smaller a particle, the longer it can remain suspended in the air. Very fine particles, made up of carbon from combustion and chemical compounds (sulphates and nitrates) can remain airborne for weeks. These particles can drift for many miles causing pollution problems across many countries. Episodes caused by this long-range transport of particulate pollution are explained below. As particulate matter is composed of such a large range of chemicals and materials from a variety of sources, the control of pollution levels is very difficult. As with ozone, local improvements will have only limited effect without international action.

The mass of sizes, shapes and chemical properties of particulate matter makes it very difficult to assess its health effects. Expert opinion is that there is no threshold concentration below which particulates have no effect on health. The aim must therefore be to reduce concentrations to a level at which minimal effects on health can be identified.