Diesel Particulate Matter
A couple of decades ago, if you rode a motorcycle through London and then wiped your face with a handkerchief, it would turn black. This is what diesel particulate matter (DPM) looks like. While carbon monoxide is a gas, DPM is a mixture of solids and liquids. The solid part consists of carbon and ash; the liquid component is made up of unburnt fuel and lubricating oil.
Diesel engines emit more particulate matter than petrol engines. This is normally invisible to the naked eye, although often can be seen during a cold start. It is the smallness of these particles that make them potentially the most harmful.
Particulate matter is often categorised by size. PM10, for example, refers to particles up to 10 microns (0.00001 mm), whereas PM2.5 only includes those up to 2.5 microns (0.0000025 mm).
The negative health effects of diesel particulate matter are due to two factors: it is small enough to get into the lungs and it is carcinogenic. Prolonged exposure to DPM is similar to the effect of smoking cigarettes.
As concern over the health implications of particulate matter have increased, regulations covering exhaust emissions from diesel engines have become progressively stricter. DPM emissions from new engines sold in the UK are currently regulated at the European level. These regulations are often referred to as Stage 3A, 3B or Stage 4.
Some local authorities also apply their own DPM emission regulations. In London, for instance, a code of practice was introduced in September 2015, which effectively bans out-of-date machinery from major sites across the capital, unless it is retrofitted with a diesel particulate filter.