Strange but true. New, cleaner, modern diesel engines could be more dangerous for human health than old, highly polluting ones. Nearly all the particulate matter found in the tailpipe is found as solid groups of carbon particles and a small amount of metallic ash. However, there may also be a significant quantity of volatile organic and sulphur compounds in the form of gases at exhaust temperatures which may be transformed to diesel particulate matter (DPM) by various physical processes as the exhaust mixes with the air and cools.

Most of the mass of volatile material that is transformed into DPM during this mixing with the air sticks onto the surface of existing groups of carbon particles. However, a small amount may break up homogeneously to form very tiny new particles which can be called nanoparticles. The probability of volatile gases forming nanoparticles increases when concentration of the groups of carbon particles is reduced by aftertreatment or cleaner combustion.

The groups of carbon particles act like sponges, soak up volatile gases and prevent them from forming nanoparticles. Nanoparticle formation is more likely with clean, low-carbon-emission engines. Here, the concentration of carbon is reduced more markedly than that of volatile gases.