Diesel cars produce high levels of soot or "particulate matter" that can cause respiratory problems leading to an increased risk of cardiovascular disease.
However, since 2009, it is mandatory for a diesel car to be fitted with a Diesel Particulate Filter (DPF) in its exhaust to prevent some 80% of this soot being ejected into the air around us.
A DPF needs to be emptied regularly and, under normal conditions, this should happen when the car exhaust temperature is high enough, ie., when it's being driven at higher speeds on motorways or A-roads. The soot itself will be burnt off and only a small amount of ash residue will be left behind.
Unfortunately, it's not possible to remove this ash, unless the DPF itself is removed for specialist cleaning. But the good news is that your diesel car's DPF should last for at least 100,000 miles.
And if you drive your diesel car mainly in stop-start, low-speed urban conditions, the soot build-up will occur much more quickly. Not having your DPF cleaned regularly or ignoring warning lights could be costly: replacing a filter could cost at least £1,000 plus labour.