It’s a sad fact of life that solving one problem almost invariably causes another. The widespread use of diesel engines in cars has helped to reduce transport carbon dioxide emissions to combat climate change. An unwelcome side effect has been the increase in urban pollution from diesel exhausts.
One ingredient in this pollution is tiny soot particles, referred to as particulate matter. They are so small, less than 2.5 microns in diameter, they can penetrate cells in the lungs and trigger cancer. Any particle that small can do the same; it’s the size that makes them a health hazard, not what they’re made of. It’s a serious concern because roughly three times as many people die prematurely from particulate related illnesses than die in crashes on the roads.
Diesel particulate filters are now fitted as standard to all diesel vehicles to remove the soot from the exhaust before it enters the air, and these are gradually starting to have a beneficial effect.
However, there have always been other sources of particulate matter, notably brake dust. Cars, trucks and buses used to have drum brakes which kept most of the dust inside the drums, but were prone to overheating under repeated use, leading to brake fade.
Disc brakes came along from the 1960s onwards and are now almost universally fitted, with great resistance to brake fade. The brake assemblies are largely open to the air and stay much cooler, but nothing now contains the dust as the brake pads wear away; it all gets into the atmosphere, apart from that which coats your wheels.
If you have ever wiped this black powder from your wheels with a finger, you’ll have felt the impalpable nature of this dust; the particles are about the same size as exhaust particulates and can do exactly the same harm. As soot levels fall, the proportion of brake dust as a pollutant rises and the government is already looking at ways of controlling brake dust.
Technology will be part of the answer; regenerative braking slows the car down by making the wheels charge a battery. As well as being good for energy efficiency, this is pollution free braking. Manufacturers are also developing brake materials which create far fewer of the harmful particles.
However, driving style can dramatically affect how much brake dust is created. Drivers who rush to the next set of lights and then stand on the brakes at the last second not only produce far more pollution from the exhaust, they are spraying large quantities of harmful brake dust into the air.
A driver who looks well ahead, releases the accelerator early to let the car slow down and only brakes from a low speed for the last few yards before a junction will produce very little brake dust. Recognise that style? Of course you do, it’s taught in our advanced courses!
Driving the IAM RoadSmart way not only reduces casualties from crashes, it is also a very effective way of tackling the high number of deaths caused by pollution in our towns and cities.By Tim Shallcross, IAM RoadSmart's head of technical policy and advice