The main source of toxic pollution that affects people in Bristol is road traffic. Although Bristol has some large industrial processes, for example in Avonmouth, these emit pollution through tall chimneys. This disperses and dilutes the pollution so that the concentrations are very much reduced before it comes into contact with anyone.
In contrast, road traffic emits pollution very close to where people travel, work and live. Although there have been considerable reductions in emissions from vehicles due to new engine and fuel technology, the sheer volume of traffic on the roads has led to high concentrations of key pollutants in some areas.
Within the traffic fleet HGV's and buses emit more than cars, although the per capita emissions of buses are lower when they operate at full capacity. Slow moving, congested traffic and very fast traffic (e.g. on motorways) cause the most emissions. Least emissions are produced by free flowing vehicles driven at moderate speed. The maintenance of vehicles is also very important. A poorly maintained vehicle will emit many times more pollution than a vehicle that is checked and serviced regularly.