What is exhaust pollution?

Your car runs by burning petrol or diesel (a complex blend of lots of different chemicals collectively called hydrocarbons) to produce harmless Carbon Dioxide and water. There are, however, some by products of this process.

Some of these hydrocarbons do not get burned in your engine and pass through the exhaust unchanged. There are two in particular, called Benzene and 1:3 Butadiene that can be harmful. Not all the fuel burns up completely, so some Carbon Monoxide (CO) is also produced. These are the main pollutants produced by older, petrol driven cars, with diesels of all kinds burning their fuel much more completely, so producing negligible amounts of them.

In the 1980s, there was a move towards "lean burn" petrol engine technology — increasing the amount of air with the fuel in the engine so that more of the fuel is completely burned up. This reduces the amount of the first two pollutants but tends to encourage the oxygen and nitrogen in the air to combine to produce Nitrogen Oxides (NOx).

If your petrol car was registered after 1993, it will be fitted with a catalytic converter which removes 95% of these three pollutants from the exhaust as compared with a similar 1976 model.

Exhausts can also contain Sulphur Dioxide (SO2) from impurities in the fuel, but only 3% of the total emissions of this substance come from transport, the rest mainly from industry and power generation.

Recently, a health threat has been identified from Particulates, or PM10s, which are microscopic soot particles produced in the combustion process. Very little particulate emission (5%) is from petrol engines, though, with much more (19%) coming from diesels, disproportionately from the larger diesels in trucks and buses. The National Environmental Technology Centre (NETCEN) recently suggested that one bus can produce as many particulates as 128 typical cars.

The final element in exhaust pollution is Ozone (O3). This is not emitted directly but made in the air by the action of sunlight on other pollutants to form "ground level Ozone", which, unlike the "Ozone Layer" in the high atmosphere, is regarded as a bad thing if levels are too high. Ozone is actually broken down by Nitrogen Oxides, so one tends to be lower where the other is higher.

How do we know what the levels of these pollutants are?

Levels of these seven pollutants are monitored across urban and rural areas around the country. The measurements are made public — the best place to see them is on BBC1 Ceefax pp 412-417 where they are updated hourly.

Are these pollutants dangerous?

Most toxic substances are only dangerous when a certain level is exceeded. Your medicine cupboard is full of chemicals which are beneficial if taken as prescribed by your doctor but which will kill you if you swallow the whole bottle. Many everyday items are poisonous if taken to excess — Vitamin A, for example, is an essential part of your diet, and lack of it is what kills hunger strikers. A Polar Bear's liver, on the other hand, contains a lethal dose of Vitamin A!

Four of the pollutants described above (Nitrogen Oxides, Carbon Monoxide, Sulphur Dioxide, and Ozone) are like this — there are accepted levels at which no harmful effects are observed even in sensitive population groups. For the other three (Benzene, 1:3 butadiene and particulates) there is no way of proving they are safe at any level, so the experts set standards where the risk to health are "exceedingly small" — in other words they can't actually measure it!

What are the levels of these pollutants?

The graphs shown below compare the safe levels for these pollutants with the average across both the urban and rural measuring sites shown on Ceefax at 7pm on Friday 27 March 1998. This is likely to be a worst case scenario, being right at the end of a Friday rush hour. The worst location is also shown. This is only one day, but if you check the figures regularly you will see that they are entirely typical of peak hour levels.

AIR QUALITY LEVELS AT 7pm, 27 MARCH 1998 vs SAFE LIMITS
Substance Levels Graph units Limit Definition Main Source Worst Location Key
NOx 104.6
80
27.8
8
ppb
single measure
104.6 ppb
hourly avg
Diesel &
Petrol Transport
Marylebone
Road
Safe
Limit

Worst

Urban
average

Rural
average

 

ppb
Parts
Per
Billion

ppm
Parts
Per
Million

SO2 100
24
4.1
1.2
ppb
single measure
100 ppb over
15 min period
Industry Urban Hull
O3 50
38
14.2
23.9
ppb 50 ppb running
8 hour avg
Secondary Rural
Pembrokeshire
PM10
particulates
50
37
18.6
11.7
ug/m3 50 ug/m3
24 hr avg
Industry and
diesel transport
Port Talbot,
South Wales
CO 10
5
0.66
0
ppm
single measure
10 ppm running
8 hour avg
Petrol transport Marylebone
Road
Benzene 5
1.74
1.1
0.39
ppb
annual avg
5 ppb
annual avg
Petrol transport,
fuel evaporation
Southampton
1:3
Butadiene
1
0.33
0.21
0.05
ppb
annual avg
1 ppb
annual avg
Petrol transport,
fuel evaporation
Central
London
Figures: The United Kingdom National Air Quality Consultation Draft, DoE 1996

What these graphs show is that the average urban levels of all of these pollutants on one particular Friday afternoon in March 1998 are well within the levels set by the World Health Organisation. Nowhere were the levels exceeded, with the worst location for each pollutant being central London for the transport related substances, industrial centres for the particulates and sulphur dioxide and a rural, low traffic area for ozone. This is exactly what would be expected.

Furthermore, emissions of the main transport related pollutants (CO, NOx, Benz, 1:3 But, PM10) are expected by the government to fall between 60 and 80 percent between 1990 and 2010 (they have already fallen 20%) even allowing for the highest predicted traffic growth (and thoroughly unrealistic that is, too) as the majority of cars, buses and lorries get cleaner and cleaner due to catalyst and fuel technology.

Note — since this report was produced, a new reading has been added to the Ceefax pages for benzene and 1:3 butadiene at kerbside on the Marylebone Road. This shows an annual average reading of 1.24 ppb 1:3 butadiene, over the limit. However, since this substance is measured on an annual average exposure, any individual would have to spend an entire year lying in the gutter by the roadside in order to be exposed to this level. Since no-one does this, the reading is of limited relevance to the health of the population.
 


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