The two gases that particularly characterize photochemical smog, O 3 and
nitrogen oxides, caused particular concern because of their potential to induce
respiratory problems. Ozone impairs lung function, while nitrogen oxides at high
concentrations are particularly likely to affect asthmatics. Oxygen-containing
compounds, such as aldehydes, cause eye, nose and throat irritation, as well as
headaches, during periods of smog. Eye irritation is a frequent complaint in Los
Angeles and other photochemically polluted cities. This eye irritation is particu-
larly associated with a group of nitrogen-containing organic compounds. They
are produced in reactions of nitrogen oxides with various organic compounds in
the smog (Box 3.6). The best known of these nitrogen-containing eye irritants is
peroxyacetylnitrate, often called PAN.
Photochemical smog is not the only pollution problem created by vehicles.
Automobiles are also associated with other pollutants such as lead (Pb) and
benzene (C 6 H 6 ). The success of lead tetralkyl compounds as antiknock agents for
improving the performance of automotive engines has meant that, in countries
with high car use, very large quantities of lead have been mobilized. This lead
has been widely dispersed, but particularly large quantities have been deposited
in cities and near heavily used roads. Lead is a toxin and has been linked with
several environmental health problems. Perhaps the most worrying evidence has
come from studies (although difficult to reproduce) which suggest a decline in
intelligence among children exposed to quite low concentrations of lead.
Unleaded petrol was introduced in the USA in the 1970s so that catalytic con-
verters could be used on cars. Since then, unleaded petrol has become used more
widely. There is evidence that blood lead concentrations have dropped in paral-
lel with the declining automotive source of lead. Nevertheless, the decrease in
atmospheric lead may not yet be enough to reduce possible subtle health effects
in children to a satisfactory level. This is because children have a high intake of
food relative to their body weight. Thus children are more likely than adults
to consume a significant amount of their intake of lead with food and water.
Although some of the lead in foodstuffs may have come from the atmosphere,
lead in foods may also result from processing.
Benzene (see Fig. 2.4) is another pollutant component of automotive fuels. It
occurs naturally in crude oil and is a useful component because it can prevent
pre-ignition in unleaded petrol (the production process is usually adjusted so that
the benzene concentration is about 5%). There is evidence that in some loca-
tions, where there has been a switch to fuels with high concentrations of aromatic
hydrocarbons, there has been a sharp increase in photochemical smog. This is
due to the high reactivity of these hydrocarbons in the urban atmosphere. This
problem should draw our attention to the way in which the solution of one
obvious environmental problem (lead from petrol) may introduce a second rather
more subtle problem (i.e. increased photochemical smog from reactive aromatic
compounds).
Benzene is also a potent carcinogen. It appears that more than 10% of the
benzene used by society (33 M tonne yr-^1 ) is ultimately lost to the atmosphere.
High concentrations of benzene can be found in the air of cities and these con-
centrations may increase the number of cancers. Exposure is complicated by the54 Chapter Three