Air pollution and health

We saw in Section 3.6.1 that the acid-laden smoke particles in the London atmosphere caused great harm to human health in the past. Pollutants in the atmosphere still cause concern because of their effect on human health, although today we need to consider a wider range of potentially harmful trace substances. The photochemical smog encountered ever more widely in modern cities gives urban atmospheres that are unlike the smoky air of cities in the past. Petrol as a fuel, unlike coal, produces little smoke.

The two gases that particularly characterize photochemical smog, O3 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 particularly 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 (C6H6). 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 converters could be used on cars. Since then, unleaded petrol has become used more widely. There is evidence that blood lead concentrations have dropped in parallel 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 locations, 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 concentrations may increase the number of cancers. Exposure is complicated by the importance of other sources of benzene to humans, for example tobacco smoke. Toluene (C6H5CH3; Fig. 3.4d) is another aromatic compound present in large concentrations in petrol. Toluene is less likely to be a carcinogen than benzene but it has some undesirable effects. Perhaps most importantly it reacts to form a PAN-type compound, peroxybenzoyl nitrate, which is a potent eye irritant.

As emphasized in the previous section, particles have increasingly come to be seen as an important influence on the environmental health of modern populations. The fine PM-2.5 is able to penetrate through the respiratory system all the way to the alveoli. Normally particles are removed from the respiratory system in the mucus which is driven upwards by fine hairs or cilia. There are no cilia in the air sacs, so roving amoeba-like cells (macrophages) engulf the particles. They can migrate upwards to the ciliated parts of the respiratory system or through the alveolar walls. The activities of the macrophages, although important, release inflammatory compounds. The inflammatory effects can easily be transmitted to the blood, such that pulmonary inflammation can readily become associated with cardiovascular problems. This explains the enhanced death rate often observed when human populations are exposed to particle-laden air. In addition to these immediate effects the particles are rich in polycyclic aromatic hydrocarbons (PAHs), which are carcinogens and could explain some of the cancer incidence seen in urban populations.

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