Air becomes polluted if the constituents are present in sufficient amounts to affect the health of humans or animals or to affect vegetation. There are a variety of air pollutants; those that follow are known as primary pollutants and are responsible for 90 per cent of all air pollution:
• particles (often referred to as particulate matter – PM)
• sulphur dioxide (SO2)
• nitrogen oxides (NOx) including nitrogen dioxide (NO2)
• carbon monoxide (CO)
• hydrocarbons (CHx) and photochemical oxidants.
Health effects of the principal types of pollutants
• Particulates and sulphur dioxide: Most attention has focused on particle fractions (and sulphur dioxide), especially particles of small diameter that can enter the respiratory tract. Evidence of adverse health effects is strongest for particles with a diameter of less than 10 micrometers (so-called PM10). Those less than 2.5 micrometers (PM2.5) are respirable, that is small enough to penetrate deep into the lung. These last particles are of particular concern as they irritate the lungs and may be composed of toxic metals which can be carcinogenic or can cause brain, kidney, liver and nerve damage (Stoker and Saeager 1976, in Gupta and Asher 1998).
• Nitrogen oxides (NOx): The health impacts of nitrogen dioxide are not very clear;
at high levels, NO2 acts as an irritant and may lead to bronchitis in children.
Nitrogen monoxide (NO) is released from car engines and is oxidized to NO2 in the atmosphere. The main effect of NOx gases is their role in the production of photochemical smog through their reaction with hydrocarbons.
• Carbon monoxide (CO): Carbon monoxide binds to haemoglobin in the blood and can reduce its oxygen-carrying capacity. This is a particular problem for people with existing cardio-respiratory limitation.
Other important pollutants are:
• Ozone (O3): Ozone has been shown to have effects on lung function in some subjects, probably through inflammatory or irritant processes.
• Lead (Pb): Lead can bring about behavioural change and brain damage. Most lead in the air is ‘organic lead’ in which it is bonded to a hydrocarbon.
108 Environment, Health and Sustainable Development
Dispersion and chemistry of pollutants
The effect of a pollutant will depend on the amount and its effectiveness. The relationship between the concentrations of pollutants found in the air and sources of emissions is complex and influenced by patterns of dispersion, air chemistry, and other factors. All emissions undergo chemical or physical transformation to some degree (Figure 8.1).
Low-level ozone and nitrogen dioxide are secondary pollutants formed by chemical reactions of primary pollutants. Although small quantities of NO2 are emitted by vehicles, most derive from the oxidation of nitrogen oxide (NO) by ozone and the hydroperoxy (HO2•) radical:
NO + O3 → NO2 + O2
NO• + HO2• → NO2• + OH•
NO2 is also the main precursor of ozone in a reaction catalysed by sunlight (hv) and volatile organic compounds:
NO2
hv→ NO + O*·
O* + O2 → O3 (where O*· represents an exited state atom)
High levels of nitrogen dioxide tend to be associated with low levels of ozone and vice versa. Ozone is typically highest in peri-urban and rural areas rather than in city centres, where the NO is thought to react with ozone-forming NO2.
The type of urban pollution seen in many countries has changed dramatically over the past century with changes in transport and domestic and industrial energy production and use. In the early part of the twentieth century, the main constituent of air pollution in British cities such as London was the emission of sulphur dioxide from burning coal and other fossils fuels; this led to acidic smog.
Since then, with the banning of such fuels in urban areas, concentrations of sulphur dioxide and larger sooty particles have declined dramatically. Now the main problem is photochemical smog from traffic-related emissions, which tend to occur under certain meteorological conditions – notably anti-cyclonic conditions that can lead to a stable layer of air forming above the boundary layer and reducing the movement of air within it, combined with strong sunlight.
Figure 8.1 Dispersion and air chemistry in relation to pollutants
Outdoor air pollution 109
Activity 8.2
This activity is to remind you of the sources and health effects of air pollution. Use the summary in Table 8.1, together with the information in the first part of the chapter, to answer the following questions:
1 How have most industrialized cities in high income countries managed to clean up their air?
2 What is thought to be the chief type of outdoor air pollutant in low and middle income countries, and what are its sources and effects?
3 How do most air-transmitted pollutants enter the body, and what differences are there in the ways in which the various primary pollutants affect human health?
4 Besides the danger to human health, what other environmental damage is caused by the pollutants described?
Table 8.1 Primary pollutants of outdoor air, principal sources, effects on health and the environment and control methods
Type Source Effect on health and
environment
Control
Particulates Industry, biomass combustion, waste incineration, car emissions
Respiratory
problems, lung cancer, eye irritation, loss of environmental visibility
Gas cleaning, use less coal and biomass fuels Separate industrial from residential areas
Controlled incineration Sulphur oxides
(SO2and SO3)
Biomass and fossil fuel combustion (coal in particular).
Acid rain damage to vegetation and buildings
Reduced use, of sulphurous coal Industrial emissions Corrosion of metals
Cardio-respiratory problems
Fluidized-bed combustion Carbon monoxide
(CO)
Incomplete combustion in car exhaust and biomass burning. Industry
Nerve and brain dysfunction Cardiac problems, low birthweight, death
Control of vehicle emissions (catalytic converters) Use of public transport Nitrogen oxides
(NOx)
Transportation, biomass and fossil fuel combustion.
Cigarette smoke
Plant damage, photochemical smog, respiratory tract problems
Catalytic converters Reduce traffic volumes; much is secondary reaction Hydrocarbons and
photochemical oxidants, e.g. ozone, aldehydes
Car emissions, industry, incineration Solvent evaporation
Respiratory problems, eye nose and throat irritation Plant damage
Emission control (catalytic converters) fuel efficiency Photochemical smog
Lead Car emissions (using
unleaded petrol)
Behavioural changes and brain damage
Use of unleaded petrol
110 Environment, Health and Sustainable Development
Feedback
1 Column 4 of Table 8.1 lists control measures used to reduce outdoor air pollution.
Many of these have required legislation. You should note that while most of the pollu- tion has resulted from industrialization, rural areas also have problems. There are means of reducing or inhibiting most forms of outdoor air pollution although there are usually economic consequences in controlling them.
2 In rural areas the burning of biomass fuel is the most significant air pollutant; in urban areas, fossil fuel (petrol, diesel, coal) emissions associated with transport and power stations are the most significant.
3 Most air pollutants act on the cardio-respiratory system via the lungs. The effect of particulate matter is a result of size and toxicity. Large particles act as an irritant, causing or exacerbating respiratory symptoms; small particles can carry toxic metals into the lungs, leading to cancer or nerve damage.
4 Particulates can reduce solar radiation and visibility; if they settle on vegetation, they can reduce photosynthesis and damage materials such as textiles and metals.
High concentrations of sulphur oxides can damage paints, metals and limestone and marble as well as being responsible for acidic precipitation (see Chapter 11). Ozone and hydrocarbons can damage plants and are involved in the production of photochemical smog.