8 / Basics of Environmental Science
(SO 2 ) persons with respiratory complaints may experience breathing difficulties, and if it contains
more than about 2.5 ppm of NO 2 or 5.0 ppm of SO 2 healthy persons may also be affected (KUPCHELLA
AND HYLAND, 1986). These are quantities that can be monitored, and there are many more. It is also
possible, though much more difficult, to determine the quality of a natural habitat in terms of the
species it supports and to measure any deterioration as the loss of species.
These are matters that can be evaluated scientifically, in so far as they can be measured, but not
everything can be measured so easily. We know, for example, that in many parts of the tropics
primary forests are being cleared, but although satellites monitor the affected areas it is difficult to
form accurate estimates of the rate at which clearance is proceeding, mainly because different people
classify forests in different ways and draw different boundaries to them. The United Nations
Environment Programme (UNEP) has pointed out that between 1923 and 1985 there were at least 23
separate estimates of the total area of closed forest in the world, ranging from 23.9 to 60.5 million
km^2. The estimate UNEP prefers suggests that in pre-agricultural times there was a total of 12.77
million km^2 of tropical closed forest and that by 1970 this had been reduced by 0.48 per cent, to
12.29 million km^2 , and that the total area of forests of all kinds declined by 7.01 per cent, from 46.28
to 39.27 million km^2 , over the same period (TOLBA ET AL., 1992). Edward O.Wilson, on the other
hand, has written that in 1989 the total area of rain forests was decreasing by 1.8 per cent a year
(WILSON, 1992). (A rain forest is one in which the annual rainfall exceeds 2540 mm; most occur in
the tropics, but there are also temperate rain forests.) Similar differences occur in estimates of the
extent of land degradation through erosion and the spread of deserts (called ‘desertification’). Before
we can devise appropriate responses to these examples of environmental deterioration we have to
find some way of reconciling the varying estimates of their extent. After all, it is impossible to
address a problem unless we can agree on its extent.
Even when quantities can be measured with reasonable precision controversy may attend
interpretations of the measurements. We can know the concentration of each substance present in air,
water, soil, or food in a particular place at a particular time. If certain of those substances are not
ordinarily present and could be harmful to living organisms we can call them ‘pollutants’, and if they
have been introduced as a consequence of human activities, rather than as a result of a natural
process such as volcanism, we can seek to prevent further introduction of them in the future. This
may seem simple enough, but remember that someone has to pay for the measurement: workers need
wages, and equipment and materials must be bought. Reducing pollution is usually inconvenient and
costly, so before taking action, again we need to determine the seriousness of the problem. The mere
presence of a pollutant does not imply harm, even when the pollutant is known to be toxic. Injury
will occur only if susceptible organisms are exposed to more than a threshold dose, and where large
numbers of very different species of plants, animals, and microorganisms are present this threshold
is not easily calculated.
Nor is it easy to calculate thresholds for human exposure, because only large populations can be
used for the epidemiological studies that will demonstrate effects, and small changes cannot always
be separated statistically from natural fluctuations. (Epidemiology is the study of the incidence,
distribution, and control of illness in a human population.) It has been estimated that over several
decades the 1986 accident at the Chernobyl nuclear reactor may lead to a 0.03 per cent increase in
radiation-induced cancer deaths in the former Soviet Union and a 0.01 per cent increase in the world
as a whole, increases that will not be detectable against the natural variations in the incidence of
cancer from year to year (ALLABY, 1995).
Where there is doubt, prudence may suggest we set thresholds very low, and in practice this is what
happens. With certain pesticide residues in food, for example, the EU operates a standard of ‘surrogate
zero’ by setting limits lower than the minimum quantity that can be detected.