are either real or potential risks to health may enter the food supply
as a result of contamination of the environment (see
Figure 30). But, in terms of the number of people affected,
environmental contamination is less of a problem than illness caused
by food poisoning from harmful microorganisms in food (see
'How to Avoid Food Poisoning').
For many environmental
contaminants, health authorities recommend maximum acceptable levels
that are considered to be safe in food. It is illegal for foods containing
higher levels to be sold although the occasional consumption of slightly
higher amounts is unlikely to be harmful. Foods are monitored to check
that they comply with the recommendations. Foods that contain more
than the permitted amounts of the contaminants being monitored are
withdrawn from sale. The effectiveness of this depends on the extent
In general, the
level of environmental contaminants in our food complies with the
limits recommended by health authorities. However, because of the
uncertainty in establishing exactly what is a safe level for many
of these contaminants, it is in the interests of our general health
to consume as wide a variety of foods as possible. By doing this,
the chances of eating large amounts of a contaminated food are minimized.
Continued and extensive surveillance and control are needed.
The common environmental
contaminants of greatest concern in food are the so-called 'heavy
metals', most notably cadmium, lead and mercury.
Almost all of
the mercury in food occurs in seafood. A dramatic instance of mercury
poisoning occurred in the Minimata Bay area in Japan. Fish and shellfish
that were heavily contaminated by industrial waste caused poisoning
in many of the people who ate them, resulting in damage to the central
nervous system and in some instances death. Surveys of the levels
of mercury and other heavy metals in food are regularly carried out
and have shown that generally the levels are below the maximum amounts
permitted by health authorities. Occasionally, higher levels are detected
and the food withdrawn from sale.
Lead occurs widely
in the environment and it can enter our bodies through drinking water
and the air we breathe, as well as through food. Children are the
group at greatest risk, because even at levels below those that produce
the usual signs of poisoning, lead can cause behavioural abnormalities.
The levels of lead that cause these effects are uncertain so it is
difficult to estimate what amount is 'safe'. In some areas, particularly
where there is heavy lead pollution in the air from leaded petrol,
lead levels may be hazardous for children. Legislation to limit the
total environmental lead burden is being enacted in many countries.
Cadmium is present
at very low levels in a wide variety of foods. Poisoning due to cadmium
in food is rare. The upper 'acceptable' limit for cadmium in food
recommended by the World Health Organisation is generally complied
with. The kidneys of animals are generally higher in cadmium than
are other foods. Contamination of rice, soya bean and seafood with
cadmium from local industrial and mining operations has caused cadmium
and industrial chemicals
Two very persistent
environmental contaminants are the pesticide DDT and PCBs (polychlorinated
biphenyls), which have been used in electrical transformers, plastics
and paints. DDT and PCBs are not easily degraded in the environment
and can concentrate in the fatty tissues of many organisms as they
move up the food chain. Recent surveys in Australia have not detected
the presence of PCBs in food. DDT has been found in many foods but
the amounts are such that the total daily intake of DDT is within
the acceptable' upper limit recommended by the World Health Organisation.