Toxicity in Food- Environmental contamination of food

Substances that 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 Foods that contain more than the permitted amounts of the contaminants being monitored are withdrawn from sale.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 of monitoring.

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.

'Heavy metals'

The common environmental contaminants of greatest concern in food are the so-called 'heavy metals', most notably cadmium, lead and mercury.

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 Occasionally, higher levels are detected and the food withdrawn from sale.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

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

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 poisoning.

Pesticides 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.