Positive Health Online

Past and current use of organochlorine pesticides has led to the pollution of the entire globe, with detectable levels in the body fat of nearly everyone in the world. Could pesticides be one of the environmental factors responsible for the rise in breast cancer?

Use of organochlorine pesticides in agriculture, forestry and public health began after World War II. Concern about persistence in the environment and accumulation in animals at the top of the food chain (including humans) led to measures to control their use. In the UK, many organochlorine pesticides including DDT, aldrin, chlordane, dieldrin and toxaphene have been banned but DDT and others are still widely used in many parts of the world. Lindane is currently approved for use in the UK.

There is some uncertainty about the causes of breast cancer. It is well established that reproductive factors affect risk (age at menarche, age at first pregnancy, age at menopause) and that diet plays a part. The role of organochlorine pesticides is not confirmed but the following arguments have been presented.

Changes over time

The increase in organochlorine pesticide use parallels the rise in the death rate from breast cancer, particularly among older women. In developed countries, the death rate has been rising steadily at about 1% a year. Most of the increase can be explained by longer life expectancy and changes in reproductive patterns but some of the increase is unexplained. Supporting evidence comes from Israel, where a ban on lindane and DDT in 1976 was followed by a drop in breast cancer deaths over the next ten years.

However, death rates from breast cancer were rising long before organochlorine pesticides came on the scene and the rate of increase did not change after pesticides were introduced. In England and Wales there has been a slow but steady increase in the death rate from breast cancer throughout this century. It levelled off after 1985 and dropped dramatically since. No one has attempted to correlate this drop with changes in pesticide use.

Geographical differences

One clue to the link between a disease and a possible cause is to look for differences in rates between countries. This has been done for dietary fat consumption and breast cancer. In Britain, where people eat a lot of fat, breast cancer is much more common. In Japan, where people's diets are less fatty, the rate of breast cancer is four to five times lower. There is not the same kind of difference in pesticide use between the two countries. The type of agriculture in Japan and Britain is highly industrialised with intensive use of organochlorine pesticides in both.

Exposure

Most of us have been exposed to minuscule amounts of organochlorine pesticides through the food we eat. The levels in our body tissues are low and have been falling. Farmers and their families are exposed to higher levels of pesticides. They appear to be at greater risk of cancers such as lymphoma and sarcoma from exposure to chlorophenoxy pesticides but not breast cancer and not from organochlorine pesticides. In tropical countries, tonnes of DDT were sprayed and continue to be sprayed into the environment to control malaria, exposing people to much higher levels of pesticides than people in Britain have ever received. Yet there is no evidence of an increase in breast cancer associated with this exposure.

Causes of breast cancer

Breast cancer is understood to be linked to lifetime exposure to natural oestrogen. Some organochlorine pesticides behave like oestrogens, even though they are different in chemical structure. DDT and lindane are oestrogenic in animal studies and in human cell culture studies. However, they are only weakly oestrogenic. Compared to natural oestrogen, DDT is thousands of times weaker. In any case, there are more than enough anti-oestrogens in our diets to counter the oestrogenic effects of low doses of chemicals (and of natural oestrogens). In rodent studies, organochlorine compounds have been shown to promote breast turnouts that have been induced by known carcinogens though none of the organochlorine pesticides initiate breast cancer in rodents.

Case-control studies

Several studies compare the levels of pesticides in women with breast cancer to the levels in women who do not have breast cancer.

In a New York study, women with the highest blood levels of DDE (a breakdown product of DDT) had four times the risk of breast cancer compared to women with the lowest level of DDE. The researchers matched the 58 women with breast cancer with women similar in all the important reproductive risk factors. Similar but not significant trends were found in 3 other case-control studies.

A weakness of these studies is that the number of cases was small and pesticide levels were measured just before or at diagnosis.

The women with breast cancer could have had higher pesticide concentrations in their blood because the cancer itself mobilises fat deposits and releases stored contaminants into the bloodstream. A better designed case-control study was done in California. Out of a large group of women whose blood had been collected in the 1960s (the peak of pesticide use), 150 women with breast cancer were matched with 150 controls. Because the women developed breast cancer on average 14 years after their blood was collected, there was less possibility of the cancer influencing the pesticide levels in their blood. In this study, there was no significant relation between DDE and breast cancer.

Conclusion

Taking all the research and arguments into account, the evidence that organochlorine pesticides increase the risk of breast cancer is weak and unconvincing. As with all human knowledge, there is uncertainty and we may never know the precise truth but as it stands, the evidence is reassuring. It should allow policy makers and researchers to direct their efforts away from pesticides and towards more important ways of preventing breast cancer.