February 20th, 2009 at 10:32 am
The following post is by Geoffrey Kabat, Ph.D., a cancer epidemiologist at the Albert Einstein College of Medicine, who has conducted studies of breast cancer and environmental factors and is the author of Hyping Health Risks: Environmental Hazards in Daily Life and the Science of Epidemiology. You can also read a post by Kabat on thirdhand tobacco smoke.
The campus at the University of California at San Diego is in turmoil over a cluster of eight breast cancer cases and what action the university should take to address a potential health threat. On Tuesday faculty, staff, and students marched to protest hazardous work conditions in the Literature Building, where the eight affected women, who were diagnosed with breast cancer since 2000, worked.
Concern has focused on some potential common exposure in the building and particularly on an elevator thought to produce strong electromagnetic fields. The university has hired a UCLA epidemiologist, who has done extensive research on the health effects of electromagnetic fields, to conduct a two-month study of the problem.
I believe that there is a need to investigate clusters like this, if only to reassure the community and to “leave no stone unturned.” There is always the possibility that something new could be learned. However, based on previous studies of cancer clusters, on what we know about breast cancer, and what we know from studies of environmental factors and electromagnetic fields in relation to breast cancer risk, it is unlikely that the crash study will turn up a common environmental exposure that could explain the cluster. Here’s why:
First, breast cancer takes perhaps two decades to develop before it is diagnosed. This raises the question of how long the eight women were working in the literature building. Although it is always striking when a number of cases of the same cancer occur in a restricted area, it needs to be remembered that there will be excesses (or clusters) as well as deficits when one looks at the distribution of cases in space. Some of these highs and lows will be due simply to chance.
Furthermore, from over forty years of study of the epidemiology of breast cancer, we know that the main determinants of breast cancer risk are: age at menarche, age at first full-term birth, number of children, age at menopause, and family history of breast cancer. Additionally, long-term hormone replacement therapy is associated with a modest increase in risk, and obesity is a risk factor for postmenopausal breast cancer. Finally, higher socioeconomic status is associated with increased risk. Thus, insofar as we have identified risk factors for breast cancer, these are things a woman carries with her wherever she moves.
The possibility that environmental exposures play a role in causing breast cancer became a focus of intense interest starting in the early 1990s, and substantial efforts have been devoted to examining such factors, including organochlorine compounds (such as DDT and PCBs); chemicals produced by the combustion of fossil fuels; and electromagnetic fields from power lines and electrical appliances. Environmental exposures, which generally are at very low levels and which vary over time, are difficult to study. Nevertheless, the results of studies that have been done to date do not provide support for a role of environmental factors in the development of breast cancer.
The only environmental exposure for which there is strong evidence of an effect is exposure to ionizing radiation from the atomic bomb dropped on Hiroshima, which is associated with substantially increased risk of breast cancer, particularly in women who were exposed during adolescence, when the breasts are developing.
The possible effect of exposure to electromagnetic fields on breast cancer risk has been the focus of three large case-control studies carried out on Long Island, NY, Los Angeles, and Washington state. These studies involved taking detailed measurements of electromagnetic fields in the homes of hundreds of women diagnosed with breast cancer as well as in the homes of similar numbers of comparison women without breast cancer. These studies showed no hint of an association.
Furthermore, a larger number of studies has examined whether use of electric blankets in the decades preceding a diagnosis of breast cancer is associated with increased risk. These studies also showed no evidence of an effect.
Electromagnetic fields produced by power lines and appliances have very low frequencies and energies and are not to be confused with “ionizing radiation.” Electromagnetic fields encountered in the course of everyday life are far below the strengths at which any biological effects have been detected. Experimental studies have also failed to yield evidence of a mechanism by which ambient electromagnetic fields could affect breast cancer risk.
If large and carefully-designed studies of electromagnetic fields (and other environmental exposures) and breast cancer have not managed to detect any evidence of a link, it is highly unlikely that the crash survey undertaken by UCLA will uncover a cause among these eight breast cancer cases.