The conclusions reached in previous chapters of this report and the summary risk assessments made above indicate that the data on the effects of exposure to electric and magnetic fields on biologic systems are either negative or so uncertain that making such an estimate would be injudicious and misleading, though a theoretic risk assessment could be performed.

As mentioned above, scientists understand cancer to be associated with damage to DNA. Thus, if an agent is found to damage DNA, the carcinogenic potential of that agent has been established as biologically plausible. That is an example of a general principle—in attempting to assess the risks of an agent, one should examine whether some biologically plausible means exists for that agent to cause disease.

No biologically plausible explanation for a putative relationship between exposure to electric and magnetic fields from power lines and an adverse effect in biologic systems has been proposed, tested, and shown to be valid.

The occurrence of a copromotional effect of exposure to very-low-frequency electric and magnetic fields, which has been reported in cell systems and animals, deserves consideration. Cancer is thought to occur after an initiator (a chemical or physical agent) has damaged DNA and started a process leading to the disease. A copromoter might not have the ability to initiate the cancer process, but the risk of cancer can increase when the test biologic system is subjected to a copromoter after the system has been exposed to an initiator.

The body of evidence, in the committee's judgment, has not demonstrated that exposure to power-frequency electric and magnetic fields is a human-health hazard. However, some epidemiologic data support an association between surrogate measurements of magnetic fields and an increased risk of childhood leukemia. Further research for understanding the various ways of measuring exposure and their possible association with adverse health outcomes in model and human systems will be needed to resolve the uncertainty.

The committee's overall conclusion is based on the weight of the evidence after review and analysis of biologic data at the molecular, cellular, and whole-animal level that are considered relevant in assessing the possibility that exposure to electric and magnetic fields in the environment causes cancer in humans. First, in vitro studies were found to observe biologic effects at field strengths that are 1,000 or more times greater than would be experienced in residential situations. Even then, the results are inconsistent and often not reproducible. The demonstration

Front Matter (R1-R22)

Executive Summary (1-8)

1 Introduction (9-20)

2 Exposure and Physical Interactions (21-51)

3 Cellular and Molecular Effects (52-72)

4 Animal and Tissue Effects (73-116)

5 Epidemiology (117-190)

6 Risk Assessment (191-199)

7 Research Needs and Research Agenda (200-208)

Appendix A: Tables (209-284)

Appendix B: Exposure Assessment in Residential Studies (285-301)

References (302-337)

Glossary (338-341)

Biographic Sketches of Committee Members (342-346)

Index (347-356)