We developed a physically based model for mean magnetic fields as a function of external sources and their distances from the residence (Bowman et al, 1999) and found it not to be as good a predictor of leukemia risk as the Wertheimer-Leeper "wire code", although it was a much better predictor of magnetic fields, suggesting that some aspect of the magnetic field other then its mean intensity must be the causal factor (Thomas et al, 1999a).

As a consultant to the California Public Health Electromagnetic Fields and Health Program, Dr. Langholz explored the question of how to characterize a factor that would explain the patterns of wire code-childhood leukemia association observed in four major United States case-control studies. In positive studies, such a factor needs to be more prevalent in "high" wire codes than "low" whereas in negative studies, the factor should not be correlated to wire code. These concepts were quantified and we used these results to characterize potential factors that might explain the wire code-childhood leukemia association.

In further work on sorting out the wire code-childhood leukemia association, Drs. Langholz and Thomas designed a case-neighborhood design with a counter-matching sub-study. This study is designed to very efficiently 1) validate the wire code-leukemia association found in the earlier Los Angeles, Denver, and Western Canada studies, 2) assess other factors that are correlated with wire code, and 3) assess the role of selection bias in population-based childhood leukemia studies of EMF. An important question that needed to be addressed in order to determine the feasibility of this design was whether a door-to-door survey of a neighborhood around the case would be reliable in identifying potential controls. To this end, Drs. Langholz and McKean-Cowdin, a post-doctoral epidemiologist in our department, undertook a pilot study, funded by the Center Pilot Project Program to 1) determine the size of the neighborhood survey-area required to identify an adequate pool of potential controls and 2) determine if a high neighborhood response rate could be obtained. We found that an eight hour walk yields a sufficient number of controls for the study with a reasonably high response rate.

As consultants to EPRI, Drs. Langholz, Thomas, and Peters did a study of traffic density (an indirect measure of automobile exhaust pollutants) and childhood leukemia using the Los Angeles case-control study of childhood leukemia. We found little evidence of a traffic density-leukemia association. In particular, traffic density did not explain the wire code-leukemia association observed in this study. This crude study does not rule out the possibility that air pollutants may cause leukemia as the traffic density metric used may be a very poor measure of pollutant levels. However, it does make such an association less likely.