The causes of child leukaemia and, more generally of malignant neoplasms in childhood have been long investigated. Various theories have been proposed on the aetiological mechanisms and risk factors for these neoplasms, including the influence of genetic, prenatal, and postnatal exposures. With a few exceptions, particularly for in utero exposure to ionising radiation and some genetic syndromes, strong and consistent evidence on the importance of specific risk factors has not been produced. One recent exception is the association of ambient air pollution with child leukaemia, an association that is also supported by experimental evidence. For example, a recent experimental study¹ showed that filtration of particulate matter in ambient air significantly reduced heritable mutation rates in laboratory mice housed outdoors near a major highway and two integrated steel mills. On the other hand, extremely low frequency electromagnetic fields (ELF-EMF) emitted by power lines have received a great deal of attention in recent years, but the evidence of an association with childhood leukaemia remains uncertain; furthermore, even if ELF-EMF are causally associated with child leukaemia they could explain only a very small fraction of all child leukaemias, perhaps around 1%.² Exposure to pesticides, both general and specific, has also been examined extensively in relation to childhood cancers, but the evidence is far from conclusive.³

The paper by George Knox in this issue⁴ follows other recent work examining proximity to industrial sources of exposure to toxic atmospheric emissions and childhood cancer.⁵ In this paper he expands on this work by evaluating the effects of specific exposures and their combinations.

Knox applies an approach that …