The outcomes of systematic investigation of such disease clusters rarely see the light of day in peer-reviewed journals. In that regard the report in this issue of the Journal by Westley-Wise and her colleagues of an investigation into a cluster of leukaemia cases among residents near a major steelworks is unusual. Of the many studies investigating close case aggregations of any type, only a few have come up with a credible explanation of why the disease cluster exists or have added to knowledge of the causes of disease. Sadly, experience tells us that almost all cluster investigations will fail to produce insights into environmental disease interactions, and no amount of time, effort and money will change this. This is for a number of reasons.
Clusters are by definition based on tiny case numbers.
Further, most reported clusters tend to be of conditions where little or nothing is known of the common causes. Finally, the supposed environmental-disease links are often impossible to investigate satisfactorily because of the lapse of time or because it is simply not clear what to investigate. The tenfold excess of childhood cancers in the United Kingdom near the Sellafield reprocessing plant 4 has been under investigation for a decade, to no avail. Most public health doctors know that having a disease cluster to investigate is a thankless task. Not to have an investigation at all would be regarded with deep suspicion, while investigations with negative results can lead to accusations of a cover-up.