CHILD LEUKEMIA

In the May issue we brought a map showing the incidence of child leukemia near three British nuclear establishments, and at first sight the clusters of above-average incidence did seem to occur in their vicinity. Yet as our map showed, a far tighter relationship (though not necessarily a significant one) could be established when the clusters (or their absence) were judged by their distance from the freeways in the same area.

The British National Radiological Protection Board has now published a thorough study after measuring the radioactive discharges from the three suspected sources and determining the dose to the bone marrow of 1-year old children in the area; they found that at a distance of 5 km (3.12 miles) it was up to 190,000 times smaller than that from the natural radioactive background, while at distances of 0.5 km (550 yards) the dose increases only 20 times. "In no way can [these contributions] be responsible for an in-creased incidence of leukemia among children," said the report, "if such an increased incidence is shown to exist."

Such an incidence has not been shown: the existence of above-average pockets is as natural as that of below-average ones. It would be quite incredible if every measurement of a rare phenomenon in small areas would yield exactly the average every time.

Meanwhile, a panel appointed by New York State suggested in July that magnetic fields of residential power lines may be linked to childhood leukemia. As usual, the papers brought this under the headline ELECTRICITY LINKED TO CANCER, though the smaller print said that the panel did not consider the evidence conclusive.

In my 15^th year of monthly superstition bashing, I have come to read such reports with a large dose of skepticism. However, in this case I withhold my judgment for a number of reasons. First, I do not want to pooh-pooh anything off-hand without having seen the full original report; second, my chief adviser on biological effects of electromagnetic fields is abroad for most of the summer; and third, this charge is new: it is not the tiresome old tale about high-voltage lines, which have been investigated for decades all over the world without revealing any serious hazard, but a suggestion that residen-tial lines (in and near homes), with large currents and their accompanying magnetic fields, might be a health hazard.

Even so there are a number of points in the newspaper reports that raise my mistrust, and one of them is the fact that the evidence is based on two studies of child leukemia in Denver: I doubt that the number of child leukemias in a city of 1,000,000 (adults) is suffi-ciently large to draw this type of conclusion.

But even if I am wrong in this particular case, I would like to warn readers about a general trap that rarely fails to claim its vic-tims. It is simply this: when you have a rare phenomenon, you need a stunningly large statistical sample before you can draw any signifi-cant conclusions. The reasons follow.