Studies that look at the relationship between environment and childhood leukemia usually consider exposure at only a single residential address, such as the child’s home at birth or at time of diagnosis, say researchers from University of California and University of Southern California. But residential mobility, they contend, can have an impact on a number of relevant factors.
For instance, mobility can affect selection through the availability of data; cases are usually required to reside and be diagnosed in the same geographic area. It can affect exposure to electromagnetic fields and overhead power lines. Residential mobility can also function as a marker for other risk factors for childhood leukemia, such as maternal place of birth and younger maternal age at birth, as well as increased exposure to viruses or other infections potentially linked to higher leukemia risk. Finally, the type of dwelling can affect not only exposure but exposure assessment. Mobile homes and apartments, for instance, are more likely to lead to poor geographic information system (GIS) matching of the residential address.
The researchers hoped with their study to “disentangle the effect of mobility.” Using the California Power Lines Study, they analyzed data from 4,879 childhood leukemia patients born in California and diagnosed between 1988 and 2008.
Many childhood leukemia cases were mobile, the researchers found: 2,982 (61%) children changed residence between birth and diagnosis. Of those who moved, 618 stayed within 2 km of their birth home; 1,992 moved outside of their birth neighborhood. Children who moved tended to be older, lived in housing other than single-family homes, had younger mothers and fewer siblings, and were of lower socioeconomic status.
However, the effects of distance to power lines and magnetic field exposure on childhood leukemia were similar for a subset of residentially stable cases, and overall results were unchanged when the researchers controlled for proxies of mobility (except for dwelling). They found an OR for childhood leukemia of 1.44 for those whose birth residence was within 50 m of a 200+ kV line, and an OR of 1.50 for the highest exposure of calculated fields, compared with 1.62 and 1.71, respectively, among children who stayed in place.
While they believe their findings on mobility are relevant to other environmental exposures and other childhood outcome studies, the researchers conclude that confounding by mobility is an unlikely explanation for the associations observed between power lines exposure and childhood leukemia.