Mom’s pesticide exposure at work increases her child's leukemia risk.

Jun 17, 2009

Wigle, DT, MC Turner and D Krewski. 2009. A systematic review and meta-analysis of childhood leukemia and parental occupational pesticide exposure. Environmental Health Perspectives doi:10.1289/ehp.0900582.

Children whose mothers were exposed to pesticides at work while pregnant are at double the risk of developing childhood leukemia.

A detailed analysis of all the available studies comparing work-related, parental pesticide exposure and childhood leukemia finds that the mother’s exposure during pregnancy increases her child’s risk of the disease. The father’s exposure before pregnancy does not.

The study emphasizes the significant contribution of prenatal exposure in developing childhood disease and shows a need for more in-depth studies of the effects of prenatal exposures to environmental factors.

Leukemia is the most common form of childhood cancer with more than 4,000 children diagnosed in the US each year. When a child has leukemia, his/her infection-fighting white blood cells start to grow abnormally and flood the blood stream. Since these white blood cells are defective, they interfere with the proper function and production of normal blood cells. White blood cells will not effectively fight infections, and the depleted red blood cells and platelets lead to anemia and bleeding problems.

However, little is known about the causes of the disease, but, like any other cancer, many factors can increase a child’s risk of developing leukemia. Currently, the known risk factors for leukemia – ionizing radiation, sex, race and genetic syndromes – account for less than 10 percent of cases. A growing body of scientific evidence suggests a link between leukemia and environmental factors such as exposure to pesticides, herbicides and insecticides.

About 165 pesticide compounds are probable or possible carcinogens, according to the US Environmental Protection Agency and other international government agencies. However, human studies of pesticide exposure and risk of childhood leukemia have shown conflicting results. Some studies have found parental pesticide exposure to be associated with increased risk of leukemia while other studies have found no effect or even decreased risk.

Now researchers have attempted to resolve some of the contradictions of previous findings by reanalyzing data from all available studies. By combining all the studies and generating a summary estimate of risk, the authors were able to detect statistically significant increases in risk that could not be detected in many of the smaller studies.

Researchers from the University of Ottawa scoured the archives to locate all studies of maternal and paternal work exposure to pesticides and childhood leukemia. Studies included foreign-language publications and unpublished dissertations. The studies were rated according to their scientific rigor – strengths and weaknesses for such things as exposure measurements and appropriate statistics – and weighted accordingly. In all, 35 studies met the criteria and were included in the analysis.

When the findings of all the paternal studies were combined, fathers’ occupational pesticide exposure did not increase leukemia risk in children. Although several previous studies had found associations of paternal exposure and leukemia, these tended to be the lower quality studies.

However, after combining all the maternal studies, mothers’ pesticide exposure during pregnancy was associated with more than a 2-fold increase in risk of leukemia. This finding persisted regardless of the quality of the articles.

Several of the maternal studies were too small to show statistically significant increases in leukemia risk on their own. However, when the results of all the studies were combined, a statistically significant increase emerged.

Generally, the strongest associations occurred with higher exposures, farm-related exposures and certain subtypes of leukemia. While more research is needed to tease apart the details of which pesticides may be most dangerous, the authors conclude that, based on their analysis, avoiding "prenatal maternal occupational pesticide exposure may be particularly important" to protect public health.

The same researchers similarily reviewed past studies of residential exposure to pesticides and its relationship to childhood leukemia and will publish those results soon.