UCLA

Previous studies have observed consistent associations between magnetic fields exposure and childhood leukemia, and in children living close to high voltage overhead power lines. Biologic plausibility, changes over time and other methodologic issues connected with observed associations with magnetic fields exposure suggests that certain factors such as other associated exposures, confounding or interaction effects may play a role in observed childhood leukemia associations. In the US, high voltage overhead power lines are located in right-of-ways (ROW), which are often locations additionally for commercial plant nurseries, a potential source of chronic pesticide exposures due to their close proximity to residencies. Pesticides are substances that are utilized to mitigate, repel, or kill unwanted pests. Pesticides are a potential childhood leukemia risk factor, due to their ability to cause molecular changes that are associated with cancer development.

We performed a large California state-wide records-based case-control study that consisted of childhood leukemia cases younger than 16 years of age that were diagnosed in California (1988-2008) obtained from the California Cancer Registry (CCR) and controls obtained from the California Birth Registry (CBR). Controls were randomly matched to cases based on their date of birth (� 6 months) and sex. We obtained commercial plant nursery and pesticide use information from the California Department of Food and Agriculture (CDFA) and the California Department of Pesticide Regulation (CDPR). We utilized CDFA reported locations for commercial plant nursery businesses that were classified as “producers,” which are plant nurseries that grow and sell a total of $1,000 or more of nursery stock annually. Additionally, we obtained Pesticide Use Reports (PUR) for pesticide applications that occurred at plant nurseries prior to 1990; for 1990 and onward, when more detailed pesticide information was available, we obtained pesticide applications for nursery plants grown in containers from the CDPR. Plant nurseries, pesticide applications and subject residences were then mapped with Geographic Information System (GIS) and then augmented with historic aerial satellite images to improve the obtained native data resolution of plant nursery/pesticide exposure information. Due to the unique size and shape footprint of plant nurseries, we selected subjects that resided within 600 m of a nursery for more detailed assessment. Additionally, many plant nurseries are located underneath high voltage power lines, therefore we selected subjects that resided within 2000 m of such power lines to ensure that we captured all nurseries in proximity to these power lines. We observed that subjects that resided very close to commercial plant nurseries had elevated childhood leukemia risks. Subjects that resided within 75 m of a plant nursery had increased risk of childhood leukemia (OR 2.40, 95% CI 0.99-5.82); this increased risk was stronger for the acute lymphocytic leukemia subtype (OR 3.09, 95% CI 1.14-8.34).

We then explored the potential relationship of commercial outdoor plant nurseries as a confounder or effect modifier for observed childhood leukemia associations with magnetic fields exposure due to high voltage power lines. Power line information was based on GIS data from electric power companies, aerial satellite images and in-person site visits. Associations with childhood leukemia and calculated fields (OR 1.51, 95% CI 0.70-3.23) slightly attenuated after controlling for commercial outdoor plant nursery proximity (OR 1.43, 95% CI 0.65-3.16) or restricting analysis to study subjects residing far away (>300 m) from plant nurseries (OR 1.43, 95% CI 0.79-2.60); similar associations were observed between proximity to high voltage power lines and childhood leukemia after controlling and restricting subjects based on their proximity to plant nurseries. Observed associations for childhood leukemia and proximity to commercial plant nurseries remained elevated after excluding study subjects exposed to high calculated fields (OR 2.16, 95% CI 0.82-5.67) or close power line proximity (OR 2.15, 95% CI 0.82-5.64). Childhood leukemia associations with plant nurseries did not materially change after controlling for calculated fields or power line proximity. Our findings suggest that close residential proximity to plant nurseries is an independent risk factor and that plant nurseries do not explain observed childhood leukemia relationships with magnetic fields exposure or close power line proximity.

Finally, we followed up our study by examining childhood leukemia relationships with more detailed pesticide exposures, including specific intended pesticide use, pesticide chemical classes and active pesticide ingredients. Elevated childhood leukemia risks were observed for exposure to several active pesticide ingredients: permethrin (odds ratio (OR) 1.49, 95% confidence interval (CI) (0.83-2.67), chlorpyrifos (OR 1.29, 95% CI 0.89-1.87), dimethoate (OR 1.79, 95% CI 0.85-3.76), mancozeb (OR 1.41, 95% CI 0.85-2.33), oxyfluorfen (OR 1.41, 95% CI 0.75-2.66), oryzalin (OR 1.60, 95% CI 0.97-2.63), and pendimethalin (OR 1.82, 95% CI 0.81-1.25). Additionally, rodenticide (OR 1.42, 95% CI 0.78-2.66) and molluscicide (OR 1.22, 95% CI 0.82-1.81) exposure had elevated childhood leukemia risks. Observed childhood leukemia associations with calculated fields or power line proximity did not materially change after controlling for pesticide exposure. Our findings suggest that pesticide exposure is an independent risk factor for childhood leukemia and that pesticide exposure does not explain observed childhood leukemia associations with high magnetic fields exposure or nearby power line proximity.