The association between exposure to environmental factors and the occurrence of attention-deficit/hyperactivity disorder (ADHD). A population-based retrospective cohort study

Abstract

Background:A number of factors contribute to attention deficit hyperactivity disorder (ADHD) and althoughthey are not fully known, the occurrence of ADHD seems to be a consequence of an interaction between multiplegenetic and environmental factors. However, apart from pesticides, the evidence is inadequate and inconsistentas it differs not only in the population and time period analysed, but also in the type of study, the control of theconfounding variables and the statistical methods used. In the latter case, the studies also differ in the adjustmentof spatial and temporal variability. Our objective here, is to provide evidence on an association between en-vironmental factors and ADHD.Methods:In our study, we used a population-based retrospective cohort in which we matched cases and controls(children free of the disease) by sex and year of birth (n = 5193, 78.9% boys). The cases were children bornbetween 1998 and 2012 and diagnosed with ADHD (n = 116).To evaluate whether there was a geographical pattern in the incidence of ADHD, wefirst represented thesmoothed standardized incidence rates on a map of the region being studied. We then estimated the probabilityof being a case by using a generalized liner mixed model with a binomial link. As explanatory variables ofinterest, we included the following environmental variables: distance to agricultural areas, distance to roads(stratified into three categories according to traffic density and intensity), distance to petrol stations, distance toindustrial estates, and land use. We control for both observed (individual and family specific variables anddeprivation index) and unobserved confounders (in particular, individual and familial heterogeneity). In addi-tion, we adjusted for spatial extra variability.Results:We found a north-south pattern containing two clusters (one in the centre of the study region andanother in the south) in relation to the risk of developing ADHD. The results from the multivariate model suggestthat these clusters could be related to some of the environmental variables. Specifically, living within 100 mfrom an agricultural area or a residential street and/or living fewer than 300 m from a motorway, dual car-riageway or one of the industrial estates analysed was associated (statistically significant) with an increased riskof ADHD.Conclusion:Our results indicate that some environmental factors could be associated with ADHD occurring,particularly those associated with exposure to pesticides, organochlorine compounds and air pollutants becauseof traffic.