A Population-Based Study of Traffic-Related and Outdoor Air Pollution during Pregnancy and Autism in Denmark

Background:

A number of previous studies reported that perinatal exposure to air pollutants may increase the risk of autism in offspring. However, most of these studies were conducted in the United States, especially in California. Here we are reporting on the first large European study of autism and traffic related air pollution in Danish children.

Objectives:

Establish the largest nationwide population and record based case-control study of childhood autism and air pollution worldwide, and test the hypothesis that exposure to traffic-related air pollution or PM air pollution is associated with childhood autism.

Methods:

We identified 20,538 autism cases from the National Hospital Registry and the Psychiatric Central Registry among all persons born between 1989 and 2013 in Denmark. For each case, we randomly selected 5 controls without autism matched by birth year and sex. Relying on AirGIS modelling, we estimated outdoor air pollution at the mother’s home NOx and particulates – for the 9 months before, during, and after her pregnancy. We calculated odds ratio (OR) and 95% confidence interval (CI), adjusting for birth year, sex, maternal and paternal age, and maternal smoking during pregnancy using conditional logistic regression analysis.

Results:

Using NO2 as a marker of traffic-related air pollution, we estimated an adjusted OR of 1.23 (95% CI: 1.19, 1.26) for autism associated with an inter-quartile-range (IQR) increase in NO2. For an IQR increase in PM2.5 the adjusted OR for autism was 1.13 (95% CI: 1.07, 1.19). Pre and post pregnancy exposure estimates were highly correlated with exposure estimates during pregnancy.

Conclusions:

Our data indicated that exposure to air pollution, especially traffic-related air pollution during pregnancy, is associated with elevated risk of autism in Danish children.