CEHN Articles of the Month, April 2015 Issue



Autism Spectrum Disorder and Particulate Matter Air Pollution before, during, and after Pregnancy: A Nested Case-Control Analysis within the Nurses’ Health Study II Cohort

Authors: Raanan Raz, Andrea L. Roberts, Kristen Lyall, Jaime E. Hart, Allan C. Just, Francine Laden, and Marc G. Weisskopf




Particulate matter, or PM, is a common, worldwide air pollutant, defined by the U.S. Environmental Protection Agency (EPA) as a mixture of very small particles or liquid droplets, including acids, organic chemicals, metals, soil, or dust. Particles vary in size, and those smaller than 10 micrometers (μm) in diameter are the most hazardous to human health because they can be readily inhaled and enter the lungs, where they can cause serious and adverse effects. These clinically relevant particles are grouped into two size-related categories. Those between 2.5 μm and 10 μm in diameter (PM2.5-10) are described as “inhalable coarse particles” and include the types of particles inhaled near dusty worksites and roadways. Those 2.5 μm in diameter and smaller (PM2.5) are described as “fine particles” and include those found in smoke and haze.
PM exposure has been associated with a number of acute and chronic adverse health and development outcomes, and even fatalities. A few studies found increased odds of having a child with autism spectrum disorder (ASD) with higher exposures to diesel PM.

This study aimed to more closely examine the relationship between PM2.5 and PM2.5-10 air pollution and the odds of having a child with ASD. The authors specifically explored the association during defined time periods before, during, and after pregnancy.

A total of 245 children with diagnosed ASD (cases) and 1,522 children without ASD (controls) were included in the study. Both cases and controls were offspring of a cohort of participants in the Nurses’ Health Study II (NHS II). NHS II surveys thousands of U.S. female nurses ages 25-43 years of age on a biennial basis to obtain comprehensive health data on that specific population. To assess the study participants’ exposures, the average PM2.5 and PM2.5-10 levels for the 9 months before, during, and after each mother’s pregnancy, at her residential address(es), were estimated using the EPA Air Quality System’s PM model. The model includes meteorological data and PM source information like emission rates from industrial sources, among other inputs.

The study found that PM2.5 exposure during pregnancy was significantly associated with a child having ASD. Associations with the nine month pre- or post-pregnancy periods were not as strong. Exposure to PM2.5 was associated with ASD in all three trimesters of pregnancy, but the highest association was seen in the third trimester.
PM2.5-10 was not found to be significantly associated with ASD in any context.

There is a significant association of maternal exposure to PM2.5 during pregnancy (especially during the third trimester) and having a child with ASD. This suggests that air pollution is a modifiable risk factor for autism, and reduced exposure during pregnancy could lead to lower incidence of ASD.


The World Health Organization Air Quality Guidelines recommend an annual maximum level for PM2.5 of 10 μg/m3. However, this guideline value is exceeded in many parts of the world, including the United States (U.S.). In 2012 EPA tightened the National Ambient Air Quality Standards (NAAQS) annual level for PM2.5 to 12 μg/m3, and there are still many Americans residing in nonattainment areas. These areas, specifically urban neighborhoods with many busy roads and near heavy industrial emitters, are often home to low-income, racial minority communities that already face disproportionate health problems. Furthermore, there is growing support among environmental health scientists that for sensitive populations such as children, the elderly, or those with certain illnesses or conditions, there may be no safe level of PM exposure.
There is substantial support for a re-evaluation and tightening of the NAAQS, but there is also the need to improve monitoring networks, compliance, and enforcement of the standards. EPA mandates that each U.S. state develop a State Implementation Plan (SIP) outlining specifically how the state will attain and maintain the NAAQS, especially in nonattainment areas. The SIP requires states to increase monitoring systems for PM, to devise strategies to reduce PM emissions, including stricter industry regulation, and to develop ways to evaluate these strategies and progress made. Unfortunately, state follow-through on SIPs is difficult to officially enforce. On March 23, 2015, EPA proposed a rule that provides details on how air agencies should meet the statutory SIP requirements that apply to areas designated nonattainment for any PM 2.5 NAAQS. Public comments on this proposed rule are accepted until May 22, 2015. Click here to read more and to learn how to submit comment.
Continued fossil fuel combustion, agricultural practices, car emissions, and construction activities continue to pose PM exposure threats in this country. Efforts to reduce our use and dependency on fossil fuels should be a priority at all levels of government. Sufficient funding is necessary to run successful public health campaigns that educate the general public on the importance of recycling, using energy-efficient appliances, planting deciduous trees, and using alternative modes of transportation.


Article available in Environmental Health Perspectives.