Monday, 26 November 2012

CHARGE, traffic pollution and autism

The headline reads "Traffic fumes linked to autism". The sub-heading describes how babies who were exposed to traffic air pollution during their first year of life "are more likely to become autistic". Welcome to the study by Heather Volk and colleagues* on traffic pollution, particulate matter and risk of autism.
Motorbiking... @ Wikipedia  

Before heading into the paper, the discerning autism research reader will no doubt remember that this is a topic which has already cropped up in autism research circles, and indeed by the same authorship group** (open-access).

The difference being that on the previous occasion, the focus was on in-utero proximity to traffic-related air pollution "as a surrogate for air pollution exposure", whereas the current study looked at both gestation and first-year estimates of exposure.

A few details of the most recent study:

  • Another CHARGE-related investigation, which estimated air pollution and air quality based on mum's address for the various stages of pregnancy and first year of life for 279 children diagnosed with an autism spectrum disorder (ASD) compared with 245 typically-developing controls.
  • The US Environmental Protection Agency (EPA) datasets on regional air pollution were cross-indexed with addresses, and some statistical wizardry applied.
  • Results: compared with control participants, exposure to traffic-related air pollution by residence was highest for the autism group particularly during the first year of life (adjusted odds ratio = 3.1). Exposure to particulates (sub 10 and 2.5 micrometres) based on regional exposure measures were also associated with autism during the first year of life.
  • The authors conclude: "Exposure to traffic-related air pollution, nitrogen dioxide, PM2.5, and PM10 during pregnancy and during the first year of life was associated with autism". The possible effect being one of inflammation (quite a regular visitor to autism shores it has to be said).

One obviously has to be quite careful with such studies of association and all that 'correlation does not equal causation' jazz. On that basis I'm not going to start any great fanfare about this research despite the intricate datasets which have been compared, and the previous body of research that it follows.

That being said, neither am I going to discard these results as being just coincidence. Air pollution is a serious environmental issue related to health; potentially acting in a multitude of ways depending on what type of pollution is present, the level of exposure*** and the age of the exposee. We've seen hints that air pollutants might play some role in cases of autism as per another study of association on meteorological factors and autism recently discussed on this blog. Definitive data is however somewhat lacking including more direct measurement of airborne pollutants and any residual traces that they may leave.

Autism is also not alone in its purported link to air pollution as exemplified by this paper by Siddique and colleagues looking at air pollution and ADHD****. Other research has attempted to link air pollution to issues with cognitive abilities*****. The primary question arising from such research is to ask whether the recent Volk data may reflect any of these comorbidities/issues above and beyond an exclusive link to autism?

Outside of just traffic-related air pollution, the air that we breathe is home to lots of other things too; some things which just don't even bear thinking about (dog poo bacteria, yep you heard me right). Questions are being asked about air quality in our cities on quite a grand scale nowadays and in conditions like autism, fast becoming a 'health priority' (and not before time), the research net should be cast as wide as possible to determine the contribution of environment (if any) to the astounding number of cases of autism estimated and being diagnosed.


* Volk HE. et al. Traffic-related air pollution, particulate matter, and autism. Arch Gen Psychiatry. November 2012.

** Volk HE. et al. Residential proximity to freeways and autism in the CHARGE study. Environ Health Perspect. 2011; 119: 873–877.

*** Valavanidis A. et al. Airborne particulate matter and human health: toxicological assessment and importance of size and composition of particles for oxidative damage and carcinogenic mechanisms. J Environ Sci Health C Environ Carcinog Ecotoxicol Rev. 2008; 26: 339-362.

**** Siddique S. et al. Attention-deficit hyperactivity disorder in children chronically exposed to high level of vehicular pollution. Eur J Pediatr. 2011; 170: 923-929.

***** Suglia SF. et al. Association of black carbon with cognition among children in a prospective birth cohort study. Am J Epidemiol. 2008; 167: 280-286.

---------- Volk, H. (2012). Traffic-Related Air Pollution, Particulate Matter, and AutismAir Pollution, Particulate Matter, and Autism Archives of General Psychiatry DOI: 10.1001/jamapsychiatry.2013.266