Autism, or more accurately Autism Spectrum Disorder (ASD), is in the news and public view a lot in recent years. According to some recent reports, it is now diagnosed in 1 out of 68 children (1.47%) in the U.S. Reasons for the apparent increase in diagnoses over recent decades are complex, but they lead us to wonder what is happening and what are the causes?
Recent scientific literature suggests that the specific causes are largely unknown, but there is a very strong genetic component (heritability of 80%). Unfortunately, even the genetic aspects are very uncertain and probably highly complex, not just a simple set of genes like the ones that determine your eye colour. Although genetics may play a large role, there are also indications that environmental factors are involved, perhaps in some sort of interaction with the genetic factors.
The popular and social media keep going in circles about vaccines, a factor for which there is no reliable scientific evidence at all. At the same time, there seems to be complete ignorance of a growing body of scientific literature linking ASD with air quality. A quick search through peer-reviewed scientific literature using the Scopus database shows at least 160 papers that mention “autism and ‘air pollution'” somewhere in the publication over the past 20 years.
I don’t know a lot about ASD, but I can comment on air pollution and so here I’ll discuss what I see from some of this literature. Much of the research literature is only fully available if you have access to a university library (like me), but I’ll try to provide some links to at least the summary or abstract of the studies. Much of this literature is highly technical however, so don’t worry if it’s not so easy to digest.
There have been an on-going set of research studies linking ASD to air quality and air pollution. For example, one study found links between ASD severity and nitrogen dioxide (NO2) levels in air. They didn’t find any link to other pollutants like ozone or particulate matter (PM), but a number of other studies have identified these links. A nice review from 2015 argues that there is good reason to think that air pollution is a causal factor in ASD, although the specific type of air pollution responsible is unclear and not consistent among studies.
A lot of studies tend to focus on the U.S., but there are indications from other countries that similar links exist. For example, a study about Denmark published in 2018 indicates an association of ASD with NO2, fine particulate matter (PM2.5) and sulfur dioxide (SO2). A study focusing on Tehran, Iran, did not find any links although the authors highlight some limitations in their work. Another study from Shanghai, China did find a significant link for ASD with fine particulate matter. A study based on Vancouver, Canada also found a significant link between ASD and another air pollutant, nitric oxide (NO). A study in Taiwan found connections between ASD and several air pollutants including ozone, carbon monoxide (CO), NO2 and SO2. So, we see that this seems to be a global phenomenon, and not just based on a limited population study.
We have to always remember that an association is not causation. We don’t know if air pollution causes ASD, but there seems to be some link between the common air quality measurements and diagnosis of ASD. So that means that the mechanisms are still highly uncertain, and to more conclusively prove that ASD is caused by air pollution (and genetics) we need to identify the chemical, physical and/or biological mechanisms. In recent years, there have been some advancements in understanding mechanisms. One study using mice identified oxidative stress and neuro-inflammation in various regions of the brain when exposed to traffic-related air pollution. In another mouse study, ASD related behavioural changes were noted after exposure to diesel exhaust. Neurotoxicity and neuroinflammation due to traffic-related emissions is discussed in another review of the field, and a rough genetic mechanism is suggested.
So, overall there seems to be reasonably good and growing evidence for air pollutants as being involved in ASD development, through some sort of triggering mechanism on the genetics of the individual. There is still a lot of uncertainty behind which specific air pollutants are involved (NO, NO2, ozone, CO, PM2.5, SO2, and others I haven’t mentioned here), or perhaps combinations of these? “Traffic-related” air pollutants seem to play a role in many studies, and these combine all the mentioned pollutants.
Another open question is whether the exposure to air pollutants matters more if it is pre-natal or post-natal. Some studies suggest one or the other, but there is not a very good consistency in these reports as far as I can see. It is certainly possible that both the fetus and the infant are susceptible to these effects, although the mechanisms might be a bit different.
A confusing part in all of this is that across much of North America, the average air quality has been improving over the past decade according to most measures. If air pollution is linked to ASD, how can we reconcile increasing ASD diagnoses with better air quality? Unfortunately there are no good answers here yet. It is possible that the air quality association is with something that is not being directly measured in standard government monitoring stations. Or, perhaps the average air quality is not the key factor, but it is related to acute exposures to poor air over a shorter or specific time period. There are still many questions, and not a lot of answers, and so research continues.
So, unlike the completely unscientific vaccine issue, there is a pretty strong scientific link between Autism Spectrum Disorder and air quality, although the details and mechanisms are still very uncertain. Since a lot of this air quality concern can be related to traffic-related emissions and diesel exhaust, there are good reasons to push for a quicker phase-out of internal combustion engines, reduction in traffic congestion, and promotion of electrified public transit.