 The prevalence of autism has increased dramatically in the US. We're not exactly sure how prevalent it used to be, but these days about 1 in 68 kids born in the United States will have it. Despite a massive influx of research funding, we still don't know what's the cause. This sharp increase in prevalence remains unresolved. Yes, changes in awareness and diagnosis can account for some of the increase, but it also might be something we're exposed to in the environment. For example, all the new chemicals we're now exposed to. Out of 80,000 agents the chemical industry has now put out, we have evidence that at least 1,000 may have neurotoxic properties. Even only a small fraction have been studied in humans during critical windows of development. The current chemical risk assessment approach is typically based on the toxicity caused by a single chemical, without acknowledging the effects of different chemical mixtures. There are hundreds of chemicals currently allowed in food that may have potential harmful effects on the developing brain. Each individual chemical may or may not have a harmful effect on its own, but we know next to nothing about their cumulative biological effects on the brain. If we really want to protect children's brains from chemicals, we first need to recognize and deal with a massive dearth of basic hazard and exposure information about any risks posed by environmental chemicals. A lot of the data we do have is from pre-clinical studies, meaning like test tubes and lab animals. Well, yeah, it's not like you can expose people on purpose. That's where epidemiological research can step in, studying exposures within populations. That's how we found out about other well-known hazards like benzene and asbestos. You can't ethically force people to smoke for a study, but you can note how many people exposed to cigarette smoke day in, day out get lung cancer compared to non-smokers. Industries spewing poison like to patronize the public that we shouldn't worry our pretty little heads, but what does the science show? What does the epidemiological evidence have to say about exposures to environmental chemicals and autism? For example, there's a bunch of studies on the effects of smoking during pregnancy on autism rates. I've talked about these so-called forest plots before, where values greater than one suggest at least a tendency towards an increased risk, whereas the values less than one suggest more of a protective association. And should the whiskers here cross the line, then that means that particular finding did not reach statistical significance. This allows you with a quick glance to interpret a huge amount of data from multiple studies. So regarding this figure here, does it look like smoking during pregnancy has any effect on autism rates? No, about half skewed to one side, half to the other, and hardly any of the results reached a statistical significance, meaning they may have just been largely chance findings. Same thing with thimerosal exposure, the mercury-containing preservative used in vaccines. Yeah, one study found significantly increased risk of autism, while three other studies found that those with more vaccine mercury exposure had lower the risk of autism. But mostly they just showed no relationship, either way. Not that it matters much at this point, since thimerosal has been removed from most vaccines. But check out the air pollution data. This is exposure to car exhaust and diesel fumes during pregnancy and infancy. See how all the data is skewed to the right, with half the findings reaching statistical significance, meaning a significant correlation between air pollution and autism. But correlation doesn't necessarily mean causation. This could be a true cause and effect relationship, or maybe it's just that poor people live in the inner cities next to polluted highways, or the traffic noise is causing stress, or some other confounding factor. The other suspicious link they found was pesticide exposure. Again, just a few second blink at this chart, and you can see where most of the evidence trends. Lots of environmental toxicants have been implicated in autism— plastics, chemicals, PCBs, solvents, toxic waste sites, and heavy metals. But the strongest evidence has been found for air pollutants and pesticides. Yeah, clinicians can advise pregnant women and parents to try to avoid exposures to harmful substances in their environment. But instead of just advising patients one by one, a more powerful strategy would be for us clinicians to band together and take a leadership role in advocating for a healthy environment and society-wide actions to help everyone.