Many of those who make the ‘vaccines cause autism’ argument often simply ignore the fact that dosage is important in determining whether a toxin has a negative impact on humans. In scanning internet sites that debate autism, you see incessant discussions on the questionability and immorality of exposing humans to toxins like mercury, aluminum, and other heavy metals. A problem with these accusations is that we humans are constantly exposed to a wide range of potential toxins every day simply resulting from the fact that we live on this planet which is composed of dozens of elements, including heavy metals such as lead, cadmium, arsenic, beryllium, and chromium. Large amounts of any of these metals can be toxic, but very small amounts of them are found in most people.

The key factor in evaluating whether we should be concerned about these toxins is the dosage to which we are exposed. For instance, low levels of many substances which would be toxic at high doses are required for human health, such as aluminum or zinc. Also, there are many substances that we are exposed to at high levels, but that are toxic to us at really high levels. Take water for instance. Human bodies are comprised mostly of water. But, you can die if you drink too much water at once (water intoxication). Rarely do people think of water as a potential toxin; but, at a high enough dose, water can be deadly.

One venue where this confusion over dosage plays out in the debate over vaccines and autism involves thimerosal. As discussed in detail in my section on thimerosal, there are numerous western countries where thimerosal has been largely removed from vaccines given to children. And, several studies have shown that autism rates have continued to rise in these countries even after thimerosal was removed, providing strong evidence that thimerosal was not responsible for the so-called autism epidemic. However, defenders of the thimerosal theory of autism have responded by arguing that in fact thimerosal has not been removed from vaccines children are exposed to, that thimerosal is still in many influenza vaccines given to children and to pregnant women, and that certain vaccines have lagged in thimerosal removal (such as the enterix Hepatitis B vaccine). There are many potential problems with this response, such as only a fraction of children actually get the influenza vaccine each year; however, the most important problem involves dosage.

Whether every microgram of thimerosal was withdrawn from childhood vaccines is largely irrelevant to evaluating the relationship between thimerosal and autism and the studies that showed that autism rates continued climbing even after thimerosal was largely removed from vaccines. What matters is that thimerosal exposure was dramatically reduced in countries like Sweden and the US, by 90% or more - endless debates about the last 10% of potential exposure is a waste of time. This is because dosage matters, particularly in heavy metal exposure where toxicity is directly linked to cumulative exposure and concentration. If thimerosal caused autism, a 90% reduction in average exposure in children should greatly reduce rates of autism. The consistent results of studies on this matter showed no dip whatsoever in autism rates after thimerosal was largely removed.