Autism has been linked to low levels of vitamin D during pregnancy (ref: here). Could modern life habits and practices be reducing the amount of vitamin D produced by sunlight that a pregnant mother receives, thereby leading to autism? It seems that humans spend an increasing amount of time behind glass windows, and glass cuts out a large percentage of the vitamin D3 producing UVB-type ultraviolet light from the sun. We work in offices behind glass windows; we drive in cars with air conditioning, so we keep the windows closed all year round; we often shop in large malls, which generally have no sunlight at all. Ultraviolet of the UVB type produces vitamin D3 on our skin, but UVB light is blocked by window glass. Window glass only lets through UVA light, which does not produce vitamin D3. What's more, according this this very interesting article by Dr Mercola on vitamin D, UVA light can actually destroy the vitamin D3 on your skin. So sunlight or daylight shining on you through a window can reduce you vitamin D3 levels. (Incidentally, the above Mercola article also mentions that it takes up to two days for your body to absorb the sunlight-generated vitamin D3 on your skin — but showering or bathing with soap removes this vitamin D3 before it can be absorbed. Thus the modern habit of showering once or twice a day may also contribute to decreasing vitamin D3 from the skin.) Furthermore, I believe that the vitamin D3 produced in your skin by sunlight is not the same as vitamin D3 you get from your diet (or from a supplement). The sunlight produced version is in fact vitamin D3 sulfate, which is water soluble, unlike the vitamin D3 from your diet. Could this water soluble vitamin D3 sulfate be better able to cross the placenta, thus giving the growing fetus ample supplies of vitamin D3? If so, then spending much of our lives indoors or in cars behind glass windows might ultimately be reducing the amount of vitamin D3 sulfate received by a fetus during pregnancy, leading to autism. It would be interesting to see a study which questioned mothers of autistic children about the amount of outdoor sunlight exposure they had during their child's pregnancy.