ADHD seems to be on a rise in the United States. According to a recent study done by the FDA, the number of children diagnosed with ADHD has increased from 4.4 million in 2002 to 5 million in 2010. In the search of an explanation, researchers have now begun to look for a wide array of fetal exposure to toxins including lead, tobacco, pesticides, and PCB (polychlorinated biphenyl). Mercury, also known as quicksilver, affects the nervous system and is one of the latest suspects to be investigated.
In a recent study done in Archives Pediatric Adolescent medicine 2012, researchers measured prenatal mercury exposure by testing mother’s hair shortly after birth. Researchers found that the child’s risk of developing ADHD increased to 40-70% at a certain threshold of mercury and that children were more likely to exhibit signs and symptoms of ADHD when 8 years old. The study also showed that there was a greater association of ADHD symptoms with boys than girls.
So how does one get exposed to mercury? Certain jobs such as mining, amalgam dental fillings, and eating certain fish contaminated with heavy metals can all cause exposure to mercury. The content of mercury in a fish depends on a number of factors that includes the size of the fish, where the fish is caught, and how far along the fish is in its life span. For example, smaller fishes such as salmon, herring, mackerel, and sardines tend to be lower in mercury levels. On the other hand, bigger fish such as shark, swordfish and fresh tuna are more likely to contain higher amounts of mercury. Although these are general guidelines, one should always remember that there are exceptions to the above rules, such as fish being a good source of omega 3 fatty acids.
This study showed that the consumption of fish was not independently related to ADHD symptoms. The authors of the same study also conducted a second analysis among the same group of mothers and children. They also showed that children of a mother who consumed more than two servings of fish per week also had a 60% reduced risk of ADHD symptoms. This being said, it would be very difficult to design a study for exposure to mercury in pregnant women due to ethical reasons and the potential link between mercury and ADHD needs to be explored further.