Does Lead Exposure Cause ADHD?

Many of these findings were based off of an original journal article regarding prenatal tobacco and lead exposure and the onset of ADHD by Braun and coworkers in the December 2006 issue of the journal Environmental Health Perspectives. For a quick synopsis of this article on lead and ADHD, please click here. Interestingly, this same group also published more recent papers on the effects of lead on conduct disorders, which are often comorbid to ADHD cases. This should be especially relevant for pregnant or nursing mothers. For more information on ADHD and pregnancy, please check out the collection of posts on this blog addressing the topic, which can be found here.

While the relevance of several studies regarding the effects of lead on ADHD and cognitive dysfunction is called into question, often because the lead-levels reflect a much higher exposure than what is often faced by the general population, a relatively large study done recently indicates that even moderately high blood lead levels show a strong correlation with ADHD. This suggests either one of two things:

• Other unknown or "hidden" factors were present in the lead-based studies which were the major contributors to impaired mental function and disorders such as ADHD. Even with lower lead levels, these under riding factors were still present, and therefore the major contributing causes to the disorder were still present.

OR

• The sensitivity to lead exposure in children is even higher than previously thought.

An important question we should be asking ourselves is "Does lead exposure beyond a certain point trigger specific ADHD symptoms, or is there an increase in ADHD behavior across the board?".

ADHD is often defined by two major components, the hyperactive/impulsive component and the inattentive component. Based on a recent publication by Nigg and coworkers in the February 2008 Journal of Biological Psychiatry, it appears that the hyperactive/impulsive component of ADHD predominates based on exposure to lead.

Interestingly, the children investigated in the study above were of the inattentive subtype or the combined subtype (both inattentive and hyperactive/impulsive) of ADHD. Based on these results, it is my personal opinion that a child who, under other circumstances may otherwise be of the ADHD inattentive subtype, could instead fall into the ADHD Combined Subtype if he/she is exposed to a specific quantity of lead during the prenatal or early childhood stages of development. Furthermore, I propose that, had the individuals in the study have been of the predominantly Hyperactive/Impulsive Subtype of ADHD, the results would have shown that lead exposure beyond a critical thresh hold would have exacerbated the already-negative hyperactive behaviors for this particular subtype.

In addition to the negative effects surrounding the hyperactive elements of ADHD, the study also found a correlation between low-level lead exposure and child IQ's. This, of course, has been a hotly debated topic for years. While other factors may clearly be at work (lead exposure is often higher in areas with lower socioeconomic status, which is also a factor often correlated with lower IQ scores), the results of numerous studies, many of them recent, still support a strong possible connection.

Theoretically, then, by significantly reducing the prenatal or early-developmental exposure to lead, a child may be at least partially shielded from negative symptoms such as a lower IQ and hyperactive behavior. However, for individuals with the predominantly inattentive form of ADHD, these lead-restrictive measures would be less effective in addressing their inattentive behaviors. Therefore, it is my opinion that reducing lead exposure due to prenatal intervention, iron therapy, or, even possibly chelation methods (both of which will be discussed in future posts), would be most effective for treating the Hyperactive/Impulsive and Combined subtypes of ADHD and less effective for the Predominantly Inattentive ADHD Subtype.

While we should be careful not to overplay or overhype the lead/ADHD connection (especially given the fact that overall lead exposure risks have gone down throughout most of the world in recent years due to the uses of unleaded gasoline and lead-based paint, among other things), it is important to recognize that there is still a statistically significant connection between the two, at least according to a number of recent studies. The Nigg paper, mentioned above, found a strong correlation with hyperactive ADHD-like behavior at much lower lead levels (much closer to the average levels found in much of the United States) than those in most previous studies. This information is particularly important to pregnant mothers, since it has been demonstrated that the negative effects of lead, and other heavy metals and toxins are more harmful on developing brains and nervous systems than to mature ones. The protective effects of reducing lead exposure to mitigate the negative symptoms of ADHD, should not, in this blogger's opinion, be overlooked.

In the next post, we will be discussing how treatment or supplementation with iron may be able to offset some of these harmful effects of early lead exposure on ADHD, should they occur.