Consider this short-ish entry a sort of extension of other discussions on this blog on the topic of lead (Pb) (see here). As per the blog title, the systematic review published by Maryam Daneshparvar and colleagues  (open-access available here) adds to the quite voluminous peer-reviewed literature indicating that lead exposure, particularly during infancy and childhood, seems to be associated with nothing but adverse outcomes in terms of development and behaviour.
Surveying the research literature "on the role of lead exposure in children with ADHD [attention-deficit hyperactivity disorder] symptoms" up to May 2014 - including some sources that I've not come across before e.g. IRAN Medex - the authors eventually boiled down the current research landscape to 18 articles "selected and entered into the data synthesis." It appears that some important guidance was followed when it came to study selection and interpretation and, all in all, some 12,000 participants were included in the selected research literature included for analysis.
Results: well as perhaps expected, nothing good seems to come from lead exposure. The authors reported that even at quite small concentrations in blood (blood lead levels, BLLs below 10µg/dL), lead is potentially linked to the presentation of ADHD in one form or another. In 16 of the 18 studies included in their analysis: "a significant association was found between BLL and one of the types of ADHD."
What's more to say on this topic? Well, as the authors note, the vast majority of the studies analysed included only one measure of BLL so one perhaps has to be a little cautious about the 'snapshot' nature of the data included. They also make some comment on how the symptoms of ADHD were a focus over and above a 'diagnosis' so that could also be considered a methodological weakness.
That all being said, and knowing what we know about 'lead being a strong poison' I'm yet again convinced that keeping kids in particular away from sources of possible lead contamination is of vital importance. I say that not just from the point of view of risk of ADHD-type behaviours or any other label (see here), but simply because nothing good comes from exposure to the stuff. When also we talk about lead exposure, we might also be inclined to note that ambient levels of the stuff might also show some interesting correlations too  (yes, correlation is not the same as causation).
Finally, in amongst all the doom and gloom about lead exposure and developing brains, there may also be a ray of hope...
 Daneshparvar M. et al. The Role of Lead Exposure on Attention-Deficit/ Hyperactivity Disorder in Children: A Systematic Review. Iran J Psychiatry. 2016 Jan;11(1):1-14.
 Dickerson AS. et al. Autism spectrum disorder prevalence and associations with air concentrations of lead, mercury, and arsenic. Environ Monit Assess. 2016 Jul;188(7):407.
Daneshparvar M, Mostafavi SA, Zare Jeddi M, Yunesian M, Mesdaghinia A, Mahvi AH, & Akhondzadeh S (2016). The Role of Lead Exposure on Attention-Deficit/ Hyperactivity Disorder in Children: A Systematic Review. Iranian journal of psychiatry, 11 (1), 1-14 PMID: 27252763