A meta-analysis of the evidence on the impact of prenatal and early infancy exposures to mercury on autism and attention deficit/hyperactivity disorder in the childhood.

Although a measurable number of epidemiological studies have been
conducted to clarify the associations between mercury exposure during
embryo or early infancy and later incidences of autism spectrum
disorders (ASD) or attention-deficit hyperactivity disorder (ADHD), the
conclusion still remains unclear. Meta-analysis was conducted for two
major exposure sources; i.e., thimerosal vaccines that contain
ethylmercury (clinical exposure), and environmental sources, using
relevant literature published before April 2014. While thimerosal
exposures did not show any material associations with an increased risk
of ASD or ADHD (the summary odds ratio (OR) 0.99, 95% confidence
interval (CI) 0.80-1.24 for ASD; OR 0.91, 95% CI 0.70-1.13 for
ADHD/ADD), significant associations were observed for environmental
exposures in both ASD (OR 1.66, 95% CI 1.14-2.17) and ADHD (OR 1.60, 95%
CI 1.10-2.33). The summary ORs were similar after excluding studies not
adjusted for confounders. Moderate adverse effects were observed only
between environmental inorganic or organic mercury exposures and
ASD/ADHD. However, these results should be interpreted with caution
since the number of epidemiological studies on this issue was limited
and still at an early stage. Further studies focused on subjects with
genetic vulnerabilities of developmental disorders are warranted for
better understanding of the effects of such environmental exposures.