Attention-deficit hyperactivity disorder and blood mercury level: a case-control study in Chinese children.

OBJECTIVE To investigate the association between blood mercury level and attention-deficit hyperactivity disorder (ADHD) in Chinese children in Hong Kong. METHODS Fifty-two children with ADHD aged below 18 years diagnosed by DSM IV criteria without perinatal brain insults, mental retardation or neurological deficits were recruited from a developmental assessment center. Fifty-nine normal controls were recruited from a nearby hospital. Blood mercury levels were measured by cold vapor atomic absorption spectrophotometry. RESULTS The mean ages of cases and controls were 7.06 and 7.81 years respectively. Boys predominated (case = 44 [84.6 %], control = 44 [74.6 %]). There was significant difference in blood mercury levels between cases and controls (geometric mean 18.2 nmol/L [95 % CI 15.4 - 21.5 nmol/L] vs. 11.6 nmol/L [95 % CI 9.9 - 13.7 nmol/L], p < 0.001), which persists after adjustment for age, gender and parental occupational status (p < 0.001). The geometric mean blood mercury level was also significantly higher in children with inattentive (19.4 nmol/L, 95 % CI 13.3 - 28.5 nmol/L) and combined (18.0 nmol/L, 95 % CI 14.9 - 21.8 nmol/L) subtypes of ADHD. Blood mercury levels were above 29 nmol/L in 17 (26.9 %) cases and 6 (10.2 %) controls. Children with blood mercury level above 29 nmol/L had 9.69 times (95 % CI 2.57 - 36.5) higher risk of having ADHD after adjustment for confounding variables. CONCLUSION High blood mercury level was associated with ADHD. Whether the relationship is causal requires further studies.