Decrease in blood inorganic mercury level and a possible increase in organic mercury level in the American population 2005-2010

Bernard M Y Cheung, Adrian J Cheung. University of Hong Kong, Hong Kong, Hong Kong

Introduction: Mercury is toxic, especially in children. The general population is exposed to mercury in fish, dental amalgam and in certain vaccines. Although total blood mercury level has been reported to be decreasing in the US, the trends in the levels of organic and inorganic mercury have not been reported.

Methods: We analyzed newly available data on blood inorganic mercury levels in the US National Health and Nutrition Examination Survey (NHANES) 2005-2010. Organic mercury level was calculated by subtracting inorganic mercury level from total mercury level. There were 8364, 8161 and 8727 participants with measurements of mercury levels in NHANES 2005/6, 2007/8 and 2009/10 respectively. We used sample weights to adjust for sampling biases. As blood mercury concentration follows a log-normal distribution, so data were log-transformed and expressed as geometric mean with 95% confidence intervals (CI).

Results: Inorganic mercury levels were 0.31 [95%CI: 0.30-0.31], 0.30 [0.30-0.30], 0.28 [0.27-0.28] µg/l and organic mercury levels were 0.76 [0.72-0.80], 0.68 [0.64-0.72], 0.78 [0.74-0.81] µg/l in 2005/6, in 2007/8, and in 2009/10 respectively. Inorganic mercury levels showed a significant decreasing trend (p<0.05). Organic mercury levels decreased in 2007/8 (p<0.05) but returned to higher levels in 2009/10. There was no significant sex difference in mercury levels; for instance, in 2005/6, organic mercury levels were 0.78 [0.69-0.88] µg/l in men and 0.74 [0.66-0.83] µg/l in women (p>0.05). Organic, but not inorganic, mercury levels were significantly lower in participants aged <20 compared to those ≥20 years. Organic mercury levels in 2005/6, 2007/8, 2009/10 were 0.41 [0.36-0.46], 0.37 [0.35-0.40], 0.43 [0.39-0.48] μg/L respectively in participants aged <20 years, and 0.87 [0.78-0.97], 0.77 [0.67-0.87] and 0.87 [0.78-0.96] μg/L respectively in participants aged ≥20 years.

Conclusions: The level of inorganic mercury, which is found in batteries, some alternative medicines and skin blanching creams, has been steadily decreasing during the study period. Organic mercury level was lower in 2007-2008 but increased in 2009-10. The increase in organic mercury level in 2009/10 in participants younger than 20 years old as well as those older would be of concern if it is the beginning of a rising trend. The increase might be due to an increase in consumption of large fishes or pollution or both. Monitoring of mercury levels in humans and the environment must be continued, and measures to reduce exposure to mercury should not be relaxed.