Locavorism in China's "mercury capital" a potentially toxic idea

by: mental_masala

Sun Apr 11, 2010 at 11:42:40 AM PDT

In a recent episode of the Colbert Report, ocean expert Sylvia Earle recommended that people rethink eating fish from the ocean, to which Colbert asked, without fish in my diet, "where will I get my mercury?" Colbert's facetious question has been answered in a new paper in Environmental Health Perspectives: you can get plenty of mercury by eating rice grown in the Wanshan region of the Guizhou province in Central China. This region is known as the "mercury capital" of China because of its rich cinnabar ore deposits and productive mercury mines.

The researchers estimate that approximately 34% of the inhabitants in Wanshan exceeded the U.S. EPA established reference dose (RfD) for methylmercury of 0.1 micrograms per kilogram of body weight per day. So how is mercury getting into their body? Is it through the air, water, or food? And where is it converted from inorganic mercury to methylmercury?  (Methylmercury is far more toxic than inorganic mercury, as it more easily passes through the blood-brain barrier and placental barriers.)

The residents of the region grow most of their own food. They eat very little fish (about 1 gram per day on average) -- the typical source of mercury in the diet. 

The researchers collected samples of agricultural products directly from fields and sampled water directly from wells and reservoirs, then submitted it for chemical analysis. With estimates of the average diet and water consumption, they could calculate the probable daily intake of mercury and methylmercury.

The researchers found that food is the source of most of the mercury intake, even though residents almost never eat fish. Locally-raised rice, vegetables and meat account for about 90% of the total mercury intake; locally-raised rice alone accounts for 94-96% of the methylmercury exposure . Rice, it turns out, is a particularly effective converter of inorganic mercury to methylmercury, as the water-saturated soils offer conditions that favor conversion of mercury to methylmercury.

Since rice is a major staple for more than half of the world's population,  the authors recommend further research on mercury contamination of rice in rice-producing areas. 

The authors conclude the following:

In summary, this study shows that the general adult population in Guizhou is exposed to low levels of MeHg that may not pose serious health risks to most members of the population. Nevertheless, a small portion of the population in heavily contaminated Hg mining areas may exceed the tolerable intake levels of MeHg for pregnant women. It is clearly shown in this study that rice consumption is the predominate pathway of MeHg exposure to the general populations. Moreover, fish consumption contributes only 1–2% of their MeHg exposure dose, which is much lower than in Japan, North America and Europe. However, inhabitants in Hg mining areas were found exposed to both high levels of MeHg and inorganic Hg. It is still unknown to what extent dose-response relationships established for MeHg through fish and seafood consumption is valid for populations being exposed through rice consumption. More studies are urgently needed to scrutinize this issue.

Image from Wikipedia's Guizhou province entry.