Continuing on the topic of heavy metal exposure, this post will discuss internal mercury and cadmium exposure levels as reported in three studies. These studies looked at potential exposure as a result of either occupational exposure to elements in e-waste or environmental exposure due to proximity to e-waste sites. A comparison group was used in all three studies in order to associate elevated mercury and cadmium levels with e-waste recycling activities.
You can learn more about which electronic devices contain mercury and cadmium, as well as an overview of the known human health risks of exposure, in my post entitled What Are You Leaving Behind?
I could only find one study that measured mercury exposure among e-waste workers and neighboring community members, and the quality of the study was quite poor. Hair samples were used as the biological measure to assess mercury exposure, which can be a good measure of the variation in mercury intake over a long period of time, if the hair is analyzed in segments . This study, to my understanding, did not analyze the hair in segments.
The study I'm referring to was conducted in Bangalore, India, and is the same article I mentioned above (containing the photos) . The aim of the study was to identify exposure to a number of different metals, mercury being one of them. Only males were looked at in this study: eleven exposed and eight control participants. As a results of the small sample size, a number of factors that may have influenced the findings were either not measured or not able to be considered in the analysis (i.e. age, diet, smoker status and years of employment). Overall, I found the methods section to be poor, as it lacked a detailed description of how the hair samples were taken and analyzed.
The results of the study did not indicate a statistically significant difference between the male workers in two e-waste recycling sites in Bangalore (means=0.4 and 0.1 micrograms per gram), compared to men from Chennai, India, the control group (mean=0.19 micrograms per gram). In fact, the concentrations reported were below the averages found in other parts of India (mean= 0.8 micrograms per gram) , and well below the levels deemed to be normal and acceptable in Canada (less than 6 micrograms per gram) . The authors suggest that this may be due to very little consumption of fish by the residents in this area, since fish consumption is seen as a major source of mercury exposure in humans [1,2].
The study also measured mercury content in soil samples from a crude backyard e-waste recycling site in Bangalore (mean=1.8 micrograms per gram; range, 0.09 to 59 micrograms per gram), and from a control site in Bangalore (<0.05 micrograms per gram). These levels are also well below the Canadian guidelines for mercury content in soil: agricultural and residential areas must be less than 6.6 micrograms per gram, and industrial areas must be less than 50 micrograms per gram .
There is clearly a gap in the current literature describing human mercury exposure associated with e-waste recycling. Since this is the first and only study, to my knowledge, that has looked at mercury, further investigation is needed.
My literature search discovered three studies that looked at cadmium exposure and e-waste disassembly. Two of the three measured concentrations in hair samples, and one measured it in blood. All three studies measured internal exposure to additional heavy metals, and consequently, have already been discussed in the sections on lead exposure (Zheng et al., 2008) and mercury exposure (above).
As already mentioned, Canada doesn't have a guidance value for safe blood cadmium levels (BCLs) . According to the literature, the threshold BCL deemed to be a risk for toxicity is five micrograms per litre [5,6]. In the study by Zheng et al., BCLs were measured in children ages one to seven from Guiyu, a well known area where primitive family-run recycling is practiced by nearly 60 to 80% of families . In the Guiyu group, the mean BCL was 1.58 micrograms per litre (range, 0 to 9.2), which was statistically significantly different from the reference group (from Chendian) mean BCL of 0.97 micrograms per litre (range, 0 to 3.49). Other studies have found high levels of cadmium in Guiyu, in samples of dust, water, and river sediments, which further supports the finding of elevated BCLs in this population of children, by comparison to those in the reference group . Furthermore, factors related to e-waste recycling showed a statistically significant correlation with BCLs. In Guiyu, e-waste materials and process residues are dumped in workshops, yards, roadsides, open fires, irrigation canals, ponds, and rivers. As a result, house dwelling location, father's occupation, and child playing areas that were close to e-waste increased one's risk of exposure to cadmium, and therefore, were all factors found to be associated with the elevated BCLs. Since smoking is a major source of non-occupational exposure to cadmium, secondary smoke exposure was assessed, but surprisingly was not found to be significantly correlated with BCLs. Even though the mean BCLs were found to be below the threshold of five micrograms per litre, recent studies have suggested that the tolerable cadmium limit be lowered since kidney damage was observed at lower concentrations . To further put these values into perspective, the Canadian Health Measures Survey from 2007/08 detected BCLs of 0.15 micrograms per litre in youth ages 6 to 19 .
In the other two studies that were looked at, a statistically significant difference in cadmium exposure was also observed, which was believed to be attributable to primitive e-waste recycling. The first study compared cadmium concentrations in hair among male e-waste workers in Bangalore, India to a control group from Chennai, India ; and, the second study compared cadmium concentrations in hair among residents in Taizhou, in southeastern China where extensive e-waste recycling takes place in family-run workshops, to two industrialized cities within the same province (Ningbo and Shaoxing) . Once again, the quality of both studies was lacking, and because there currently is no standardized protocol for hair analysis, a comparison of the results from the two studies was not made .
A paper that I had come across in my literature search was referred to in the Taizhou study, which looked at cadmium levels in locally grown rice. The researchers calculated that Taizhou rice would alone contribute to 67% of the World Health Organization's tolerable daily intake for cadmium . Given this, it was not surprising to see that the Taizhou participants showed approximately twice as high levels of cadmium than the control participants . Other sources of cadmium exposure that are unrelated to e-waste, such as smoker status, food and drinking water levels, were not measured in this study. However, their methods for determining that the high levels of cadmium content in hair were likely due to e-waste activities were quite sophisticated (Pearson's correlation and principal component analysis).