Wednesday, December 9, 2009

Flame Retardants: Here, There... Everywhere

"We're definitely eating them and probably inhaling a small amount" said Dr. Arnold Schecter, professor of environmental sciences at the University of Texas Health Centre[1].

What Dr. Schecter is referring to are the unseen but widely found polybrominated diphenyl ethers (PBDEs) that are almost guaranteed to be present in any home or area where high-tech electronics like TVs, computers or cell phones have been in use [1,2]. PBDEs are synthetic chemical compounds that are used as flame retardants (chemicals that are added to polymers to prevent fires) in upholstery furniture, carpet backing textiles, foam, plastics, and electrical and electronic equipment [1,2]. Great concern has been expressed by a number of researchers about the escalating volume of PBDEs that are persistent in the environment, and their potential to disrupt endocrine function, neurodevelopment and increase ones risk of cancer [3,4]. It was incredible learning about the various environmental media that PBDEs have been detected in: air, water, sediment, soil and biota from literally across the globe [1,3,4,5]. PBDEs have been detected in human adipose tissue, blood, and breast milk, and repeat measures have proven that these concentrations are clearly increasing over time [1,2,5].

PBDE Exposure
and E-Waste
In my literature search I came across two studies that looked at both human internal exposure to PBDEs associated with e-waste disassembly, and two health outcomes thought to be related to exposure: increased cancer incidence and altered thyroid function. Both studies conclude with a plea for more research on the human health impacts of exposure to PBDEs, since there is an apparent gap in the present scientific literature. A number of in vitro and in vivo studies have been conducted, however, few studies have involved humans.

In the study that looked at cancer and PBDE exposure, the researchers assessed the level of internal exposure to PBDEs among a convenience sample of cancer patients living around an e-waste disassembly site [6]. After an increasing incidence of cancer (such as liver and lung cancer) was noted among residents surrounding the e-waste disassembly site, the researchers decided to look at the body burden of PBDEs (through kidney, lung and liver tissue samples) among local cancer patients who visited a surgical ward. Unfortunately, the study did not have a comparison group; therefore the association between cancer and PBDE exposure in e-waste dumping grounds could not be measured. The study could only conclude that the PBDE internal exposure levels for the study participants (174.1-182.3 ng/g lipid) were notably higher than those reported among the European population, but comparable to those reported in the USA population (based on previous exposure studies). The study also identified the main congeners of PBDEs in the tissue samples (PBDE47, PBDE28 and PBDE209), which was consistent with previous measures of food, air particles, e-waste residues and soil samples collected from the disassembly site in an earlier study. These findings suggest that the congeners can enter the body through three main routes of exposure: inhalation, ingestion and skin penetration. Clearly, a more in-depth investigation of the potential association between cancer and PBDE exposure is needed. After a quick literature search of PBDEs and cancer on MEDLINE I could only find one case control study of non-Hodgkin lymphoma published in 1998 [7].

The second study I looked at was a cross-sectional study of residents in two villages: a village close to an e-waste recycling site (exposed group) and a village located 50 km away from the e-waste site (control group) [8]. Due to PBDEs’ structural similarity to thyroid hormone and polychlorinated biphenyls (PCBs), researchers have been investigating their ability to cause thyroid hormone disruption and DNA damage (can lead to cancers and developmental disorders) [8,9,10] . In the following study, the researchers measured serum levels of PBDEs as an indicator of internal exposure, serum levels of thyroid stimulating hormone (TSH) as an indicator of thyroid function (host response), as well as urinary 8-hydroxydeoxyguanosine (8-OHdG) and frequencies of the cytokinesis-block micronucleus (CBMN) as indicators of the presence of DNA damage. Not only was the median concentration of total PBDEs in the atmosphere of the e-waste recycling site 47 times higher than the concentration in the control site (7149 vs. 150 pg/m3, respectively), but also the median serum PBDEs level for the exposed participants was more than twice that in the controls (382 vs. 158 ng/g lipid, respectively). These median serum levels in both villages were notably higher than those previously reported from studies in Spain (median, 12 ng/g lipid)[11], New Zealand (median, 7.17 ng/g lipid) [12], and Japan (median, 2.89 ng/g lipid) [13].

The results also showed that the levels of serum TSH among the participants exposed to e-wastes were significantly increased (p<0.01). A dose response trend was noted between the levels of PBDE exposure and TSH levels; however, the study did not account for other pollutants within the e-waste dismantling site that may have impacted thyroid hormone homeostasis. The study did account for some factors that can affect the balance of TSH in the human body (ie. BMI, age and sex), however, a number of endogenous and exogenous factors were not measured, reducing the strength of these findings.

With respect to DNA damage, there was no evidence of excess oxidative DNA damage (8-OHdG) that may have been caused by PBDE exposure. The researchers suspect that this may have been because the study was underpowered. The CBMN assay did however find that having a history of working with e-wastes increased ones risk for micronucleated binucleated cells (MNed BNC) 28-fold. Current evidence suggests that these MNed BNC can lead to cancer (14,15). Although two in vitro studies were cited as connecting PBDEs to an increased frequency of MNed BNC, exposure to other genotoxic agents found in e-waste but not included in the study, must be accounted for in future studies.

The study findings do suggest that PBDE exposure may indeed interfere with the thyroid hormone system and cause genetic damage, but further studies are needed. The alarmingly high level of exposure in both villages in Zhenjiang emphasizes the need for enhanced surveillance and control of the occupational and environmental exposure to PBDEs, particularly in areas where e-waste recycling is taking place. Even though human health effects of PBDEs exposure are only starting to be increasingly explored, a number of countries and organizations are recommending the mitigation of environmental exposure. The US Environment Protection Agency has already classified decaBDEs (one of three mixtures of PBDEs) as possible human carcinogens [16], and two PBDE congeners have already been banned in the European Union since 2004 [17].

What is Canada doing?
- As of December 2006, Environment Canada and Health Canada have prepared screening assessments on several PBDEs.
o Health Canada’s screening assessment found human PBDE exposure estimates to be well below the levels believed to cause health effects in laboratory animals.
- The Government of Canada now prohibits the production, use and importation of specific PBDEs.
- Health Canada has implemented ongoing food monitoring and will continue to conduct research on PBDEs [2].

1 comment:

  1. I really like the way you describe both investigators and you following up on leads! Can you also include the TSH levels, so we can get an idea of effect size, not just significance. Glad to see the links to enhanced surveillance – for a blog audience, you might describe this more. Very glad to see links back to Canada re actions on PBDEs.