Last month, two significant cracks were found in the cornerstones of conventional thinking about how chemicals in the environment make their way into the human body and other living organisms, possibly leading to damaging effects. The timing is particularly apropos, as the first registration deadline of November 30 approaches for the EU’s regulation on Registration, Evaluation, Authorization and Restriction of Chemical substances (REACH), and the U.S. Congress, under H.R. 5820, considers the first real revamp of the Toxic Substances Control Act (TSCA) since its adoption in 1976.
The issue specifically pertains to bioaccumulation, an important component of the exposure hazard assessment and risk assessment of organic chemicals. In an article published in Env. Sci. Tech. (Aug. 11, 2010), Michael S. McLachlan and a team of researchers challenge two of the central paradigms that form the backbone of bioaccumulation assessment in existing regulations. They assert the following: 1) The assessment of chemical bioaccumulation should be done from a multimedia perspective (by referencing the chemical quantity/concentration in an organism to the chemical quantity/concentration in its total environment, not just one medium of that environment, such as water; and 2) The biotransformation rate is a more important determinant of bioaccumulation than partitioning properties.
The first point applies to chemicals in general, McLachlan says. The thinking in the past has been that bioaccumulation should be assessed by comparing the chemical’s concentration in an organism to its freely dissolved concentration in the medium in which the organism lives. Under that thinking, the bioaccumulation factor for a fish would be Cfish/Cwater, for instance. More recently air has been introduced as the reference medium for calculating bioaccumulation for air-breathing organisms. McLachlan says that this approach leads to an incorrect ranking of the relative bioaccumulation of different chemicals. “It does not take into consideration that many chemicals in the environment are sequestered into media other than the exposure medium. It overvalues the likelihood of such contaminants being transferred from the environment to biota, and hence undervalues the bioaccumulation of chemicals that are primarily found in the exposure medium and that are not sequestered into other media.” The distortion can amount to many orders of magnitude, he adds.
Meanwhile, the second point applies to non-ionic substances that are not highly volatile or very water soluble. Currently partitioning properties, in particular KOW (the octanol/water partitioning coefficient), are used in many regulatory frameworks as screening criteria to identify potentially bioaccumulative substances. “We are not aware of any regulations that utilize measures of biotransformation to screen for bioaccumulative chemicals,” McLachlan says. “But — for chemicals that aren’t highly volatile or water soluble — our work demonstrates that the rate of biotransformation is what will determine whether a chemical is bioaccumulative or not.”
McLachlan does not expect a rapid impact. However, he does predict that this work will, in the mid-term, lead to changes in how bioaccumulation is assessed. He notes that there are scientific challenges that have to be overcome before better screening assessments for bioaccumulative chemicals can be adopted in regulations. “We hope that our work will encourage the development of methods to screen for biotransformation, and that when they are ripe these methods will be incorporated into the lower tiers of bioaccumulation assessment frameworks in regulations such as REACH and TSCA.”