Wednesday, July 25, 2007

Doh! There's more to bioaccumulation than we thought!

Here’s one for the “why didn’t we figure this out sooner” file, or maybe the “gee – those of us air-breathers really are different from our gilled cousins!” You see, for years one of the primary methods of determining the ability of a chemical to accumulate in living creatures was to study the accumulation (or bioaccumulation) of the chemical in fish. The model is based on the idea that fat-loving chemicals, which includes most bioaccumulative chemicals, are essentially absorbed from the surrounding water by fish, or, more or less technically, by “swimming bags of lipid.” Those that are not rapidly metabolized are retained in the fat, allowing not only for accumulation in our little fish, but also for the proverbial big fish that eats the little fish all the way up the food chain to polar bears, bald eagles and homo sapiens. Some infamous lipid-loving chemicals that we all know and fear include certain PCBs, dioxins, and DDTs.

Most governments, including the U.S., have thankfully learned (after…umm decades) to consider carefully a chemical’s potential for persistence, or ability to hang around the environment, and bioaccumulation when evaluating and regulate commercial chemicals.

Great! No more bioaccumlative chemicals climbing up the food chain. Problem solved. Or is it? A recent report by Barry Kelly, Frank Gobas and others, published in Science (Volume 317, pages 236-239) suggests that our current method for evaluating bioaccumulation may miss – and in a big way. According the study, some chemicals that don’t accumulate in fish, or chemicals that might pass the “swimming lipid bag” test with flying colors, can accumulate land mammals and marine mammals.

What’s the difference? After all fat is fat – be it a swimming or walking bag of lipid (which I must admit sometimes I’ve felt myself as I struggle to squeeze into my favorite jeans at the end of the summer.) Turns out, as with anything, there’s more to bioaccumulation than hanging out in fat. Living organisms are dynamic creatures, and most things that enter the body have the potential to be metabolized and/or excreted. Even chemicals that hide out in fat can be eliminated given enough time. But what’s different between fish and polar bears or fish and humans (among other things) is that according to Kelly and others, “…air-breathing organisms in this analysis exhibit higher [biomagnification factors] than those in water-respiring organisms because of their greater ability to absorb and digest their diet, which is related to differences in digestive tract physiology and body temperature.” Additionally, note the author, air-breathers may be less efficient when it comes to eliminating certain chemicals from their bodies than water-respirers.

Go figure. This is where, as a toxicologist who bought into the “bag of lipid” model years ago without question, now wonders – what was I thinking? Chemicals that might pass (and have passed) the fish bioaccumulation test, wouldn't pass a mammalian test, according to the authors who note that these chemicals, “representing a third of organic chemicals in commercial use, constitute an unidentified class of potentially bioaccumulative substances that require regulatory assessment to prevent possible ecosystem and human-health consequences.”

Time once again, to reconsider how we evaluate and regulate, and release chemicals into our environment.


Anonymous said...
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Joe said...

>"Time once again, to reconsider how we evaluate and regulate, and release chemicals into our environment."

This reminds me of a comment I heard at a seminar once. The speaker made reference to "industrial hygiene's dirty little secret." And that is, we all know the recommended/regulated exposure limits are going to be lowered as we learn more...
It's scary being in this business - doing what we can with what we know, but knowing that what we know might not be good enough...always having to challenge our models...hmmm...I suppose that's true for every profession.

Emily Monosson said...

Thanks for your comment Joe.

Unfortunately as you point out there are lots of examples where once "acceptable" exposures are no longer acceptable, TCE - is an example of one of the more recent and contentious issues in terms of acceptable concentrations in groundwater; then there is lead, mercury, and radiation.

Partly this is a result of fine tuning and increased understanding of what happens to chemicals in the environment and in living systems, as the science and methodology develops (really, toxicology is a relatively young science compared to some other disciplines.)

The flip side is (playing the devil's advocate)that though we can now measure very small amounts of chemicals, and we can observe (directly or indirectly) how these chemicals interact with biological systems in some cases at the molecular level - one might ask how important these exposures and interactions are? What is the biological impact? But that brings us back to your point - just because we can't measure or don't understand it right now doesn't mean its not relevant!

Though this might be true for every profession, in this case there is a lot at stake for those exposed individuals and populations.