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The Researchers Perspective - Podcast Interview with Dr. Shanna Swan
Narrator: EHP presents The Researchers Perspective.
Ernie Hood: Welcome once again to The Researchers Perspective. Im your host, science writer Ernie Hood.
On this episode of The Researchers Perspective, we welcome the lead author of EHPs Paper of the Year for 2009, Dr. Shanna Swan. Dr. Swan is Professor of Obstetrics and Gynecology, and of Environmental Medicine, at the University of Rochester School of Medicine and Dentistry, where she is also Director of the Center for Reproductive Epidemiology. Paper of the Year recognition honors the EHP article with the highest number of citations in the literature over the previous five years.
The 2009 Paper of the Year is Decrease in Anogenital Distance among Male Infants with Prenatal Phthalate Exposure, which was published in the August 2005 edition of EHP.
Dr. Swan, welcome to The Researchers Perspective, and congratulations on your study being named EHPs Paper of the Year.
Dr. Swan: Thank you, Ernie.
Ernie Hood: The article has undoubtedly been widely read in the field, but for those who may not have seen it, would you very briefly summarize it for us?
Dr. Swan: Sure. We found that the concentration of several phthalate metabolites in urine samples taken from mothers when they were pregnant were significantly linked to a measure of male genital development in their offspring, and that measurement is called anogenital distance, or AGD, which reflects the size of the entire genital area in the male.
Ernie Hood: Many in our audience will recall that the paper was quite controversial at the time it was published. Why it was so controversial?
Dr. Swan: Well, it was controversial because we used a measure which had almost never been used in humans before, although its widely used in rodent studies. Its a standard in toxicology for assessing reproductive toxicity, but it was new for humans. Also, we were the first to show a link between prenatal phthalate exposure and reproductive development in humans, although that had been shown also in rodents, in fact termed the phthalate syndrome.
Ernie Hood: In your estimation, where do things stand today with regard to acceptance of your methods and your findings?
Dr. Swan: Well, I think theyre still controversial. I think theyre much more widely accepted, and they have actually been brought into ongoing studies now, which shows some degree of acceptance. Theyve been used to support legislation, which shows some degree of acceptance. But undoubtedly they have to be published again by other authors. Others have to examine this. And we actually have been funded to pretty much re-do that study in a larger population using improved methods. So hopefully well be able to bolster our own findings.
Ernie Hood: That sounds terrific. Where does the science itself stand today? Obviously the high citation rate also implies that there has been significant follow-up to your initial research
Dr. Swan: Theres quite a bit of active research in the area. I think that paper did stimulate a lot of that, but also concern about the risks from phthalates, because theyre so prevalent in the environment. So as I said before, we are following up in a large, multi-center, cohort study thats just being funded now. Theres a study going on in Canada that will be looking at anogenital distance; one in Sweden, and our methods for anogenital distance will be piloted in the National Childrens Study. So theres a lot of interest in using these methods to assess not only the risks of prenatal phthalate exposure, but also AGD as a developmental endpoint.
Ernie Hood: Dr. Swan, I recall that during the considerable publicity generated in the mass media by your 2005 study, you struggled to advise members of the public on what, if anything, they should do in light of your findings. You wanted to be responsible and not overstate your results while still pointing out that there might be a cause for concern. Given the subsequent knowledge that has emerged since 2005, has anything changed since then in terms of your public message about phthalates?
Dr. Swan: I dont think so, I think that actions can be taken which are precautionary, which I suggested at the time, and I think thats pretty much where we are now. Until we have several replications of this study, I think we have to view this only as a precautionary message. But there are things that people can do to reduce their exposure to phthalates, although we need to know more of those things, because we dont really understand sources of exposure. Thats another area where the science has developed; theres been a lot more work on trying to figure out what are the sources of exposure; how could people limit their exposure if they wanted to, and I think that has been very promising as well.
Ernie Hood: In August 2008, President Bush signed into law a ban on the use of six phthalates in childrens productshas the legislation gone far enough at this point, or do you believe we need to further reduce the use of phthalates in consumer products?
Dr. Swan: I think that legislation was important and historic in many ways. It doesnt address, however, exposure to phthalates from sources other than toys, and in particular it doesnt address exposure to phthalates by pregnant women. And since we now believe that the source of exposure to at least two of the more toxic phthalates is through food, we really dont know how to reduce that risk, and we cant buy phthalate-free food, short of going to our farmers market and getting something, you know, the carrots with the tops on them. But any processed food has the potential to be contaminated by phthalates. Not that they all are, but that needs to be examined. So I think we havent gone far enough to understand what are the sources of most phthalates, in particular the diethylhexyl phthalate thats in PVC and that were all most concerned about. We dont really know how thats getting into our bodies.
Ernie Hood: Dr. Swan, population studies have shown that we all have a certain low level of phthalates in our bodies, but its several different phthalates. What is the effect of that, if any? That cumulative exposure. Does it add up, or are the low levels relatively safe at those extremely low levels?
Dr. Swan: Thats a really important question. Thats probably the number one question thats confronting people working in this area today. And the evidence from rodent studies is that it does add up, that you have what some have called the new math, where you have things that are apparently safe at very low doses, and then you get three or four of them together at those low doses and you add up to considerable risk. That may very well be whats happening to people, and why we see measurable effects in our studies when people are exposed to only low levels of each individual phthalate.
Ernie Hood: Dr. Shanna Swan, thank you so much for joining us on this edition of The Researchers Perspective.
Dr. Swan: Good to talk to you.
Ernie Hood: And thank you for listening to this edition of The Researchers Perspective, the EHP podcast. Join us again next time as we explore another unique perspective in the environmental health sciences!