by
Michael A. Jeannot, Department of Chemistry, St. Cloud State University
Case Overview and Background
This case is based on an actual article entitled "Baby Alert" which appeared in Consumer Reports (May 1999). The article raises some concerns about the safety of polycarbonate baby bottles, and recommends that parents dispose of them as a precaution. However, the American Plastics Council and the Food and Drug Administration have raised concerns about the experimental methodology used as well as the recommendations made in this article.
This is an excellent case for the development of critical thinking skills for students in an introductory chemistry course, particularly a non-majors course. It may be described as a decision or dilemma case, and has been taught using the discussion method in a class of about 30 students; however, the teaching method may be readily adapted to group work if desired. In terms of content, the case may be used to illustrate applications in polymer chemistry, quantitative chemical analysis, toxicology, endocrine disruption [1], and risk-benefit analysis.
Objectives of Case
Analysis of Major Issues
One of the most important issues in this case is a basic understanding of polymer chemistry, using polycarbonate as an example. The text I use in the non-majors course has a good chapter on polymer chemistry, which has been covered in class prior to this case study. Alternatively, students can be assigned such reading and quizzed immediately before the case. Another option is to incorporate a brief lecture on polymer chemistryduring the discussion, with particular emphasis on polycarbonate. An excellent on-line description of polycarbonate chemistry is also available at http://www.psrc.usm.edu/macrog/pc.htm. However, this may be considered supplemental material for the purposes of this case.
The chemical structure of the monomer, bisphenol-A, is shown below:
>Bisphenol-A reacts with phosgene gas (COCl2) to produce polycarbonate:
An important consideration is the method by which the monomer bisphenol-A is released from the polycarbonate. This is a good opportunity to discuss complete vs. incomplete reactions (residual bisphenol-A in the polymer matrix) and the concept of diffusion of monomers through a polymer matrix into an external solution (leaching). Another possible source of the bisphenol-A monomer is via hydrolysis (reversal of the polymerization process, which may be induced by heating and/or the presence of acids or bases).
The idea of finite detection limits is extremely important in this case, as many of the published works indicate that "no" bisphenol-A was found in the experiments. Clearly, the word "no" does not mean that there are zero molecules of bisphenol-A; rather that the concentration is too low to measure by a particular method. Since detection limits on the order of a few parts per billion (ppb) are typical, it may be useful to illustrate how many molecules this corresponds to in some volume. Students may be surprised how large this number is.
While Consumer Reports does not state concentrations specifically, the Chemical & Engineering News article states that the Consumers Reports scientists found about 1 ppb of bisphenol-A under their test conditions. Other published studies [6] report values of less than about 5 ppb. Thus, it would be reasonable to assume, as a worst-case scenario, the presence of 5 ppb bisphenol-A in the bottle contents.
So what does this mean? Is 5 ppb significant? First of all, the total amount of bisphenol-A consumed per kg body weight, per day must be calculated. Here, the students must be given some data: The FDA normally uses the figure of 750 g of formula per day for a 4 kg infant (188 g formula/kg/d). Thus, the amount of bisphenol-A consumed would be 5 ng bisphenol-A/g formula (i.e., 5 ppb) x 188 g formula/kg/d = 0.94 μg/kg/d or about 1 μg/kg/d . This is an excellent exercise in dimensional analysis.
Is 1 μg/kg/d significant? Now, we get into the issue of toxicity and the concepts of LOAEL and safe doses. The LOAEL (lowest observed adverse effect limit) must be measured experimentally via animal studies. The students may be told that the Environmental Protection Agency (EPA) LOAEL value for bisphenol-A is 50 mg/kg/d. Thus, it may be presumed that consumption of this LOAEL amount of bisphenol-A is "borderline" in terms of estrogen-mimicking effects. In mice, such effects may include disruption of mammary function, and decreased size of the testes and decreased sperm count in male offspring of pregnant mice exposed to bisphenol-A. The reference or safe dose is usually defined as 1/1000 of the LOAEL, or 50 μg/kg/d. The actual dose (~1 μg/kg/d), based on experimentally measured concentrations of bisphenol-A, is about 50 times lower than the "safe" dose, according to the EPA values for the LOAEL and safe dose.
At this point, it will be worthwhile noting that the Consumer Reports article reports a bisphenol-A dose of about 40 times higher than the "safe" dose. Thus, it would appear that the "safe" dose referred to in the Consumer Reports article must be much lower than the EPA version. This point should be sought out in the discussion, and it may spark further discussion about the determination of "safe" doses. The extrapolation of animal tests to humans, possible differences in the effect of bisphenol-A during different stages of development, and cumulative effects of bisphenol-A are all controversial issues that make risk assessment more difficult.
When the students are asked to make a decision as to whether or not Julia should continue to use the polycarbonate bottles, they should consider whether or not there is enough information to make a good scientific judgement. Nevertheless, options should be discussed and a decision must be made. It may be emphasized that even though it might be possible to quantitatively measure the risk involved, the ultimate judgement of safety is a subjective one.
Classroom Management
This case is being used with a group of about 30 students in a discussion format. The students should bring a calculator to do some simple calculations. It is assumed that the students have some background in the concept of concentration, and some rudiments of polymer chemistry. The following readings and articles should be handed out to the students (with permission) prior to the case discussion:
In the classroom, an appropriate opening question would be "What is Julia concerned about?" The name of the monomer (bisphenol-A) should be drawn out, as should the idea of toxicity. This leads to the question, "What makes something toxic?" It may be helpful to allow the students to think about this for a couple of minutes, write down their ideas and then share them with their neighbor ("think, pair, share"). It is important here to elicit the concept of dosage or concentration. After all, it may be pointed out that almost any chemical is toxic if ingested in sufficient amount.
Once the idea of toxicity has been explored, a reasonable direction to go in is, "If the bottles are made from polycarbonate (non-toxic), then why are we concerned?" Again, a "think, pair, share" approach works well. Here, it is important to elicit the concepts of leaching (from residual bisphenol-A in the polymer matrix), reversal of the polymerization, and speculation on the effects of heat and composition of the bottle contents.
The students should be asked about the concentrations (ppb) of bisphenol-A found in the Consumer Reports article and the other three published works in the handout "Summary of Research & Migration Research." They should then be directed to calculate the dosage (μg/kg/d) based on the supplied feeding data (see above). Finally, they should be asked to compare this to the EPA LOAEL and safe dose. This should then be reconciled with the Consumer Reports data, and conclusions drawn about "safe" levels used in Consumer Reports compared to EPA values.
As a concluding exercise, students should be asked to brainstorm what Julia's options are. A "vote" on these options will force the students to make a decision, based on the limited data (not an uncommon situation in real life). In doing so, students are performing a risk-benefit analysis, whether they realize it or not!
Instructors may wish to have their students hand in a brief written assignment delineating the reasons for their decision(s). As a follow-up exercise, students may be assigned readings on the endocrine issue. Portions of Colborn’s book [1] provide a very readable account of the endocrine issue. More advanced students may refer to reviews on endocrine disruption [7, 8].
References
Update (4/21/01): A new study conducted by the Research Triangle Institute (RTI) in Research Triangle Park, North Carolina, examines whether extremely low doses of BPA could have any effect on human reproductive health. The results were presented at the Low-Dose Peer Review Workshop conducted by the National Toxicology Program of the National Institute of Environmental Health Sciences in October 2000. The NTP Statistics and Dose-Response Modeling Subpanel called this study "arguably the most comprehensive of the studies we evaluated." The RTI study director Dr. Rochelle W. Tyl, an internationally respected reproductive toxicologist, described it as "the most definitive study conducted to date on possible effects of both low and high doses on endocrine-sensitive endpoints." For more information, go to http://www.rti.org/news/news_full.cfm?NR_ID=249.
An article in Science magazine also addresses the RTI
study:
Kaiser, J. 2000. Endocrine disrupters. Panel cautiously confirms
low-dose effects. Science 290(5492):695-7, Oct. 7, 2000.
Acknowledgements: This case was developed as part of a National Science Foundation-sponsored Case Studies in Science Workshop (NSF Award #9752799) held at the State University of New York at Buffalo on June 7-11, 1999.
Posted: 9/3/99
nas Last Revised: 04/07/05
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