36276nl_r2

8/10/04

10:17 AM

Page 2Harvard
Center forRisk
Analysis

Risk in

AUGUST 2004

Using Decision Sciences toPromote Public HealthVolume 12Issue 3

Perspective

WEIGHT OF THE EVIDENCE EVALUATION OF
LOW-DOSE REPRODUCTIVE AND DEVELOPMENTAL
EFFECTS OF BISPHENOL A
Introduction

GEORGE GRAY, PH.D.

JOSHUA COHEN, PH.D.

"…the evidence
considered by
the panelsuggests thatthe weightof the evidence
for low-dose
effects is veryweak."

Bisphenol A (BPA) is a plastic monomer
that is the starting material for themanufacture of polycarbonate plastics used inplastic bottles, jugs, baby bottles and manyother products. It also lines the inside of foodand beverage cans. As a result, exposure toBPA in the general population is widespread.
Assessments based on evaluations by theU.S. Environmental Protection Agency(EPA) indicate that these exposures are well
below the levels generally considered to bewithout harm. However, some scientists
believe that BPA can act as an artificial
estrogen, causing adverse reproductive anddevelopmental toxicity at levels far lower
than previously believed. Some laboratory
animal studies support this hypothesis, whileothers do not.
A panel convened by the Harvard Center
for Risk Analysis (HCRA) evaluated theweight of evidence in studies of BPA
published as of April, 2002. Focusing on
potential male reproductive impacts but also
considering other end points associated withsuspected hormone-like effects, the panel
found no consistent affirmative evidence of

low-dose BPA effects for any endpoint. It
found that inconsistent responses by various
rodent test species also raised doubts as to thegeneralizability of results to humans. And
differences in the pattern of responses to BPA
compared to known estrogenic compounds
cast doubt on estrogenicity as a low-dose
mechanism of action for BPA. Not only does
this systematic evaluation provide regulatory
decision makers with useful information in thecase of BPA, but it also offers an example of
how such issues can be addressed when thescientific data remain less than conclusive, asituation that is common.
The panel was chaired by Donald Mattison,
former medical director of the March ofDimes and currently Senior Advisor to theDirectors of the Center for Research forMothers and Children at the National Instituteof Child Health and Human Development. The
work was funded by the American Plastics
Council. This Risk in Perspective summarizes
the full report, which has been peer-reviewed
and appears in the journal Human andEcological Risk Assessment.

For more information on HCRA visit our website at:www.hcra.harvard.edu

36276nl_r2

8/10/04

10:17 AM

Page 3________________________________
2 Weight of the Evidence Evaluation of Low-Dose Reproductive and Developmental Effects of Bisphenol A — continued

Human Exposure and the Suspected Hazard
Because BPA is used in the production of plastics,
including food containers, there is widespread exposure
to this substance. In addition, there may be local
environmental exposure to BPA in the vicinity ofpolyvinylchloride manufacturing facilities or otherindustrial sites that use BPA in their production
processes. BPA is also a component in some dental
sealants and papers, although these products have beenfound to make little or no contribution to exposure.
European Union scientific bodies have estimatedexposure from food and wine ranging from 0.0005 to0.009 mg/kg-day, well below the reference dose (RfD) of0.05 mg/kg-day identified by U.S. EPA as the averagelevel of daily exposure expected to be free from harm.
EPA identified their RfD from experiments using lab rats,
where the lowest level of exposure associated with adverseeffects was 50 mg/kg-day. The RfD was calculated by
dividing this 50 mg/kg-day lowest observed adverse effectlevel (LOAEL) by a safety factor of 1,000.
Concerns have been raised that the identification of aLOAEL at 50 mg/kg-day might not be the end of thestory and hence that EPA’s RfD may not be sufficiently
protective. In particular, some believe that BPA might
be an endocrine modulating compound, specifically, a
low-dose environmental estrogen. Experiments inanimals and in vitro experiments using isolated tissues,
cells, or cellular components from a number of species,
including humans, have suggested that BPA exhibits
estrogen-like activity, although it is considerably lesspotent than natural estrogen.

Recent investigations of live laboratory animals havesuggested a broad range of effects from BPA exposure,hypothesized to be due to endocrine action. Studies fromone group of investigators have received the mostattention (see, for example, vom Saal et al., 1998). Thesestudies report changes in the weight of male reproductive
organs (the prostate and epididymis) and changes insperm production in offspring whose mothers had
exposures as low as 0.002 to 0.02 mg/kg-day, levels thatare comparable to estimated human exposures, far lessthan the LOAEL of 50 mg/kg-day in rats, and even lessthan EPA’s RfD of 0.05 mg/kg-day.
The reporting of positive findings (or negativefindings) from such studies, however, represents only thebeginning of the scientific debate. The HCRA panel wasconvened because there are a multitude of factors thatcan make it unclear whether such a relationship exists.
Subtle problems with study design can make it appear
that BPA is causing an effect even if it is not. Otherfactors can disguise the existence of such a relationship.
For example, errors in making fine measurements oforgan weight can obscure an impact on this quantity.
Finally, even if effects are verified in laboratory animals,
it is not clear if they can be extrapolated from animals tohumans or, when necessary, from the high doses
sometimes used in studies to the relatively low levels ofexposure typically experienced by people. The panel’stask was to look at the scientific literature as a whole inan effort to sort out the puzzle in the case of BPA.

Methods of Assessing the Evidence
To do so, the Harvard panel reviewed the 19
laboratory animal studies published through April, 2002
that investigated any health effects in laboratory animals
following low dose exposure to BPA (i.e., no more than
5 mg/kg-day, which is a factor of 10 below the LOAEL).
Although some of the studies reported that BPA
exposure was associated with health effects in thelaboratory animals, many others did not. Comparing
different studies was complicated by differences in thetype of animals used, differences in the health effects
investigated, as well as other differences in methodology.
Even the type of feed administered to the animals inthese experiments could have affected the results.
The panel chose to use a framework developed by anexpert panel, and described in Gray et al. (2001), toguide their assessment of the weight of evidence forlow-dose effects of BPA and the potential relevance
given common exposures.

In order to evaluate whether the effects observed
were really occurring in laboratory animals, thepanel placed weight on findings based on thefollowing criteria:• Corroboration – Replication of findings among
similar studies and the observation of similar effects
under relevant conditions increases the confidence
that the findings represent a real effect inexperimental animals. Lack of corroboration isgrounds to doubt the validity of a single finding. In
multi-generational studies, corroboration is supported
if the effect appears across generations, and ischallenged if it does not.
• Rigor – Studies must be evaluated for their conduct
and analysis. Greater weight is given to betterconducted studies and those that follow the codes ofgood laboratory practice (GLP).

36276nl_r2

8/10/04

10:17 AM

Page 4________________________________
3 Weight of the Evidence Evaluation of Low-Dose Reproductive and Developmental Effects of Bisphenol A — continued• Power – The statistical power of an experimental
design should be examined for its ability to detect
effects of a certain magnitude, especially for"negative" studies, where a low level of response
could be mistaken for lack of response.
As to the question of whether positive findings in test
animals can be generalized across species to humans, andfrom high experimental doses to relatively low
environmentally relevant exposures for humans, thecriteria are:
• Universality – The degree to which an effect isconsistently reproduced in multiple valid test species
increases confidence that the result might apply tohumans. If, however, the result shows up in only one
species or one strain, or one route of administration(oral, subcutaneous injection, respiratory, etc.), andnot in others, generalizability is challenged.
• Proximity – Effects in species closer to humans, atdoses and by routes of administration similar tohuman exposures, weigh more heavily in favor ofgeneralizability.

Finally, a key component to the evaluation of any
scientific hypothesis is its plausibility. In the case ofBPA, the question is whether this substance sharescharacteristics with other estrogenic substances, andwhether its molecular mechanisms would operate inhumans. The formal criteria for evaluating biologicalplausibility were:• Cohesion – The extent to which all the data areconsistent and are subject to a single, biologicallyplausible explanation increases weight compared toa situation where inconsistencies require ad hoc
explanations and exceptions to general patterns.
• Relevance – From what is known about theunderlying biological basis for a toxic response in
test animals and the biology of humans, it may bepossible to judge whether similar metabolism,
mechanisms of damage and their repair, andmolecular targets of toxic action should be expected
to operate in humans as they do in the test animals.

Findings
In general, the findings in the literature the panel
examined failed to meet one or more of the above
criteria, which is why the panel found that there is no
consistent affirmative evidence of low-dose BPA effects
for any endpoint.
As noted earlier, the potential association between
BPA and prostate weight has been regarded as
particularly important. Some of the studies of this effecthave not found any association with BPA exposure. The
panel found a number of factors that may have masked
this association, including:• Differences in the way the studies weighed theprostates.
• Inadequate statistical power, although the studies that
did not find an association were larger and hence
more statistically powerful than the studies that did
find an association.
• Differences in the animal strains used, with thepossibility that some studies used animals that arenaturally more sensitive to BPA’s effects than otherstrains. Whether humans share the characteristics ofputatively sensitive strains is not known.
• Other study design factors, such as the type of feed
used and the age at which the animals were sacrificed
and tested.
On the other hand, the panel identified factors that
may have given rise to the appearance of an effectassociated with BPA exposure even if no such effectactually exists. These factors include:• Inadequate control for confounders like body weightof individual animals, and individual vs. grouphousing of test animals.
• Failure to account for potential effects of intrauterine
position (variations in natural hormonal exposuresdue to proximity to males or females during prenataldevelopment)
Given the contradictory nature of the studies and thepotential problems clouding their findings, the biologicalplausibility of the low-dose estrogen hypothesis wasparticularly important to the panel. The panel’s review ofthe literature indicated that BPA does not exhibit several
key characteristics that are typical of estrogenic agents.
For example, natural estrogen causes cancer at highdoses, whereas BPA does not. The panel concluded that
because BPA does not share these other well-establishedestrogenic characteristics, it is unlikely to be exhibiting
estrogenic characteristics in the case of the disputed
impacts on the male reproductive tract.
In any case, the panel noted that the inconsistency ofeffects across species casts doubt on whether even a realeffect observed in mice could be extrapolated to humans.
Rat studies, including two large, well-conducted multiple
generation reproductive and developmental studies in
rats, have generally reported an absence of effects at low
levels of BPA exposure.

36276nl_r2

8/10/04

10:17 AM

Page 1________________________________
4 Weight of the Evidence Evaluation of Low-Dose Reproductive and Developmental Effects of Bisphenol A — continued

Harvard Center for
Risk Analysis
Harvard School of
Public Health718 Huntington Avenue
Boston, Massachusetts
02115-5924
617 432-4497
www.hcra.harvard.edu
100% recycled paper,
all post-consumer fiber.
FURTHER READING/REFERENCES:
Gray, GM, Baskin, SI, Charnley, G, etal. (2001) The Annapolis accords on
the use of toxicology in risk
assessment and decision-making: An
Annapolis Center workshop report.
Toxicol Methods 11(3): 225-231
Gray, GM, Cohen, JT, Cunha, G,
Hughes, C, McConnell, EE,
Rhomberg, L, Sipes, IG, andMattison, D (2004) Weight of theevidence evaluation of low-dose
reproductive and developmental
effects of bisphenol A. Humanand Ecological Risk Assessment
10: XX-XX
Vom Saal, FS, Cooke, PS, Buchanan,DL, et al. (1998) A physiologically
based approach to the study ofbisphenol A and other estrogenic
chemicals on the size of reproductiveorgans, daily sperm production, andbehavior. Toxicol Ind Health14(1-2): 239-60

PEER REVIEWER:James K. Hammitt, PH.DConclusions
Many important public health questions
confront conflicting evidence andinterpretation. Decisions often must be
made before the answers are clear. We
believe that the organized and systematic
approach for assessing the weight ofevidence described here can provide amore reliable characterization of thecurrent ‘best estimate’ of the scientificevidence, and do more, sooner, to help
inform risk management decision making
in the face of uncertainty. The
characterization of the state of the scienceis an important step in the risk assessment
process. It should be recognized,
however, that the risk management
process may evaluate other factors, suchas the relative significance of false
positive or false negative results, incoming to a decision.
In the case of BPA, the evidence
considered by the panel suggests that theweight of the evidence for low-dose

effects is very weak. Studies areconflicting, the effects are subtle withquestionable functional importance even
if real, and there are conflicting data as tothe proposed mode of action (i.e., whether
BPA acts as an estrogen).
The panel suggested follow-up researchon the low-dose effects of BPA that:• Uses similar species and strains of test
animals.
• Homogenizes methodologicalapproaches such as dose range, routeof exposure, housing, feed, and manyother design issues.
• Develops models to further determine
applicability of rodent results tohumans. A widely available, peerreviewed physiologically-based
pharmacokinetic (PBPK) model
would be helpful in understanding
potential similarities and differencesin the responses of different species.

Other authors of the full panel report include Gerald Cunha, Department of Anatomy,
University of California, San Francisco; Claude Hughes, Quintiles Inc.; Ernest E.
McConnell, ToxPath Inc.; Lorenz Rhomberg, Gradient Corp.; I. Glenn Sipes, Dept ofPharmacology and Toxicology, University of Arizona; and Donald Mattison, Center for
Research for Mothers and Children, National Institute of Child Health and HumanDevelopment. Other members of the panel who chose to be acknowledged in the paper
for their contributions but not to be listed as authors are Paul Foster, NIEHS, Marvin
Meistrich, University of Texas at Houston, Heinz Nau, Veterinary Medical University ofHannover, and Richard Sherins, Genetics and IVF Institute.

Upcoming courses by the Harvard Center for Risk Analysis
Analyzing Risk: Science, Assessment, and Management, Boston
September 21-24, 2004
http://www.hsph.harvard.edu/ccpe/programs/RISK.shtml
Analyzing Regulations: Health, Safety and the Enviroment, Washington D.C.
Aril 14-15, 2005
http://www.hsph.harvard.edu/ccpe/programs/BCA.shtml
The Risk Communication Challenge, Boston, May 11-13, 2005
http://www.hsph.harvard.edu/ccpe/programs/RCC.shtml