DHHS Eagle graphic
ASL Header
Mission Nav Button Division Nav Button Grants Nav Button Testimony Nav Button Other Links Nav Button ASL Home Nav Button
US Capitol Building
HHS Home
Contact Us
dot graphic Testimony bar

This is an archive page. The links are no longer being updated.

Testimony on Haze and Pollution by Barry L. Johnson, Ph.D.
Assistant Surgeon General
Assistant Administrator
Agency for Toxic Substances and Disease Registry
U.S. Department of Health and Human Services

Before the Senate Committee on Environment and Public Works, Subcommittee on Clean Air, Wetlands, Private Property, and Nuclear Safety
October 1, 1998

Good afternoon. I am Barry Johnson, PhD, Assistant Administrator of the Agency for Toxic Substances and Disease Registry (ATSDR). The subcommittee invited ATSDR to testify on regional haze and mercury pollution. Our agency has had no involvement with regional haze but has worked on various mercury issues. Much of our work on the scientific issues of mercury pollution is reflected in the ATSDR draft Toxicological Profile for Mercury.

ATSDR has developed a toxicological profile on mercury in compliance with a mandate in the Comprehensive Environmental Response, Compensation, and Liability Act, as amended (CERCLA, or Superfund). Section 104(i)(3) directs us to develop toxicological profiles for priority hazardous substances released from Superfund sites. Our priority list of hazardous substances is developed jointly with EPA and updated every 2 years. Mercury is number three on the 1997 list. ATSDR is also required by CERCLA (.104(i)(3)) to revise and republish individual toxicological profiles as necessary, but no less often than once every 3 years.

CERCLA also requires that ATSDR's toxicological profiles contain "An examination, summary, and interpretation of available toxicological information and epidemiologic evaluations on a hazardous substance in order to ascertain the levels of significant human exposure for the substance and the associated acute, subacute, and chronic health effects." This language directs ATSDR to develop numerical estimates of health risks posed by hazardous substances. Health assessors and risk managers use numerical values to characterize the toxicities of hazardous substances. Risk assessment methods are most often used for carcinogenic substances. This results in point estimates or ranges of health risk that are based on various exposure scenarios. ATSDR uses the numerical minimal risk levels (MRLs) for noncarcinogenic toxicities of substances.

An MRL is an estimate of what level of daily human exposure to a hazardous substance is likely to be without appreciable risk of adverse noncancer health effects over a specified duration and route of exposure. These substance-specific estimates are intended to serve as screening levels. Public health assessors use MRLs to identify contaminants and potential health effects that may be of concern at hazardous waste sites. It is important to note that MRLs are not intended to define clean-up or action levels for EPA or other agencies.

ATSDR develops MRLs during the preparation of toxicological profiles. They are derived when ATSDR determines that reliable and sufficient data exist to identify the target organs of effect or the most sensitive health effects for a specific duration of exposure. MRLs are based only on noncancer health effects and not on a consideration of cancer effects.

ATSDR first published a toxicological profile on mercury in 1989. The mercury profile was then updated in 1994, and a second update was released in a draft version in October 1997.

ATSDR's 1994 mercury profile contained a chronic duration, oral exposure MRL that was based primarily on a 1989 analysis of data collected on persons in Iraq who had been accidentally exposed to methylmercury in grain during the early 1970s. That MRL was 0.1 micrograms of mercury per kilogram body weight per day ( g/kg/day). It was numerically equal to EPA's reference dose (RfD), and was based on the same neurodevelopmental endpoint (children's delayed walking and talking) that EPA used to derive their RfD for methylmercury.

The CERCLA mandate to update the mercury profile, coupled with the need to increase our knowledge of the health effects associated with mercury, led us to convene a series of meetings in Atlanta in 1994 and 1995. We invited peer scientists to join us in panel meetings to review the direction we should take in our continuing assessment of the health impact of methylmercury. At that time, we were cognizant of a number of ongoing studies, including studies in the Faeroe Islands and the Seychelles Republic.

In 1995 our panel of experts recommended that we await the development of the Seychelles data and use them as a starting point in our mercury reassessment efforts. So we waited until 1996, when published data from the Seychelles study began appearing in the scientific literature, to begin updating our mercury toxicological profile. In October 1997, ATSDR released for public review and comment our current draft profile. The document remains in draft, pending further discussions with EPA, other federal agencies, and the public. An upcoming interagency workshop in November to evaluate the major scientific studies on methylmercury and its developmental effects in children will be a key forum for resolving some remaining points of science and public health.

ATSDR's MRL for chronic, oral exposure to methylmercury in the October 1997 draft toxicological profile is derived from a study conducted by University of Rochester investigators in the Seychelles Islands that reflects multiple generations of human exposure to organic mercury through fish as a primary route of exposure. Because of the long-term nature of this exposure, the large sample size, and the rigorous study design, this data set was used as the primary basis of ATSDR's evaluation. The Seychelles study overcomes several of the limitations in the Iraqi study. For example, there is a rather large sample size of 779 mother-infant pairs before the application of the exclusion criteria. This was a prospective study, with the goals and objectives stated before data collection.

In the Seychelles study, children were evaluated at 6.5, 19, 29, and 66 months of age. Through the age of 29 months, no effects attributable to methylmercury exposure were found using a battery of neurobehavioral and neurophysiological tests. The only endpoint that was correlated in any way with mercury exposure was the subjective observation by several examiners that some boys, but not girls, showed a decreased activity level during the testing period. This decrease in activity associated with an increase in maternal hair mercury levels was, however, not considered by the Rochester team to be attributable to mercury exposure, and was observed only in boys whose mothers had hair mercury levels above the median value (5.9 ppm) used as a no-observed-adverse-effect level (NOAEL). This value is 2 4 times less than the level at which the transient depressed activity levels in young boys were noted (i.e., >12 26 ppm). Similarly, the Rochester researchers found no statistical association between prenatal exposure to mercury and the age at which the children in the Seychelle Islands study walked or talked.

Since October 1997, there have been additional scientific publications on the human health effects of mercury. For example, results of the 66-month testing of children in the Seychelle Islands are now available. Further, a study of the Faeroe Islands population published in December of 1997 will need to be examined by ATSDR in the context of our draft toxicological profile.

A scientific evaluation of studies like those conducted in Iraq, the Faeroe Islands, and the Seychelles is complicated. All studies like these have particular strengths and limitations. One must evaluate each study for its statistical power, the adequacy of data collection and analysis, the relevance of exposure data, biological plausibility, and relevance of health findings. Examination of all currently available scientific information must be concluded and throughly debated by peer scientists before final pronouncements on made MRLs and similar health guidance values. The results from studies published since October 1997 will be carefully reviewed by ATSDR and incorporated in our final version of the toxicological profile on mercury.

As previously noted, our toxicological profiles require developing MRLs, the derivation of which is a rather straightforward algorithm. It is deliberately analogous to what the EPA and the Food and Drug Administration (FDA) do for RfD and ADI derivations, respectively. You have a benchmark of toxicity, no-observed-adverse-effect level (NOAEL), or some other surrogate for that, and an uncertainty factor. The higher the uncertainty factor, the lower the overall quality of the data set. The higher the quality of the data set, the lower the uncertainty factor.

Although the operational derivation of an MRL is straightforward, any derivation involves a substantial amount of professional judgment. In the case of mercury, we must consider the fact that mercury is ingested by mothers, yet we know the fetus is the target of concern. The mercury concentrations are measured in the mothers' hair, but we are really concerned about the concentration of mercury in the blood reaching the fetus. So we have to convert the maternal hair mercury concentration to a blood mercury concentration, and subsequently convert that blood mercury concentration to an oral daily intake.

In addition, we have the issue of uncertainty. ATSDR considers four general areas of uncertainty in the derivation of MRLs: cross-species extrapolation, 1 to 10; intra-species variability, 1 to 10; the use of an adverse effect level as opposed to a non-adverse effect level, 1 to 10; and a factor to account for the quality of sufficiency of the overall database, 1 to 10, sometimes referred to as a modifying factor by ATSDR.

In our draft 1997 mercury profile, we looked at the uncertainty and the available mercury data. We have the most sensitive population, the developing fetus. We have a known and relevant route of exposure, consumption of mercury-contaminated fish. We have identified a NOAEL in the most sensitive subpopulation, and we see an absence of any neurodevelopmental deficit at similar exposure levels in other populations. That led us to select an uncertainty factor of 1. This uncertainty factor is consistent with the standard application of those factors in the derivation of health guidance values. This is not to suggest that there is no uncertainty remaining about any threshold for the health hazards of methylmercury, or that ATSDR's proposed MRL does not reflect a margin of safety consistent with adequate precautionary approaches and good public health practice.

From the foregoing considerations, ATSDR derived an MRL for chronic, oral exposure to methylmercury of 0.5 g/kg/day. MRLs for both elemental mercury and inorganic mercury (salts) are also presented in the draft mercury toxicological profile. The significant neurotoxic (nervous system) and nephrotoxic (kidney) health hazards posed by these forms of mercury are often addressed by ATSDR in response to spills or other unplanned releases of mercury in schools, hospitals, and homes. In fact, mercury has been the single most frequently encountered chemical in our emergency response program for the last 8 years. For this reason, ATSDR and EPA jointly developed and released a health alert in the summer of 1997 that has been widely distributed to schools and other potential targets of mercury spills.

In July of 1998, ATSDR assembled a panel of 18 experts from the scientific and medical communities to assist the agency in reviewing key issues in the areas of toxicology and human heath risk assessment as they relate to metallic, inorganic, and organic mercury compounds. The purpose of the meeting was to review all relevant science on mercury and its compounds, including methylmercury, and to react to ATSDR's proposed response to comments we received during the public review and comment period. The meeting of the panel was held as an announced public meeting. It consisted of scientists from the EPA, FDA, the Centers for Disease Control and Prevention (CDC), academia, and the private sector, including the teams of scientists who conducted the human epidemiological studies in Iraq, the Seychelles, and the Faeroe Islands. The discussions from this expert panel are being used by ATSDR and other federal agencies to help form part of the agenda for the November 1998 interagency workshop on mercury.

ATSDR continues to work with other federal agencies to reach a consensus on mercury issues. The Committee on Environment and Natural Resources (CENR), Subcommittee on Toxics and Risks, established a Mercury Working Group in September of 1997 to look at a number of issues concerning mercury. This workgroup, which includes representation from ATSDR, EPA, FDA, CDC, NIH, NOAA, USDA, and the Office of Science and Technology Policy, has already come to a consensus on a range of issues, and continues to work towards agreement on all public health issues concerning mercury.

The CENR Working Group is sponsoring the November 18 20 interagency workshop in Research Triangle Park, North Carolina, to further elucidate issues regarding methylmercury exposure and public health. The research teams responsible for the Seychelles and Faeroes studies, as well as a team conducting similar studies along the Amazon River basin, will present and discuss their findings at this meeting. ATSDR considers this meeting to be an important step toward resolving remaining scientific issues. The meeting will be used by ATSDR to help bring closure to its toxicological profile on mercury.

A challenge for health officials is to balance the known public health benefit of consuming more fish in the diet, and the known dangers of excess mercury exposure. To mitigate adverse health effects of excessive exposure to mercury, ATSDR supports efforts to reduce or eliminate exposure to mercury in the environment. Such efforts must be pursued through pollution prevention strategies, including health education for both health care providers and the citizens who may be at risk due to high levels of exposure to not only organic, but also inorganic and metallic mercury. Throughout the profile revision period, ATSDR has advised the public that it would be premature to predicate any risk management decisions on information in a draft document. That needs to await the sorting out of all relevant issues and the finalization of the profile.

I thank you for this opportunity to present to you some of the conclusions in ATSDR's Toxicological Profile for Mercury and to discuss our ongoing efforts to enhance the accuracy and utility of our mercury MRLs, including the chronic oral MRL for methylmercury.

Mr. Chairman, we would be pleased to answer any questions that you or subcommittee members may have.

Privacy Notice (www.hhs.gov/Privacy.html) | FOIA (www.hhs.gov/foia/) | What's New (www.hhs.gov/about/index.html#topiclist) | FAQs (answers.hhs.gov) | Reading Room (www.hhs.gov/read/) | Site Info (www.hhs.gov/SiteMap.html)