Mechanisms of toxicity/carcinogenicity and Superfund decisions.

Heavy metals that contaminate soils and water usually exist in various oxidation states and form a number of compounds with different physical and chemical characteristics. These differences are often reflected in dramatic variation in toxicokinetic and biologic properties. Such variation in properties, critical in determining intrinsic toxicity, often causes a great deal of uncertainty in analyses of public health risks at sites where metal exposure is evaluated. In the Superfund program, such uncertainties may substantially undermine attempts to characterize potential impacts to populations exposed to metals from improperly disposed waste. In the case of chromium, risk assessment uncertainties can be considerable and fall generally into two categories. First, there is almost no information on potential health effects due to chronic oral exposure to chromium-containing compounds, and a nonquantifiable and probably large uncertainty exists in establishing no-effect levels. In fact, reference doses (RfDs) for CrIII and CrVI are based on chronic studies in which no adverse effects were seen even at the highest dose. Considerations of bioavailability, deduced from site characterization data, and acute toxicity indicate that general application of these RfDs may lead to highly inaccurate estimations of risk. Second, because of the ready reduction of CrVI in biological systems, it has not been possible to separate effects of CrVI from those of CrIII. Thus, data on the relative toxicity and carcinogenicity of these two species is sparse and difficult to interpret. Moreover, kinetic considerations make it difficult to determine the site and rates of reduction of CrVI. This makes prediction of target site concentrations of the two species difficult.(ABSTRACT TRUNCATED AT 250 WORDS)


Introduction
Risk assessment activities at hazardous waste sites that have been placed on the National Priorities List (Superfund sites) involve both assessment of current and potential exposures related to contaminants at the site and an evaluation of the relationship between exposure to these contaminants and possible adverse health effects. Confidence in the latter evaluation is often dependent on the quality and quantity of experimental evidence for toxic effects of a contaminant when specified doses are administered to laboratory animals. Low confidence in these dose-response relationships implies a great deal of uncertainty in quantitative estimates of risk. Since uncertainties often are contentious in interactions among the public, potentially responsible parties, and EPA, efforts to reduce uncertainties in risk assessments should be given high priority. *U.S. Environmental Protection Agency,8HWM-SM, 999 18th Street, Suite 500, Denver, CO 80202.
In the case of chromium, dose-response relationships are ielatively poorly defined, and this is the source of considerable uncertainty in quantitation of risks due to chromium exposure. Several problem areas, particularly those involving carcinogenic mechanisms, were extensively examined by other investigators during this symposium and will not be considered in this discussion. Instead, the focus of this presentation will be doseresponse relationships involving noncarcinogenic end points. For such end points, EPA generally considers the reference dose (RfD) to be the appropriate critical toxicity value for purposes of risk assessment. The RfD is intended to represent that amount of a substance that can be consumed daily for a significant portion of a lifetime without inducing adverse health effects. For chromium, RfDs have been established for "insoluble" compounds where chromium is found in the +3 valence state and for "soluble" compounds where chromium is found in the +6 valence state. Generally, EPA considers a daily intake, estimated as part of an exposure assessment, that exceeds the RfD as posing the poten-tial for induction of adverse effects in at least a portion of the exposed population. Whether the RfD accurately reflects the actual potential of a compound to produce toxicity is dependent at least in part on how accurately experimental studies measure the dose-response relationship.

Oral RfD for Insoluble Chromium(il) Compounds
The RfD for insoluble Cr"' compounds, as described in EPA's Integrated Risk Information Service (IRIS) (1), is provided in Table 1. It is worthwhile to note that the RfD is based on a single study in which there are no toxic effects seen at any dose. The only way to derive an RfD from this study is to assume that the highest dose approximates a no-observable-adverse-effect level (NOAEL) and to apply safety factors to arrive at an RfD for humans. Recently, a site with the potential for exposure to chromium in contaminated soil has provided an opportunity to consider the soundness of this assumption.
At the site in question, both Crl", primarily as enriched chromite ore, and Crvy, primarily as soluble chromate salts, were present from uncontrolled releases from a chromate production process. In particular, there is, on-site, a large pile of chromite ore which has been used as a sandbox by neighborhood children in the past. Thus, one exposure scenario used in evaluating risk on the site involved the incidental ingestion of chromium in the ore during trespassing events by children. Because chromite is highly insoluble in water, it seemed reasonable to use the RfD for insoluble Cr"' compounds in quantifying risk. However, when the chromite ore was sampled and analyzed for total chromium, the concentration was found to be only 16 mg/kg. Since chromite ore is a concentrate ready for use in dichromate pro-duction, the expected concentration of chromium would be between 25 and 40% or 25,000 to 40,000 mg/kg. Thus, the Contract Laboratory Procedures method, using a digestion in hot concentrated nitric acid, apparently solubilized only a small fraction of the total chromium (U.S. EPA, unpublished report). If a similar small fraction were solubilized in the digestive tract after ingestion, and 2% of the soluble Cr"' were absorbed (2), ingestion at the RfD rate would correspond to an absorbed dose of Cr"' of a few nanograms per kilogram per day. Even highly toxic metals such as lead are not measurably toxic at such low doses. In fact, the average daily absorbed dose of lead from dietary sources is estimated to be about 325 ng/kg/day for a 6-year-old child (3). Faced with these data, one must ask if the current RfD for insoluble Crl" compounds is appropriate for chromite ore and perhaps other insoluble Crl" forms.
Clearly, additional chronic studies are needed to adequately define the chronic oral toxicity, if any, of the commonly encountered Cr"' compounds.

Oral RfD for Soluble Chromium(VI) Compounds
The IRIS entry for soluble Crvy compounds is provided in Table 2. As with Crl", the RfD is based on a study in which no adverse effects were seen, even at the highest administered dose. In this study, Crvi was added to drinking water in concentrations up to 25 ppm. It is interesting to note that this is five times the concentration of CrVI in water that produced nausea in a single volunteer. This volunteer used chromate-containing water in place of normal drinking water for a single day (4). This is by no means a definitive study, but it does suggest the possibility that toxicity following ingestion of chromate may occur only at levels of exposure that cause some acute symptoms. Again, chronic toxicity studies are neededl in which doses are sufficiently high to produce some adlverse effects. Such stu(lies might need to addcress the potential problem of nausea by using animals that are able to regurgitate.

Pharmacokinetics of Chromium(VI)
CrV' is clearly reduced to Cr"' in the body. In theory, this reduction could profoundly affect the delivered dose of chromium in either valence state to sites for toxic action. For example, given IP, Cr"' causes extensive damage to the kidney (2). Slightly higher (loses of Cr'" cause a similar effect, but most of the chromium recoveredl in the urine is in the form of Cr"'. The inference is that Cr'I is riapidly reduced in experimental animals and that Cr"' is responsible for the toxicity seen. It follows that, if substantial (lifferences exist in the ability of humans and animals to reduce Cry', or if there is substantial individual variation in the ability of humans to reduce Cr"i, there will be considlerable (lifficulty in extrapolating animal results to humans or in interpreting human epidemiological studies. Some evidence that such differences may exist is priovidedl by Korallus (5). In his paper, clear differences in dlistribution and excretion of chromium were foundl in workers occupationally exposed via inhalation to Cr"1.
In fact, the data suggest two distinct populations, perhaps genetically based, exist in the study populations (Fig. 1). One group apparently reduced Cri' rapidly, an(d large amounts of Cr"' were excrete(d in the urine. The other group apparently reducedl Cry' more slowly andl excreted less Cr"ll. Differences in reducing capacity wvere thought to be based extracellularly in the blood. Apparently, this finding has not yet been (luplicate(l in other worker populations or by other investigators. Nevertheless, the study at least raises the question of how differences in the ability to reduce Cr\' might influence toxicity. For example, one might imagine that rapidl reduction of Cr"' in the bloocl would produce relatively high concentrations of Cr"'. After filtration at the glomerulus, this Cr"' might concentrate in the urine, delivering a higher dose of the toxic chromium form to the kidney tubules. On the other hand, slow riecluction of Cry' in the bloocl could allow the Cr"' to penetrate into cells. Once there, reduction woulcl produce Cr"' intracellularly. Since Crl"' crosses membranes very poorly (2), reduction would effectively trap the chromium inside of cells. Cells susceptible to the carcinogenic actions of chromium (or other adverse effects) might thus receive a higher dose in individluals that reduced Cr"' slowly in the blood. Such indlividluals might be less susceptible to kidney dlamage because of lower bloodl Cr"' levels. However, tissue Cr"' levels might be higher, making slow reducers susceptible to other toxic effects, includling cancer.
Finally, if the ability of blood to reduce chromate is saturable in the riange of (loses that might be expected in human exposures, there may be (lifferences in expecte(l toxicity when the dose absorbedl exceecls the reducing capacity of the blood. That is, doses that saturate the reduction capacity of the bloodl wouldl be expecte(l to increase tissue chromium levels regarclless of an indlivi(lual's reducing capacity. Thus, at high (loses, the concentration of chromium at target sites may be better predicted by administer ed or exposed dose.
In any event, it seems clear that studlies on the pharmacokinetics of chromium need to be carried out. Such studies should include both human exposures and the use of experimental animals and shouldl focus on estimating (lose deliveredl to specific target sites, such as the kidlney. In addition, more information is needed to define the variation in blood-reducing capacity in the human population stu(lies.

Summary
In summary, available (lata on chromium make risk assessments using EPA critical toxicity values (RfDs) very uncertain. The basic problem lies in our current inability to confidently predict the (lose of chromium that actually r-eaches its site of toxic action in the body. Examination of the RfD for insoluble Cr"' compoun(l suggests that the RfD could overestimate risks by at least a few orlders of magnitude for some Cr"' compoun(ls. Similar consi(leration of the RfD for soluble Cr"' compounds reveals the possibility that toxicity might occur only at doses high enough to cause acute symptoms, such as nausea. Finally, the pharmacokinetics of Cr'' after absorption may be complex and variable in the human population. This suggests that ther e could be human subgroups with greater sensitivity to some of the potential toxic effects of chromium. It seems clear that risk assessments based on exposure to chromium compounds could be improved dramatically with increased attention among researchers to the kinetics of chromium in mammalian systems. 1'29