Estimates of Human Exposure

Determination of reliable indoor exposure estimates includes considerable uncertainty and reliance on unfounded conjecture [2], Routine exposure estimates have ranged from toxicologically negligible levels to amounts that would produce frank systemic toxicity if they ever occurred. 

In practice, if excessive resident exposures occur, they are traceable to misuse or exposure to unpleasant or obnoxious odors not attributable to the active ingredient In spite of considerable posturing by campaigners and activists it remains that chlorpyrifos was withdrawn by the registrant rather than banned by the U.S. 

Safe indoor use of chlorpyrifos continues in parts of the world outside of the United States. The confusion and anxiety generated by the many disparate opinions represents the consequences of non-validated models, invalid toxicology testing procedures, ultraconservative default assumptions, and regulatory concerns dominated by worst-case assumptions. 

More extensive use of situational exposure monitoring and development of biological exposure indices are means to minimize some scientific uncertainties about potential resident pesticide exposures (Keenan et al. [15]). 

Drift at its lowest confirmable levels is a consequence of the Laws of Conservation of Matter and, perhaps, careless application at higher levels of exposure. Levels of drift exposure possibly represent nanograms to micrograms per kg body weight. Inhalation and deposition on exposed skin would be the prominent routes of exposure. Drift can usually be distinguished in dose and time from those exposures that occur by overspray or result from accidental pesticide application 

---

Table 3 Estimates of Human Exposure Derived from Whole-Body Dosimetry, Biomonitoring, and Routine Use of Pesticide Fogger...

Under contract to the Systems and Strategy Development Division of the OAQPS/EPA, Systems Applications developed and applied modeling methods for the estimation of human exposure and dosage from airborne materials. The model is intended for a screening analysis of the impacts of chemicals under EPA review as potentially hazardous by the definitions of the NESHAPS program. 

The estimation of human exposure/dosage to atmospheric concentrations of the studied chemicals involved three computational tasks ... 

Estimates of human exposure by route and subpopulation can be used directly, without comparison to health effects data, to evaluate potential pollutant problems in an area. For example, the following analyses can be performed ... 

McKone TE, Layton DW (1986) Screening the potential risks of toxic substances using a multimedia compartment model estimation of human exposure. Regul Toxicol Pharm 6 359-380... 

Exposure Levels in Environmental Media. Reliable monitoring data for the levels of di- -octylphthalate in contaminated media at hazardous waste sites are needed so that the information obtained on levels of di-ra-octylphthalate in the environment can be used in combination with the known body burden of di-w-octylphthalate to assess the potential risk of adverse health effects in populations living in the vicinity of hazardous waste sites. Di-u-octylphthalate has been detected in ambient air, rain, surface water, groundwater, and sediment. However, as a result of the confusion about the nomenclature for octylphthalate esters, much of the historical monitoring data available actually pertain to the branched isomer, di(2-ethylhexyl)phthalate (Vista Chemical 1992). Therefore, little current information specific to the /1-octyl isomer is available regarding concentrations of the compound in foods, drinking water, and environmental media, particularly with respect to media at hazardous waste sites. The lack of monitoring data precludes the estimation of human exposure via intake of or contact with contaminated media. 

Information on disposal practices for wastes containing 2-hexanone is necessary for estimations of human exposure from this source. No regulations govern the disposal of 2-hexanone. 

Exposure Levels in Environmental Media. Data are available on the occurrence of hexachlorobutadiene in air, water, and foodstuff. The majority of the monitoring data on hexachlorobutadiene are outdated and therefore more recent information on the levels typically found in the environment would allow for more accurate estimation of human exposures, and could also serve to indicate time- dependent trends when compared with older data. No data were located regarding the occurrence of hexachlorobutadiene in groundwater or soil. 

Quantitative estimation of human exposure, environmental release/emissions rates and environmental distribution. This may be achieved through modelling or by field monitoring. 

W. Tang, I. Hemm, and G. Eisenbrand, Estimation of human exposure to styrene and ethylbenzene, Toxicology, 144(l-3) 39-50, April 2000. 

Where there is usable estimation of human exposure, the lowest level should exceed this. Ideally, the intermediate-dose level(s) should produce minimal observable toxic effects. If more than one intermediate dose is used, the dose levels should be spaced to produce gradation of toxic effects. In the low and intermediate groups and in the controls, the incidence of fatalities should be low to permit a meaningful evaluation of the results. If application of the test chemical produces severe irritation, the concentration may be reduced. This reduction may result in a subsequent reduction in, or absence of, other toxic effects observed at the high-dose level. If the skin shows severe damage, it may be necessary to terminate the study and undertake a new study at lower concentrations. 

For repeated inhalation toxicity, at least three concentrations of the test chemical with a control and (when appropriate) a vehicle control (corresponding to the concentration of vehicle at the highest exposure level) should be used. Except for exposure to the test chemical, the control group should be handled in a manner identical to the test group. The highest concentration of test chemical should result in toxic effects but not produce an incidence of fatalities, which would prevent a meaningful evaluation of results. The lowest concentration should not produce evidence of toxicity. When there is a usable estimation of human exposure, the lowest concentration of test chemical should exceed this. 

Estimates of human exposure to JP-8 do not provide documentation of exposures to individual components of JP-8. Studies have not been done to determine which components of JP-8 might account for its toxicity. Thus, the value of calculation of a margin of exposure for JP-8 is questionable because knowledge of the composition of JP-8 might not accurately predict the relative exposure to components of JP-8 at the tissue level. 

The following discussion centers on uncertainty related to the estimation of human exposure. Many of the general principles will be applicable to both exposure and toxicology, and the connection and context of this treatment of uncertainty associated with toxicological determinations will be made later in this discussion. 

A particularly important component of the risk assessment concerns the assumptions and data used to develop an estimate of human exposure - the human dose. Elements of the dosimetry problem were described in Chapter 2 and we do not intend to go into them in more detail here. Certain additional aspects of this issue should be brought out, however, because they critically affect the meaning that is to be attached to the risk estimates produced under the regulatory principles. 

Exposure Levels in Humans. A population exists that is potentially exposed to creosote through contact with contaminated media at hazardous waste sites and with treated wood products. A second potentially exposed workforce population exists at wood treatment facilities and in other industries in which creosote-derived products are produced or used. Currently, no information exists that demonstrates tissue levels of any components of the mixture in these populations. Although exposure is now estimated in occupationally exposed workers using urinary concentrations of biomarkers, such as 1 -hydroxypyrcnc, actual exposure levels are harder to determine. Estimates of human exposure to creosote constituents, or body burdens of creosote components, are complicated by the lack of information on exposure to creosote constituents and levels of creosote-derived components in the environment. Collecting information on tissue levels of creosote components in humans would be necessary to examine the relationship between levels of creosote-derived compounds in the environment, human tissue levels, and subsequent development of health effects. This information is necessary for assessing the need to conduct health studies on these populations. 

The carcinogenic effects in rodents probably only relevant to man at elevated dose-levels. The in vitro genotox suggests that estimations of human exposure are worth deriving, leading to risk estimations using models that accommodate thresholds. 

The following example shows how TEQ values are calculated. 2,3,7,8-TCDD has TEE = 1 (asdefined) 2,3,4,7,8-pentachlorodibenzofuran(pentaCDF)has TEF =0.5. Therefore, a mixture of 2 ng TCDD + 6 ng pentaCDF has an assumed toxicity equivalent to 2ng TCDD + (6 x 0.5) ng TCDD, or 5 ng TCDD altogether. Estimates of human exposure to PCDDs and PCDFs in the environments are ca. 0.1 ng TCDD TEQ per person per day, mainly from food. 

The toxicrdogical implications of all these related compounds have not been elucidated entirely, except for DDE and DDD (TDE) metabolites. Hie toxicological efiiects are important not only for the specific case of DDT, as these are related to the estimation of human exposure. 

The accuracy of any model to predict VOC concentrations in test chamber experiments mostly depends on the accuracy of the source (term ER in Eq. 1) and sink submodels (terms A and D in Eq. 1) incorporated into the lAQ model. In other words, a realistic estimate of human exposure to VOCs emitted from indoor materials and products requires knowledge not only of their emission rates but also of their adsorption/ desorption capacity or the buffer effect on VOC concentrations in indoor air. Small environmental test chambers are increasingly used in order to characterize the emission of VOCs (i.e. the source term in Eq. 1) from materials and products present indoors, whether they are used to realize the building environment, maintenance work (includ-... 

While all of the above emission and deposition values are given in the form of TEQs, it should be noted that neither emission nor deposition is equivalent to exposure or intake. Significant changes in composition can occur to complex mixtures of CDDs, CDFs, and PCBs as they move through the environment. Measurements at or near the point of human contact provide the best estimates of human exposure. TEQs are most relevant to potential for hazard and risk when they represent intake values. 

---