The Environmental Media Evaluation Guides (EMEGs), presented in Appendix A, provide health assessors with a means of selecting contaminants that need to be further evaluated for their potential impact on public health. This evaluation should include a detailed analysis of site-specific exposure pathways and conditions.
EMEG values are derived using methodology that incorporates standardized exposure assumptions. At some sites, the existing conditions may result in exposures that differ from those used to derive the EMEG values. In these situations, the health assessor can use the methodology presented in this section to define site-specific exposures more accurately . These exposure doses can then be compared to the appropriate toxicity values (e.g., Minimal Risk Levels) to determine if the exposures pose a potential health hazard.
The following generic equation can be used to estimate the exposure dose resulting from contact with a contaminated medium:
ED = (C * IR * EF) /BW
where,
ED = exposure dose;
C = contaminant concentration;
IR = intake rate of contaminated medium;
EF = exposure factor;
BW = body weight;
Some standard values that may be useful in estimating exposures are shown in Table D.1.
| Body Weight 70 kg - adult, average (1) 16 kg - children 1 through 6 years old, 50th percentile (1) 10 kg - infant (1)
|
Some exposure may occur on an intermittent or irregular basis. For these kinds of exposures, an exposure factor (EF) can be calculated to average-out the dose over the exposure interval. The exposure factor is calculated by multiplying the exposure frequency by the exposure duration, and dividing by the time period over which the dose is to be averaged. For example, if a child comes into contact with contaminated soil twice a week over a five-year period, the exposure factor would be: EF = (2 days/week * 52 weeks/year * 5 years) / (5 years * 365 days/year)
EF = 0.28
Therefore, in this example, the dose resulting from one exposure event would be multiplied by 0.28 to yield the average daily dose over the 5-year period.
The use of an exposure factor gives the dose averaged over the period of exposure. However, the health assessor should recognize that some health effects may not depend on the average dose, but rather on the peak dose or some other measure of the dose rate.
The following discussion provides an overview of quantitative evaluation of human exposure through the following pathways: inhalation, water ingestion, soil ingestion, food ingestion, and dermal exposure to water and soil.
Inhalation is an important pathway for human exposure to contaminants that exist as atmospheric gases or are adsorbed to airborne particles or fibers. Inhalation exposure to contaminants from hazardous waste sites can occur as a result of direct release of gases and particles from an on-site facility, volatilization of gases from contaminated soils or water bodies, or resuspension of dust and particles from contaminated soil surfaces.
In order to estimate an inhalation exposure dose, the ventilation rate must be determined. Ventilation rates are often expressed as a minute volume, which is the volume of air inhaled in one minute (liters/minute). Minute volumes vary little with gender before age 12. However, adolescent males have a 50% higher minute volume than their female counterparts; and adult men, under heavy exertion, have a 35-40% higher minute volume than adult females (2).
A person's level and frequency of physical activity are major factors affecting minute volume. Values for average levels of activity have been established based on eight hours per day spent at each of the following: work, rest, and light-to-moderate activity (Exhibit D.1.). Although these values are applicable to the general population, individual persons may exhibit large variations based on their levels of physical activity.
Other factors influencing minute volume include: temperature, altitude, conditions which may aggravate air quality background levels, and a person's weight, height, health, smoking status, and pulmonary disease status (2).
Exhibit D.1. illustrates how inhalation exposure doses can be estimated and provides inhalation minute volumes.
Ingestion of contaminated water is often the most significant source of exposure to hazardous substances from a site. To estimate exposure to a contaminant from the ingestion of potable water, the contaminant concentrations in tap water samples from individual homes should preferably be analyzed. In the absence of data on individual wells, the assessor may consider using data from monitoring wells to estimate upper limits for exposures to contaminants.
The oral ingestion dose is ideally computed using site-specific information for the populations at risk (i.e., bodyweight and consumption rates). Exhibit D.2. illustrates how exposure doses via drinking water can be estimated.
Soil ingestion can occur by the inadvertent consumption of soil on hands or food items, mouthing of objects, or the ingestion of nonfood items (pica). All children mouth or ingest non-food items to some extent. The degree of pica behavior varies widely in the population, and is influenced by nutritional status and the quality of care and supervision. Groups that are at an increased risk for pica behavior are children aged 1-3 years old, children from families of low socioeconomic status, and children with neurologic disorders (e.g., brain-damage, epilepsy, mental retardation) (1).
For non-pica children, a soil ingestion rate of about 50 to 100 mg per day is supported by recent studies using tracer metals in soil (4,5). Soil ingestion by adults has not been well studied, but limited evidence suggests a tentative value of 50 mg/day (6). For children with obvious pica behavior, soil ingestion rates of 5-10 grams per day are possible (1).
Both use of and accessibility to the site and surrounding areas must be considered when evaluating a site's soil exposure pathways. Sites with abandoned buildings, standing water, or streams may attract children, and exposures may occur at sites near playgrounds or school yards despite fencing and other efforts to restrict access.
Workers at commercial or industrial properties could ingest contaminated soil at rates which depend upon the type of employment.
Both residential and recreational areas are likely to provide access for exposure. Contaminated soil can be brought into homes on the feet of family members and pets. Suspended soil particulates in outdoor air can also enter a house through indoor-outdoor air exchange. A young child playing on the floor will have the maximum opportunity both for ingestion and for dermal exposure to soil and dust accumulated on the floor.
Exhibit D.3. illustrates how soil ingestion exposures can be estimated, and it provides soil ingestion rates for various age groups.
Assessment of the human health risk from ingestion of contaminated food requires information on the quantities of contaminated foodstuffs consumed and the extent of contamination present in foodstuffs. The most reliable method of assessing the extent of human exposure to contaminants in food is direct measurement of concentrations in foodstuffs. Such measurements should be conducted on foodstuffs prepared for consumption or portions of contaminated plants and animals that are representative of those portions used as food.
If food chains appear to be a significant pathway for human exposure and the appropriate information on contaminant levels is not available, that lack of information should be explicitly identified in the health assessment and a recommendation should be made that the appropriate information be obtained.
Estimation of exposure dose through food chains requires knowledge of the consumption rate of specific food items in the human diet. Nationwide daily consumption rates by food group are presented in Appendix E.
The consumption rates of the population in the vicinity of a hazardous waste site may differ considerably from national average consumption rates. For example, regional consumption rates of fish--freshwater, saltwater, and shellfish--may vary widely from national averages. Consumption rates of subpopulations within the contaminated area may also vary significantly from the national averages. For example, consumption of fish increases with the age of the consumer (9), and fish consumption for sport fishers is likely to be higher than the average for the United States population (10). When local consumption patterns are available, and different from national averages, they should be used in calculations to determine exposure estimates.
In the case of residential soil contamination, the consumption rate of home-grown foods and local wild plants is of interest. Appendix E presents consumption rates of home-grown foods as determined by the U. S. Department of Agriculture (11). These data are organized into four groups: urban, rural non-farm, rural-farm, and all households. Table E.7. in Appendix E contains the average percentage consumption of home-grown foods.
To estimate the total daily intake of a particular contaminant, daily intakes of contaminants from all affected foodstuffs should be considered. Exhibit D.4. illustrates how food ingestion exposure doses can be estimated.
Dermal absorption of contaminants from soil or water is a potential pathway for human exposure to environmental contaminants. Dermal absorption depends on numerous factors including the area of exposed skin, anatomical location of exposed skin, length of contact, concentration of chemical on skin, chemical-specific permeability, medium in which the chemical is applied, and skin condition and integrity.
The area of skin that is exposed will be influenced by the activity being performed and the season of the year. Skin surface area also varies with age. EPA (1,12) provides data on skin surface areas of different parts of the body for adults and for children.
Dermal absorption of contaminants in water occurs during bathing, showering, or swimming and may be a significant route of exposure. Worker exposure via this pathway will depend on the type of work performed, protective clothing worn, and the extent and length of water contact. The permeability of the skin to a chemical is influenced by the physicochemical properties of the substance, including its molecular weight (size and shape), electrostatic charge, hydrophobicity, and solubility in aqueous and lipid media. In general, chemicals that demonstrate high skin permeability are low molecular weight, non-ionized, and lipid soluble.
Chemical-specific permeability constants should be used to estimate dermal absorption of a chemical from water. Values for dermal permeability constants may vary over a large range, spanning at least five orders of magnitude (13). Dermal permeability constants are available for relatively few chemical substances (see reference 3 for a summary of reported values). Before using a dermal permeability constant, the original reference should be checked to ensure the applicability of the experimental study. In some studies, the permeability constants were determined using neat liquids or concentrated aqueous solutions. Exposure of skin to high concentrations of organic solvents can cause delipidation of the skin, which can profoundly alter the skin's permeability. Dermal permeability constants derived from animal studies may not be applicable for human assessment purposes because of substantial differences in their skin permeability.
Dermal absorption can be significantly increased by skin abrasions which remove the outer stratum corneum layer of the skin. Pathological skin conditions, such as psoriasis or eczema, can also result in increased penetration of chemical substances into the skin (14).
When the permeability constant for a chemical is known, the dermal absorption of a chemical from water can be estimated as illustrated in Exhibit D.5.
Dermal absorption of contaminants from soil or dust depends on the area of contact, the duration of contact, the chemical and physical attraction between the contaminant and the soil, and the ability of the contaminant to penetrate the skin. Chemical specific factors, such as lipophilicity, polarity, volatility, molecular weight, and solubility also affect dermal absorption.
Many organic chemicals bind to organic matter in soil, thereby decreasing their absorption by the skin. In addition, only the fraction of the contaminant that is in direct contact with the skin is amenable to absorption . Therefore, the ability of a soil contaminant to be dermally absorbed depends on the diffusion of the contaminant through the soil matrix. Experimental studies have confirmed that dermal absorption of a contaminant may be reduced when the contaminant is applied in soil as compared to direct dermal application of the compound (15).
A soil-specific factor involved in dermal absorption is adherence, the quantity (mg/cm2) of soil on the skin. Hawley (16) reports soil adherence values of 0.5 mg/cm2 for children and 3.5 mg/cm2 for adults. EPA (1,3) has reported soil adherence values of 1.45 mg/cm2 for commercial potting soil and 2.77 mg/cm2 for kaolin clay. Data on dust adherence to skin are limited; however, Hawley's work (16) also provides a dust adherence value of 1.8 mg/cm2 for adults. Based on this data, a soil adherence value of 2 mg/cm2 is proposed.
To calculate the average lifetime dermal dose, divide a 70 year lifetime exposure period into the time intervals shown in Exhibit D.6. For each exposure time interval, dermal absorption is estimated as the soil concentration times the soil adhered times the fractional lifetime exposure. This product is divided by the appropriate body weight for each exposure time interval. Exhibit D.6. illustrates how soil dermal absorbed doses can be estimated.
1. EPA Office of Health and Environmental Assessment. Exposure factors handbook. Washington, DC: Environmental Protection Agency, March 1990; EPA/600/8-89/043.
2. National Council on Radiation Protection and Measurements (NCRP). Radiological assessment: predicting the transport, bioaccumulation, and uptake by man of radionuclides released to the environment. Bethesda, MD: National Council on Radiation Protection and Measurements, 1984. NCRP Report no. 76.
3. EPA Office of Emergency and Remedial Response, Office of Solid Waste and Emergency Response. Superfund exposure assessment manual. Washington, DC: Environmental Protection Agency, 1988; EPA 540/1-88/001.
4. Calabrese EJ, et al. How much soil do young children ingest: an epidemiologic study. Regulatory Toxicology and Pharmacology 1989; 10:123-37.
5. Davis S, et al. Quantitative estimates of soil ingestion in normal children between the ages of 2 and 7 years. Archives of Environmental Health 1990; 45:112-22.
6. Calabrese J, et al. Preliminary adult soil ingestion estimates: results of a pilot study. Regulatory Toxicology and Pharmacology 1990; 12:88-95.
7. International Commission on Radiological Protection (ICRP). Report of the task group on reference man. New York: Pergamon Press, 1975.
8. National Academy of Sciences (NAS). Drinking water and health. Washington, DC: NRC Press, 1977.
9. Rupp EM, Miller FL, Baes CF III. Some results of recent surveys of fish and shellfish consumption by age and region of U. S. residents. Health Physics 1980; 39(2):165-75.
10. Office of Toxic Substances, Exposure Division. Methods for assessing exposure to chemical substances. Vol. 8, Methods for assessing environmental pathways of food contamination. Washington, DC: Environmental Protection Agency, September 1986; EPA 560/5-85-008 and PB87-107850.
11. U.S. Environmental Protection Agency. Dietary consumption distributions of selected food groups for the U.S. population. Washington, DC: Environmental Protection Agency, February 1980, EPA 560/11-80-012 and PB81-147035.
12. EPA Office of Health and Environmental Assessment. Development of statistical distributions or ranges of standard factors used in exposure assessments. Washington, DC: Environmental Protection Agency, March 1985; OHEA-E-161.
13. Duggard PH. Absorption through the skin: theory, in vitro techniques, and their applications. Food Chemistry and Toxicology 1986; 24:749-53.
14. Hensby CN, Schaefer H, Schalla W. The bioavailability and toxicity of topical drugs related to diseased skin. In: Chambers PL, Gehring P, and Sakai F, eds. New concepts and developments in toxicology. New York, NY: Elsevier Science Publishers, 1986.
15. Yang JJ, et al. In vitro and in vivo percutaneous absorption of benzo[a]pyrene from petroleum crude-fortified soil in the rat. Bulletin of Environmental Contamination and Toxicology 1989; 43:207-14.
16. Hawley JK. Assessment of health risk from exposure to contaminated soil. Risk Analysis 1985; 5(4):289-302.