PETITIONED PUBLIC HEALTH ASSESSMENT
MCFARLAND STUDY AREA
MCFARLAND, KERN COUNTY, CALIFORNIA
Completed Exposure Pathway
|Medium||Exposure Route||Time of Exposure||Exposure Activities||Estimated Exposed*||Chemicals||Public Health Concern|
|Water||Oral, Dermal||Past, Current, Future||Drinking/eating||1-2000||1-1200||metals, pesticides||No|
|Soil||Oral, Dermal||Past, Current, Future||Recreational, working, trespassing||1-2000||1-1200||metals, pesticides||No|
ATSDR's Comparison Values
ATSDR comparison values are media-specific concentrations that are considered safe under default conditions of exposure. They are used as screening values in the preliminary identification of "contaminants of concern" at a site. "Contaminants of concern" may be an unfortunate term because "concern" may be misinterpreted as implying "hazard." For ATSDR, however, a "contaminant of concern" is merely a site-specific chemical substance that the health professional has selected for further evaluation of potential health effects.
Generally, a chemical is selected as a "contaminant of concern" because its maximum concentration in air, water, or soil at the site exceeds one of ATSDR's comparison values. However, comparison values are not thresholds of toxicity. While concentrations at or below the relevant comparison value are considered safe, any environmental concentration that exceeds a comparison value does not automatically mean it will produce adverse health effects. Indeed, the purpose behind highly conservative, health-based standards and guidelines is to enable health professionals to recognize and resolve potential public health problems before they become actual health hazards. The probability that adverse health effects will actually occur as a result of exposure to environmental contaminants depends on site-specific conditions, individual lifestyle, and genetic factors that affect the route, magnitude, and duration of actual exposure, and not on environmental concentrations alone.
Screening values based on noncancer effects are obtained by dividing NOAELs or LOAELs determined in animal or (less often) human studies by cumulative safety margins (also called safety factors, uncertainty factors, or modifying factors) that typically range from 10 to 1,000 or more. By contrast, cancer-based screening values are usually derived by linear extrapolation from animal data obtained at high doses, because human cancer incidence data for very low levels of exposure do not exist. In neither case can the resulting screening values (i.e., EMEGs or CREGs) be used to make realistic predictions of health risk associated with low-level exposures in humans.
The following list describes the various comparison values used by ATSDR to select chemicals for further evaluation.
|Cancer Risk Evaluation Guide|
|MRL||Minimal Risk Level|
|IMRL||Intermediate Risk Level|
|CMRL||Chronic Risk Level|
|EMEG||Environmental Media Evaluation Guide|
|aEMEG||Environmental Media Evaluation Guide based on acute Minimal Risk Level|
|IEMEG||Intermediate Environmental Media Evaluation Guide|
|RMEG||Reference Dose Media Evaluation Guide|
|RfC||Reference Dose Concentration|
|EPAIII||EPA Region III|
|DWEL||Drinking Water Equivalent Level|
|CLHA||Child Longer-Term Health Advisory|
|LTHA||Drinking Water Lifetime Health Advisory|
|MCL||Maximum Contaminant Level|
|MCLG||Maximum Contaminant Level Goal (µg/L)|
|Maximum Contaminant Level Action|
|NAAQS||National Ambient Air Quality Standards|
|PEL||Permissible Exposure Limit (OSHA)|
|REL||Recommended Exposure Limit (NIOSH)|
|TLV||Threshold Limit Value (ACGIH)|
|FDA||Food and Drug Administration|
|ppm||parts per million, e.g., mg/L or mg/kg|
|ppb||parts per billion, e.g., g/L or g/kg|
|kg||kilogram (1,000 grams)|
|mg||milligram (0.001 grams)|
|µg||microgram (0.000001 grams)|
|m3||cubic meter (used in reference to a volume of air equal to 1,000 liters)|
Cancer Risk Evaluation Guides (CREGs): estimated contaminant concentrations in water, soil, or air which, based on EPA's cancer slope factors. CREGs would correspond to a hypothetical risk of one excess cancer in one million persons exposed over a lifetime. However, "the true risk is unknown and may be as low as zero" (EPA, 1986).
Minimal Risk Levels (MRLs): estimates of daily human exposure to a chemical (for example, doses expressed in mg/kg/day) that are unlikely to be associated with any appreciable risk of harmful noncancer effects over a specified duration of exposure. MRLs are derived for acute (under 14 days), intermediate (between 15 and 364 days), and chronic (over 365 days) exposures. MRLs are published in ATSDR's toxicological profiles for specific chemicals.
Environmental Media Evaluation Guides (EMEGs): concentrations of a contaminant in water, soil, or air that are unlikely to be associated with any appreciable risk of harmful noncancer effects over a specified duration of exposure. EMEGs are derived from ATSDR minimal risk levels by factoring in default body weights and ingestion rates. Separate EMEGs are computed for adults and children, and where the corresponding MRLs are available, acute (under 14 days), intermediate (between 15 and 364 days), and chronic (over 365 days) exposures.
Intermediate Environmental Media Evaluation Guides (IEMEGs): media-specific concentrations that correspond to a minimal risk level, factoring in body weight and ingestion rates for intermediate exposures (over 14 days and under one year).
Reference Dose Media Evaluation Guide (RMEG): the concentration of a contaminant in air, water, or soil that corresponds to EPA's RfD of RfC for that contaminant when default values for body weight and intake rates are taken into account.
EPA's Reference Dose (RfD): estimate of the daily exposure to a contaminant unlikely to cause noncarcinogenic adverse health effects over a lifetime of exposure. Like ATSDR's MRL, EPA's RfD include safety factors and are expressed in terms of dose (mg/kg/day).
Reference Concentration (RfC): concentration in air expected to be associated with no hazardous health effects over a lifetime of exposure, assuming default body weights and inhalation rates.
Risk-Based Concentrations (RBCs): values are similar to ATSDR's RMEGs and CREGs because they are derived for carcinogens and noncarcinogens from RfDs and cancer slope factors, respectively, assuming default values such as body weight, exposure duration, and frequency. However, unlike EMEGs, they are available for fish, water, soil, and air.Environmental Protection Agency Region III (EPA III) derives RBCs.
Drinking Water Equivalent Levels (DWELs): based on EPA's oral RfD. DWELs represent corresponding concentrations of a substance in drinking water that are estimated to have negligible hazardous effects in humans over a lifetime of exposure, at an intake rate of two liters daily, and assuming that drinking water is the sole source of exposure to the contaminant. DWELs are similar to ATSDR's RMEG for drinking water.
Child Longer-Term Health Advisories (CLHAs): contaminant concentrations in water that EPA deems protective of public health (taking into consideration the availability and economics of water treatment technology) over a period of seven years, using a child's weight (10 kilograms) and ingestion rate (one liter daily).
Lifetime Health Advisories (LTHAs): calculated from the DWEL. LTHAs represent the concentration of a substance in drinking water estimated to have negligible hazardous effects in humans over a lifetime of 70 years, assuming two liters of water per day for a 70-kilogram adult, and taking into account other sources of exposure. In the absence of chemical-specific data, the assumed fraction of total intake from drinking water is 20%. Lifetime HAs are not derived for compounds that are potentially carcinogenic for humans.
Maximum Contaminant Levels (MCLs): contaminant concentrations in drinking water that EPA deems protective of public health (considering the availability and economics of water treatment technology) over a lifetime of 70 years assuming two liters of water per day.
Maximum Contaminant Level Goals (MCLGs): drinking water health goals set at levels at which no known or anticipated adverse effect on the health of persons occurs, and which allow an adequate margin of safety. Such levels consider the possible impact of synergistic effects, long-term and multi-stage exposures, and the existence of more susceptible groups in the population. The MCLG is typically set equal to zero for known and possible carcinogens.
Maximum Contaminant Level Action (MCLA): levels set by EPA under Superfund that trigger a regulatory response when the contaminant concentration exceeds this value.
National Ambient Air Quality Standards (NAAQS): established by the EPA, as mandated in the Clean Air Act, for six criteria pollutants (carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, particulate, and lead). NAAQS are classified as either primary, which define levels deemed protective of public health, or secondary, which in some instances establish lower levels to prevent adverse effects on vegetation, property, or other elements of the environment.
Permissible Exposure Limits (PELs): air standards developed by the Occupational Safety and Health Administration (OSHA) for the workplace. They are time-weighted average concentrations of contaminants considered safe for healthy workers over the course of an 8-hour workday and a 40-hour workweek. A PEL may be exceeded for brief periods, but the sum of the exposure levels averaged over eight hours must be equal to or below the PEL.
Recommended Exposure Limits (RELs): established by the National Institute for Occupational Safety and Health (NIOSH) and are similar to OSHA's PELs. They are time-weighted average concentrations for the workplace deemed to be safe over a course of a 10-hour workday and a 40-hour workweek.
Threshold Limit Values (TLVs): established by the American Conference of Governmental Industrial Hygienists (ACGIH). The TLV is the time-weighted average concentrations for a normal 8-hour workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. Many of ACGIH's TLVs were adopted by OSHA for use as PELs. TLVs and PELs, which were designed to protect healthy workers, are usually much higher than the health-based values of ATSDR and EPA, which were designed to protect the health of the general population, including the young and the elderly. Although ATSDR does not base any of its community health decisions on TLVs or PELs, it sometimes cites such values in public health assessments to concentrations of site-specific contaminants into a meaningful perspective for the reader.
The Food and Drug Administration (FDA) has recommended concentration levels for certain substances in food, including fish. Levels above FDA levels mean that the food may be unsafe for human consumption.
Comparison Value References
1. Agency for Toxic Substances and Disease Registry. Health Assessment Guidance Manual. Atlanta: ATSDR, October 1992.
2. National Institute for Occupational Safety and Health. Pocket Guide to Chemical Hazards. Washington D.C.: U.S. Department of Health and Human Services, June 1994.
3. U.S. Environmental Protection Agency. New Interim Region IV Guidance for Toxicity Equivalent Factors Methodology for Carcinogenic PAHs. February 11, 1992.
4. EPA, 1986. Environmental Protection Agency. Guidelines for Carcinogenic RiskAssessment. Fed. Reg., 51: 33997-33998, September 24, 1986.
5. Williams, Gary M., and Weisburger, John H. (1991). "Chemical Carcinogenesis". Chapter 5 in: Casarett and Doull's TOXICOLOGY: The Basic Science of Poisons. Mary O Amdur, John Doull, and Curtis Klaassen, Editors. Pergamon Press pp 127-200, pg 154-155.
6. Pitot, Henry C., III and Dragan, Yvonne P. (1996). "Chemical Carcinogenesis". Chapter 8 in: Casarett and Doull's TOXICOLOGY: The Basic Science of Poisons. Curtis D. Klaassen, Mary O Amdur, and John Doull, Editors. McGraw-Hill, pp 201-267.
7. Cunningham, M.L., Elwell, M.R., and Matthews, H.B. (1994). Relationship of carcinogenicity and cellular proliferation induced by mutagenic noncarcinogens vs carcinogens. Fundam. Appl. Toxicol. 23: 363-369.]
ATSDR released the McFarland Public Health Assessment for review and comment during the period from September 27 through November 13, 2000. No written comments were received during this period and the document has been finalized.