Drinking-Water Standards History Key Definitions How Standards are Developed Current Issues Confronting Developers of Standards
Water Quality First Linked to Disease Cholera epidemic in London linked to Broad Street Pump Typhoid (Lawrence, Mass.) Cholera (Hamburg, Germany) Jersey City, NJ first community to disinfect water with chlorine Cholera epidemic in London linked to Broad Street Pump Typhoid (Lawrence, Mass.) Cholera (Hamburg, Germany) Jersey City, NJ first community to disinfect water with chlorine
Drinking Water Standards Development in U.S. st Federal standards (applied only to interstate carriers) U.S. Public Health Service Standards (revised 1925, 1946, 1956, 1962) Federal Safe Drinking Water Act passed following EPA report of 66 potential carcinogens found in New Orleans water supply (act applies only to public supplies)
Safe Drinking Water Act Amended Public skeptical of EPA and water industry Studies showed many systems with one or more toxic chemicals Congress prescribes rigorous schedule for establishing standards 83 contaminants named for standards development filtration of surface water supplies mandated EPA directed to establish 25 new standards every three years
Growth in Number of Drinking Water Standards
Definitions Primary drinking water contaminant - health-related, enforced Secondary drinking water contaminant - non-health- related, not enforced
Definitions Maximum Contaminant Level Goal (MCLG) - A non-enforceable regulatory goal designed to prevent adverse human health effects and allow an adequate margin of safety (MCLG = 0 for any carcinogen) Maximum Contaminant Level (MCL) - maximum permissible level of a contaminant in water delivered to any user of a public water system (set as close to MCLG as is technically and economically feasible) Lifetime Health Advisory Level (HAL) - non-regulatory concentration of drinking water contaminant that is not expected to cause any adverse effects over a lifetime of exposure.
Standards for Non-Carcinogens Based on Dose/Response Studies Assume a response “threshold” can be identified Uses a “Safety Factor” approach to calculate the standard
Dose/Response Testing Threshold or NOAEL (no observed adverse affect level) Dose Response
Definitions Reference Dose (RfD) - the daily exposure without deleterious effects over a lifetime Drinking Water Equivalent Level (DWEL) - drinking water concentration assuming RfD for 70 kg adult is dissolved in 2 liters of water assumed to be consumed daily Lifetime Health Advisory (HAL) - determined by applying relative source factor (generally 20% for organics, 10% for inorganics) to the DWEL
Calculating Reference Dose RfD(mg/kg/day) = NOAEL(mg/kg/day) / Safety Factor Safety factor of 100 usually used factor of 10 for human/animal response differences factor of 10 for inter-individual response differences additional safety factor of 10 applied if data are questionable
Calculating DWEL & HAL HAL(mg/L) =DWEL(mg/L) X RSC Relative Source Factor Daily Water Consumption (2 L) DWEL(mg/L)= RfD(mg/kg/day) X Body Wt (kg) 20% for organics 10% for inorganics
Example Calculation NOAEL for Aldicarb = mg/kg/day RfD = NOAEL/Safety Factor = 0.125/100 = mg/kg/day DWEL = [RfD X Body Wt.] / 2 Liters = [0.001 mg/kg/day X 70 kg] / 2 L = mg/L Lifetime HAL = DWEL X Source Factor = mg/L X 0.2 = mg/L
Drinking Water Standards for Carcinogens 5 EPA Cancer Groups A - Known human carcinogens B - Probable human carcinogens C - Possible human carcinogens D - Not classifiable E - No evidence of human carcinogenicity
Drinking Water Standards for Carcinogens Based on Dose/Response Studies But assume that NO response threshold can be identified (ie any dose poses some risk) Use mathematical models to extrapolate animal D/R data to the low risk levels considered acceptable for humans
Why Assume No Theshold for Carcinogens ? High natural incidence of tumors in all species makes threshold hard to define (requires large number of animal studies) Practical doses that lead to identifiable numbers of excess tumors in small animal populations are much higher than doses of interest in minimizing cancer risk to humans
D/R Modeling for Carcinogens Several math models have been proposed...risk estimates from different models can vary by as much as 1,000,000 fold
Drinking Water Standards for Carcinogens D/R relationship generally treated as linear risk = dose X constant so if dose increases 10-fold... risk also increases 10-fold EPA sets lifetime health advisories at 1 in 1- million risk level, but MCL’s often set at higher risk level due to technical or economic considerations
Current Scientific Issues in Drinking Water Standards Development Can cancer “thresholds” be identified? How to set “standards” for mixtures of contaminants Highly sensitive (and inexpensive) new water testing methods (ELISA) making pesticide testing more affordable Establishing standards for new chemicals and chemical metabolites