2.1 Lifetime cancer cases and cumulative cancer risk due to drinking water contaminants

U.S. Environmental Protection Agency, 2018a United States Environmental Protection Agency Technical Support Document. −6, 10−5, or 10−4 correspond to contaminant concentrations that, following a lifetime of exposure, would cause one cancer case in a population of one million, 100,000 or 10,000 people, respectively. Our cumulative risk assessment methodology follows the approach used in the National Air Toxics Assessment (), whereby the overall cancer risk metric represents a statistical probability of developing cancer over a lifetime of exposure to an individual carcinogenic contaminant or a mixture of contaminants at specified levels. Risks of 10, 10, or 10correspond to contaminant concentrations that, following a lifetime of exposure, would cause one cancer case in a population of one million, 100,000 or 10,000 people, respectively.

U.S. Environmental Protection Agency, 2000 United States Environmental Protection Agency Supplementary Guidance for Conducting Health Risk Assessment of Chemical Mixtures. European Commission Directorate-General for Health and Consumers, 2012 European Commission Directorate-General for Health & Consumers Scientific Committee on Health and Environmental Risks, Scientific Committee on Emerging and Newly Identified Health Risks. U.S. Environmental Protection Agency, 2003 United States Environmental Protection Agency Framework for Cumulative Risk Assessment. Cedergreen, 2014 Cedergreen N. Quantifying synergy: a systematic review of mixture toxicity studies within environmental toxicology. In the U.S. EPA framework for air toxics assessment and in the present study, cancer risks for individual contaminants are treated additively, and cumulative risk is estimated by mathematical addition of single contaminant risks. This is a conservative approach which assumes that individual contaminants are exerting their toxic effects independently of each other, and that the mixture of contaminants is characterized by response additivity (). As summarized in extensive research and policy analyses, chemicals present in mixtures may act interactively, with cumulative effects that may differ from summed responses for individual chemicals (). Some studies noted that chemical mixtures can show toxic effects exceeding those predicted from response additivity for single components (). Yet, in the absence of information on the interactions of water contaminants, response additive approach serves as a valuable first step towards cumulative risk assessment.

U.S. Environmental Protection Agency, 2005 United States Environmental Protection Agency Guidelines for Carcinogen Risk Assessment. 630-P-03-001F. −6 lifetime cancer risk. For chemicals included in this study, published cancer risk benchmarks are based on a linear dose-response model which assumes that any amount of exposure carries some risk. Table 1 Estimated lifetime cancer cases for drinking water contaminants in 48,363 community water systems in the United States. Contaminant 1 1 Of 22 contaminants analyzed here, 17 have national drinking water standards in the U.S., either as individual chemicals (1,2-dibromo-3-chloropropane, arsenic, benzene, bromate, carbon tetrachloride, tetrachloroethylene, trichloroethylene, uranium, vinyl chloride) or as a group (dibromoacetic acid and trichloroacetic acid are regulated as a part of a group of 5 haloacetic acids, or HAA5; chloroform, bromoform, dibromochloromethane and bromodichloromethane are regulated as a group. Ra-226 and Ra-228 have a standard set for the sum of these two elements). Drinking water concentration corresponding to 10−6 lifetime cancer risk 2 2 −6 lifetime cancer risk were obtained from the websites of the California Office of Environmental Health Hazard Assessment ( Drinking water concentrations corresponding to 10lifetime cancer risk were obtained from the websites of the California Office of Environmental Health Hazard Assessment ( https://oehha.ca.gov/ ) and the U.S. Environmental Protection Agency Integrated Risk Information System ( https://www.epa.gov/iris ). Government agency defining 10−6 lifetime cancer risk level Year published Population exposed over 10−6 lifetime risk level, (millions) 3 3 For the exposure metric, arithmetic means for contaminant concentrations for each individual water utility were calculated for all available test results for a contaminant within the 2010 to 2017 data range. Test results reported as “non-detects” were assigned a value of zero and included in the overall data array. Estimated number of lifetime cancer cases 4 4 Estimated population exposed and estimated lifetime cancer cases for chloroform, bromoform, dibromochloromethane and bromodichloromethane are based on detection and concentration data for these individual contaminants. Estimates incorporating additional data for community water systems that measure and report the group of four trihalomethanes as a single total rather than individual trihalomethane levels are reported Table 3 Arsenic 0.004 μg/L California OEHHA 2004 141 45,300 Hexavalent Chromium 0.02 μg/L California OEHHA 2011 42 2,817 Disinfection Byproducts Bromodichloromethane 4 4 Estimated population exposed and estimated lifetime cancer cases for chloroform, bromoform, dibromochloromethane and bromodichloromethane are based on detection and concentration data for these individual contaminants. Estimates incorporating additional data for community water systems that measure and report the group of four trihalomethanes as a single total rather than individual trihalomethane levels are reported Table 3 0.06 μg/L California OEHHA 2018 (proposed) 211 22,461 Chloroform 4 4 Estimated population exposed and estimated lifetime cancer cases for chloroform, bromoform, dibromochloromethane and bromodichloromethane are based on detection and concentration data for these individual contaminants. Estimates incorporating additional data for community water systems that measure and report the group of four trihalomethanes as a single total rather than individual trihalomethane levels are reported Table 3 0.4 μg/L California OEHHA 2018 (proposed) 203 8,687 Dibromochloromethane 4 4 Estimated population exposed and estimated lifetime cancer cases for chloroform, bromoform, dibromochloromethane and bromodichloromethane are based on detection and concentration data for these individual contaminants. Estimates incorporating additional data for community water systems that measure and report the group of four trihalomethanes as a single total rather than individual trihalomethane levels are reported Table 3 0.1 μg/L California OEHHA 2018 (proposed) 198 8,076 Trichloroacetic Acid 0.5 μg/L U.S. EPA IRIS 2011 155 2,452 Dichloroacetic Acid 0.7 μg/L U.S. EPA IRIS 2003 163 2,146 Bromoform 4 4 Estimated population exposed and estimated lifetime cancer cases for chloroform, bromoform, dibromochloromethane and bromodichloromethane are based on detection and concentration data for these individual contaminants. Estimates incorporating additional data for community water systems that measure and report the group of four trihalomethanes as a single total rather than individual trihalomethane levels are reported Table 3 0.5 μg/L California OEHHA 2018 (proposed) 88 641 Bromate 0.1 μg/L California OEHHA 2009 24 287 Radioactive Elements Radium-228 0.019 pCi/L California OEHHA 2006 134 3,134 Radium-226 0.05 pCi/L California OEHHA 2006 100 985 Uranium 0.43 pCi/L California OEHHA 2001 57 336 Strontium-90 0.35 pCi/L California OEHHA 2006 8 20 Tritium 400 pCi/L California OEHHA 2006 0.2 7 Carcinogenic Volatile Organic Compounds (VOCs) 1,2,3-Trichloropropane 0.0007 μg/L California OEHHA 2009 11 290 Tetrachloroethylene 0.06 μg/L California OEHHA 2001 13 91 1,2-Dibromo-3-chloropropane 0.0017 μg/L California OEHHA 1999 4 63 1,4-Dioxane 0.35 μg/L U.S. EPA IRIS 2013 5 23 Trichloroethylene 0.5 μg/L U.S. EPA IRIS 2011 2 10 Carbon tetrachloride 0.1 μg/L California OEHHA 2000 2 6 Vinyl chloride 0.05 μg/L California OEHHA 2000 0.8 7 Benzene 0.15 μg/L California OEHHA 2001 0.2 1 Here we included contaminants with cancer risk benchmarks established by an authoritative government agency such as the U.S. Environmental Protection Agency or the California Office of Environmental Health Hazard Assessment ( Table 1 ). As described in detail in the U.S. EPA guidelines for carcinogen risk assessment, carcinogenic potency of different chemicals represents an upper bound estimate for cancer risk expected from a defined dose of substance per kilogram of body weight per day, following a lifetime of exposure to this dose of a contaminant (). For water contaminants, carcinogenic potency estimates can be translated into a benchmark concentration in water that corresponds to a risk of one cancer case per population of one million people, or 10lifetime cancer risk. For chemicals included in this study, published cancer risk benchmarks are based on a linear dose-response model which assumes that any amount of exposure carries some risk.

Stoiber et al. (2019) Stoiber T.

Temkin A.

Andrews D.

Campbell C.

Naidenko O.V. Applying a cumulative risk framework to drinking water assessment: a commentary. LCR = C LTA / C RISK

EC = Σ [LCR × P CWS ], summed for all systems in a state or across the country

EC TOT = Σ [LCR × P CWS ], summed for all contaminants

CumR = EC TOT / Σ P CWS

Where: LCR = lifetime cancer risk corresponding to a specified concentration of a contaminant

C LTA = long-term average contaminant concentration in a community water system, calculated as an arithmetic average of all test results for the specified time period.

C RISK = cancer risk benchmark that represents a contaminant concentration corresponding to 10 −6 lifetime cancer risk

EC = estimated number of cases attributable to a contaminant

P CWS = population served by a community water system

EC TOT = estimated number of lifetime cancer cases due to multiple contaminants

CumR = cumulative lifetime cancer risk on a state or national level due to drinking water contaminants The cancer risk calculations are based on the formulas described byWhere:

We carried out an additional analysis for the group of four trihalomethanes (THM4), a cluster of four disinfection byproducts regulated in the United States as a single group with a legal limit of 80 μg/L. The THM4 group, defined by the U.S. EPA for regulatory purposes as “total trihalomethanes” or TTHM, includes chloroform, bromoform, dibromochloromethane and bromodichloromethane. While the majority of water systems in the United States monitor and report the levels of individual trihalomethanes in their water, approximately 9,359 water utilities in the United States only reported the combined THM4 concentration and not the levels of individual trihalomethanes for at least some years during the 2010–2017 period.

California Office of Environmental Health Hazard Assessment, 2010 California Office of Environmental Health Hazard Assessment (OEHHA)

Draft Public Health Goal for Trihalomethanes in Drinking Water. −6 risk benchmarks for the individual trihalomethanes listed in California Office of Environmental Health Hazard Assessment, 2018 California Office of Environmental Health Hazard Assessment (OEHHA)

First Public Review Draft, Trihalomethanes in Drinking Water: Chloroform, Bromoform, Bromodichloromethane, Dibromochloromethane. C RISK (group) = Σ C PWA / EC (group)

C PWA = Σ [C LTA × P CWS ] / Σ P CWS

EC (group) = Σ [C PWA / C RISK ]

Where: C RISK (group) = cancer risk benchmark representing a 10 −6 lifetime cancer risk for the group of contaminants

C PWA = population-weighted average concentration for a contaminant, summed for all water systems

EC (group) = estimated number of cancer cases attributable to a group of contaminants present at a defined concentration, such as the population-weighted average Therefore, to incorporate the data for community water systems that only reported combined THM4 levels and more accurately estimate the number of cancer cases due to the national occurrence of these contaminants, we followed a previously published approach for deriving a cancer risk benchmark for the THM4 group (). This approach combines 10risk benchmarks for the individual trihalomethanes listed in Table 1 ) and factors in the national population-weighted average concentration for individual trihalomethanes to estimate their contribution to the overall THM4-attributable cancer risk, according to formulas below:Where:

−6 risk benchmark for the THM4 group corresponds to 0.15 μg/L and represents a benchmark that is lower than the 10−6 risk levels for chloroform and bromoform (0.4 and 0.5 μg/L, respectively), but higher than the 10−6 risk levels for dibromochloromethane and bromodichloromethane (0.1 and 0.06 μg/L, respectively). Applying this benchmark to the contaminant occurrence data for community water systems after including the additional THM4 data, we estimate that a further 8,047 lifetime cancer cases could be due to disinfection byproducts in the water, in addition to the estimates reported in The concentration-weighted 10risk benchmark for the THM4 group corresponds to 0.15 μg/L and represents a benchmark that is lower than the 10risk levels for chloroform and bromoform (0.4 and 0.5 μg/L, respectively), but higher than the 10risk levels for dibromochloromethane and bromodichloromethane (0.1 and 0.06 μg/L, respectively). Applying this benchmark to the contaminant occurrence data for community water systems after including the additional THM4 data, we estimate that a further 8,047 lifetime cancer cases could be due to disinfection byproducts in the water, in addition to the estimates reported in Table 1

−4, equivalent to 4 lifetime cancer cases per 10,000 people. This risk level is two orders of magnitude greater than the one-in-a-million, or 10−6 risk benchmark that is often considered by regulatory agencies in the United States as the de minimus risk ( Castorina and Woodruff, 2003 Castorina R.

Woodruff T.J. Assessment of potential risk levels associated with U.S. Environmental Protection Agency reference values. Overall, tap water exposure to the carcinogenic contaminants analyzed in this study corresponds to 105,887 estimated lifetime cancer cases. For approximately 279 million people served by community water systems, or 86% of the U.S. population, this number of cases represents an overall cumulative lifetime risk of approximately 4 × 10, equivalent to 4 lifetime cancer cases per 10,000 people. This risk level is two orders of magnitude greater than the one-in-a-million, or 10risk benchmark that is often considered by regulatory agencies in the United States as the de minimus risk (). Estimated cancer cases due to disinfection byproducts and arsenic account for 87% of the total number of cases. The remaining cancer cases are due to radioactive chemicals in drinking water, hexavalent chromium, and carcinogenic Volatile Organic Compounds (VOCs).

U.S. Environmental Protection Agency, 2019 United States Environmental Protection Agency Enforcement and Compliance History Online. U.S. Environmental Protection Agency, 2019 United States Environmental Protection Agency Enforcement and Compliance History Online. It is important to highlight that the vast majority of the community water systems analyzed in this study were in compliance with U.S. national drinking water standards. For illustration, as the U.S. EPA data show, between 2014 and 2017, between 4.5-5.5% of community water systems had serious violations of national drinking water standards (). For 2017, the last data year included in this study, half of the 2,222 community water systems considered by the U.S. EPA to be a serious water quality violator were very small groundwater systems serving communities of less than 500 residents, and 86% of serious violations for 2017 were in water systems serving communities of less than 3,300 people (). However, as analysis in this paper shows, compliance with national drinking water standards does not mean that water contaminant levels are reduced to concentrations that, according to the latest research, are entirely without health risk. The majority of the cancer risk and estimated lifetime cancer cases correspond to community water systems that are in full compliance with drinking water standards.

Two considerations suggest that the present analysis is conservative and that the overall cumulative risk might be greater than what is reported here. First, contaminants included in this analysis are those for which robust national occurrence data are available. Numerous other contaminants, such as nitrosamines, unregulated disinfection byproducts, per- and polyfluoroalkyl substances (PFAS) and a variety of industrial and agricultural chemicals, are not monitored as frequently or are not monitored at all, precluding their inclusion in our study.

U.S. Environmental Protection Agency, 2019 United States Environmental Protection Agency Enforcement and Compliance History Online. Further, even for nationally regulated water contaminants for which monitoring is required, not all utilities fully follow the monitoring and reporting regulations. For example, in 2017, of the over 50,000 community water systems in the U.S., 32 percent of systems had some non-compliance with monitoring and reporting requirements (). This lack of monitoring means some exposure information for water contaminants was either not collected or not reported to state and national drinking water authorities, and therefore is not included in this study.