1,4-dioxane is an organic chemical used in a number of industrial and commercial applications, including as a solvent and as a stabilizer of other solvents, particularly, 1,1,1 trichloroethane (TCA) (ATSDR, 2007; NTP, 2011).

1,4-dioxane causes cancer in laboratory animals and is reasonably anticipated to be a human carcinogen, first listed in the Annual Report on Carcinogens in 1981 (NTP, 2014). In 1988, 1,4-dioxane was added to the list of chemicals known to the state to cause cancer, pursuant to California’s Safe Drinking Water and Toxic Enforcement Act (Proposition 65). It is also considered to pose a cancer risk by U.S. EPA (US EPA, 2010; 2013).

1,4-dioxane is an emerging contaminant.  In response to occurrence data and potential adverse health effects, an initial notification level was established in 1998 and subsequently revised in 2010 in accordance with Health and Safety Code section 116455.

In January 2019, the Division of Drinking Water (DDW) requested the Office of Environmental Health Hazard Assessment (OEHHA) to establish a 1,4-dioxane public health goal (PHG).

• Public Health Goal Memo.

In September 2025, OEHHA released a draft PHG of 0.04 µg/L. More information on the status of the 1,4-dioxane public health goal development can be found on the OEHHA website.

The 1,4-dioxane drinking water notification level for is 1 microgram per liter (μg/L). Certain requirements and recommendations apply to a water system if it serves its customers drinking water containing a contaminant greater than its notification level. The response level, the level at which removal of the source from service is recommended, is 35 μg/L.

In 1998, the Drinking Water Program, now known as DDW, established a 1,4 dioxane notification level of 3 μg/L for based on a U.S. EPA (1990) drinking water concentration that corresponded to a 10^-6 theoretical lifetime cancer risk. In 2010, U.S. EPA revised its 1,4-dioxane risk evaluation, such that a 10^-6 risk level corresponds to 0.35 μg/L (US EPA, 2010a, 2010b, 2013).  The notification level was revised to 1-μg/L in November 2010.

The notification level is greater than the de minimis (10^-6) level commonly used for notification levels based on cancer risk, reflecting difficulty detecting 1,4-dioxane at very low concentrations at the time of issuance.

More information about notification and response levels is available at https://www.waterboards.ca.gov/drinking_water/certlic/drinkingwater/NotificationLevels.html. 

In early 2026, DDW staff began a series of surveys to evaluate the current cost, capacity, and technical capacity for detection of synthetic organic chemicals in drinking water. Preliminary results show that 50% of responding laboratories can detect 1,4-dioxane in drinking water down to 0.04 μg/L and all respondents can detect down to at least 0.5 μg/L.  

DDW has approved EPA Method 522 to analyze for presence of 1,4-dioxane in drinking water. More information about DDW approved methods and laboratory accreditation is available at: https://www.waterboards.ca.gov/drinking_water/certlic/labs/.

In 1998, the Drinking Water Program was were notified about a 1,4-dioxane detection in a groundwater well; it has since been found in a number of wells, mostly in southern California. In 2002, the presence of 1,4-dioxane in wastewater became problematic for a groundwater recharge project in southern California, prompting a need for additional water treatment.

Drinking water systems are not required by state regulations to monitor for 1,4-dioxane. Nevertheless, because of concerns about possible contamination, a number of systems have been directed by DDW to or have chosen to sample their supplies for 1,4-dioxane.

• DDW’s water quality database reported findings of 1,4 dioxane from January 2013 through April 2026 (.xls) *

Water quality data for individual public water systems is available at Drinking Water Watch. Detections included in the accompanying spreadsheet should not be considered to reflect 1,4-dioxane in drinking water served to customers, since water from the listed sources may be blended, treated, or not used to provide drinking water. For more information about specific drinking water supplies, see public water systems' annual Consumer Confidence Reports.

*Date range chosen to reflect the year U.S. EPA established a third unregulated contaminant monitoring rule for 1,4 dioxane. 

ATSDR (2012). Toxicological Profile for 1,4-Dioxane. Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service.

NTP (2021). 1,4-dioxane. In Report on carcinogens, fifteenth edition, U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program RoC Profile: 1,4-Dioxane; 15th RoC 2021 (replace the NTP 2014 13th edition)

US EPA (2024) Final Risk Evaluation for 1,4-Dioxane | US EPA, Unreasonable Risk Determination for 1,4-Dioxane, Supplement to the Risk Evaluation for 1,4-Dioxane. Washington, DC: Office of Chemical Safety and Pollution Prevention, US Environmental Protection Agency.

• More information regarding State Water Board actions on 1,4-dioxane may be found in the 1,4-Dioxane Fact Sheet.
• OEHAA Overview of 1,4-Dioxane