By Kelly A. Reynolds, MSPH, PhD

N-nitrosodimethylamine (NDMA) is a toxic, semi-volatile organic chemical found commonly in drinking water and food. Even at low doses, repeated studies show the compound causes serious health effects in chronically exposed rats but the question is: What are the health effects in humans?

Where did it come from?
NDMA is used in a number of industrial processes, such as a solvent and plasticizer, and low levels can even be found in cured foods (i.e., fish, bacon), meats, beer, milk and cheese. Levels in common foods has been reported to range from 50-5000 ng/kg. NDMA is not currently produced or commercially used in the U.S., except for research, however it is naturally produced from a variety of common activities, including digestion in the stomach of certain drugs and foods. According to the Agency for Toxic Substances and Disease Registry (ASTDR), humans are also exposed from certain personal care (i.e., shampoos and cleansers) and household products (i.e., detergents and pesticides). Infants may be exposed from contaminated formulas, rubber bottle nipples and pacifiers, and from human breast milk.

The prevalence of NDMA in drinking water sources was first identified in the late 90’s in California where groundwater was impacted by industrial sites where rocket fuel was being produced. NDMA was used as in the synthesis of rocket fuel components and concentrations were detected as high as 40,000 ng/L (parts per trillion). The greatest concern, however, is related to a much more common process: wastewater treatment. The use of chlorine, chloramine and anion exchange resins in environmental water treatment can lead to the production of NDMA at potentially harmful levels. NDMA has also been detected at sites where chlorinated wastewater effluent was used for the recharge of groundwater aquifers at levels of hundreds of ng/L[1]. Areas not associated with effluent recharge have also tested positive at low levels (10 ng/L)[2].

Risks and Regulation
The United States Environmental Protection Agency’s Integrated Risk Information System (IRIS) has given NDMA the classification of B2, meaning it is a suspected human carcinogen based on animal studies but that direct evidence of these risks in humans has not been established.

Animal studies show that once ingested, NDMA enters the bloodstream and spreads rapidly (within minutes) to many organs in the body. The liver breaks down NDMA and it is excreted in the urine and exhaled from the lungs within 24 hours of exposure. Currently there are no reports of NDMA causing cancer in humans but acute exposures in humans have resulted in death due to severe liver damage and animals with short-term and long-term exposures at levels between 5 and 50 ppm in water developed serious health effects, such as liver disease and death. Similarly, the effects of the compound on human fetuses is not known but the offspring of mice who ingested NDMA during pregnancy were either dead at birth or died shortly thereafter. (ASTDR 1989)[3].

Although water is listed as an important source of human exposure to NDMA, a Canadian study found that water actually contributes to only 10% of the overall exposure[4]. A latter study conducted by the USEPA found that trace levels of NDMA in water account for less than one percent of the overall human exposures (reviewed in OEHHA, 2006)[5]. The FDA has set action levels for NDMA in barley malt (10 ng/L) and malt beverages (5 ng/L) certain foods but water remains a target for control.

Currently there is no maximum contaminant level (MCL) for NDMA in water but levels of 0.7 ng/L have been estimated for an additional cancer risk of one in a million, in a lifetime. The limit of detection in water is currently at a range of 0.5 to 2 ng/L[6]. California health agencies have set a notification level of NDMA in drinking water at 10 ng/L, with a public health goal of 3 ng/L, and a response level of 200 ng/L. The response level correlates to an estimated 1 in 10,000 risk and a recommendation of removing the water source from service.

Experts expect that legally enforceable drinking water standards (MCLs) will be established for NDMA in the future. Given the consistent evidence of carcinogenic effects in animals and the general knowledge of adverse human health effects related to other nitrosamines, there is a strong indication of the need to reduce exposures of humans to NDMA.

Tracking the Contaminant
In 2000, California officials found NDMA in two drinking water supply wells, at levels of 30-40 ng/L and thus over the action level of 10 ng/L. The contamination was linked to activities at a water reclamation plant nearby where chlorinated wastewater was being used to recharge the groundwater aquifer. Population increases and increasing drinking water needs have led to widespread activities of wastewater reuse in the United States, particularly in the arid southwest. Therefore, the emergence of NDMA as an unintended drinking water contaminant has caused quite a stir.

Henceforth, scientists at the United States Geological Survey (USGS) and others studied the degradation potential of NDMA in the environment by applying treated wastewater to designated soils and allowing it to seep below ground to recharge a groundwater supply. This recharge site, known as the Montebello Forebay Water Conservation System, consists of more than 600 acres of with ground basins between 4-10 feet deep were wastewater and storm flows can be deposited to allow for percolation of water back into the underground aquifer. The hope was that during the time spent in the basin and percolating, microbial populations would aid in breaking down the NDMA compound in the water supply. This process is generally referred to as biodegradation. The result was that this natural, environmental treatment process utilizing indigenous soil microbes was effective at reducing the levels of NDMA by 54%[7].

In 2001 the California Department of Health Services conducted a survey of various drinking water effluents and found 15% (3/20) of chloramine treated drinking water was positive for NDMA, above the action level. None of the eight supplies utilizing free chlorine as a disinfectant was positive, however one of four water supplies using anion exchange treatment tested positive above the 10 ng/L limit[8].

Water Treatment Options
Although highly soluble in water and easily transported in the environment, NDMA does not bioaccumulate and is not persistent in the environment. In air, NDMA breaks down within minutes upon exposure to sunlight and is believed to break down within a few months in the soil subsurface. Little is known about the degradation of NDMA in water, however. Municipal treatment of drinking water contaminated with NDMA is complicated due to the low levels of the contaminant that must be monitored. Numerous cities in the U.S. have implemented NDMA monitoring and studies are underway to better evaluate the treatment processes and environmental precursors associated with NDMA.

The compound does not rapidly degrade in the environment and absorbs poorly to media and thus passes easily through soils and activated carbon. Ultraviolet light can be used to degrade NDMA by breaking the structural bonds of the compound and sunlight can reduce NDMA in the environment. In addition, reverse osmosis has been shown to remove NDMA from drinking water at an efficiency of about 50%. In recent decades, potable water reuse has gained in popularity around the U.S. as we face the challenges of decreased potable water supplies and increasing demand. Contaminants will continue to emerge as we change processes and increase exposure potentials. It is important to continue research efforts to evaluate the health impacts of these emerging contaminants, realizing that conventional treatment of drinking water and wastewater does not remove all contaminants of concern. Point-of-use treatment technologies may help to reduce exposure risks from the source that travel to the tap.


  1. Mitch, WA et al., (2003) N-nitrosodimethylamine (NDMA) as a drinking water contaminant: a review. Environmental Engineering Science. 20: 389-404
  2. Siddiqui, M and Atasi, K. (2001) NDMA occurrence and formation- a review. Proceedings of the 2001 Annual American Water Works Association Conference, Washington, D.C.
  3. ASTDR (1989) Public Health Statement for n-Nitrosodimethylamine, Agency for Toxic Substances and Disease Registry (ATSDR). Atlanta, GA
  4. IPSC (International Programme on Chemical Safety), (2002)
  5. OEHHA (2006) Public health goals for chemicals in drinking water, N-nitrosodimethylamine. December.
  6. Water Quality (2005) NDMA
  7. Bradley, PM et al., (2005) Biodegradation of N-nitrosodimethylamine in soil from a water reclamation facility. Bioremediation Journal. 9: 115-120.
  8. Califormia Department of Public Health (2002) Studies on the occurrence of NDMA in drinking water.

About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected].


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