Summary: We have received many inquiries lately about the water contaminant NDMA and whether it poses a health risk in filtered water. The answer is no, based on levels found in the recent studies, there is no significant cancer risk from NDMA in drinking even unfiltered tap water.
Why is NDMA in the news recently?
Recent news articles focused on a Tsinghua University School of Environment study which listed levels of the chemical NDMA found in Chinese water. The majority of both Chinese and English media articles misconstrued the original study findings, with the worst articles giving the false impression that this is a significant finding of a major cancer risk. Additional misinformation was presented regarding international guideline levels and the ability of various filtration technologies to remove NDMA.
Most of the inquiries we received were from the ChinaWire wechat posting which was copied from a China Daily article “Cancer agent found in 44 cities’ drinking water”
To clear up the confusion surrounding the matter, we will address recent questions. To simplify things, this article will list all values in nanograms per liter (ng/L)
1 ug (microgram) = 1000 ng (nanogram)
What is NDMA?
NDMA (N-Nitrosodimethylamine) is a chemical, usually formed in water supplies from the interaction of chloramine (used as a water disinfectant in many Chinese cities) with nitrogen compounds in water, or with equipment (such as rubber seals) in the water distribution system. See the Resources & References section at the end of this article for more information.
Is NDMA dangerous?
Yes, NDMA is associated with cancer risk in humans, based on animal studies. There are also cases of individual developing cancer after being given massive doses of NDMA. However, risk is based on concentration. At the levels found in Chinese tap water, there is no significant cancer risk from NDMA.
What NDMA levels were reported in the recent Chinese study?
The following is paraphrased from the original study, with a focus on tap water: Nitrosamine levels were measured in 164 water samples of finished water, tap water, and source water from 23 provinces, 44 cities from large cities to small towns, and 155 sampling points all over China over the previous three years. NDMA was found in 41% of the tap water samples. The average NDMA concentration in tap waters was 13 ng/L, across all tap water samples, with a mean concentration of 32.3 ng/L among the detected samples.
What are the actual guideline levels for NDMA?
WHO guidelines are 100 ng/L. Japan and Australia also adopted 100 ng/L, Canada is the lowest at 40 ng/L. China and the US governments have not yet adopted national guidelines (but certain US states have). The media articles incorrectly implied that California has a maximum permitted level of 10 ng/L. Actually that is California’s Notification Level which simply means that at 10 ng/L, it is recommended (not required) that the water utility inform the consumers, either in an annual report or other method, that NDMA levels have exceeded 10 ng/L. The Response Level is when it is recommended (not required) that the utility change their supply water, or make other steps to notify the public. The Response Level for NDMA in California is 300 ng/L.
What is the actual risk of NDMA at the Chinese study levels?
The Canadian guidelines of 40 ng/L are the closest to the Chinese study averages of 32.3 ng/L, so I will use them to approximate the study risks. Where did the Canadian guidelines come from? First, an analysis was done of the rat cancer studies to find a conversion to calculate risks to human. Conservative adjustments (allometric scaling) were made, acknowledging the high toxicity of NDMA and the possibility that it may be more damaging to humans than to rats. Human exposures were based on an average human adult bodyweight of 70kg, drinking 1.9 liters of contaminated water per day, over the course of 70 years. The resulting calculations determine that NDMA drinking water contamination levels of less than 40 ng/L (drinking 1.9 L per day over 70 years) will only produce a 10^-5 (1 in 100,000) increased cancer risk, which is defined by the government as “essentially negligible”. WHO followed the same calculations but didn’t using allometric scaling, resulting in a higher guideline of 100 ng/L. California law also defines “no significant risk” as 10^-5 with their resulting calculations arriving at 30 ng/L. From the standards we can see that the Chinese study’s mean concentration of 32.3 ng/L holds no significant risk according to even the most stringent current international standards. The average of 13 ng/L across all tap water samples is close to a 1 in a million increased cancer risk which is considered de minimis or “essentially zero” by the US EPA.
OK, so the study NDMA averages show no real risk. Will my water filters remove NDMA anyway?
The WHO states:
“The most common process for NDMA removal is UV irradiation. A concentration below 0.005 µg/litre should be achievable by UV irradiation provided that the water is not grossly contaminated. NDMA is not removable by air stripping, activated carbon adsorption, reverse osmosis or biodegradation.”
Reverse osmosis has partial removal in some studies, but will almost certainly not work in China due to the use of chloramines (which are a major cause of the NDMA in the first place), the resulting RO membrane degradation, localized hard water and high sediment levels which cause RO membrane fouling.
All the reference sources say that high powered UV will work for NDMA removal. Is there a version for my home?
UV at an intensity of 1000 mJ/cm2 will remove ~90% of NDMA. Most non-municipal UV systems have a target intensity of 30 to 40 mJ/cm2 for disinfection. However, oversizing a UV system to match water flow rates will allow you achieve 1000 mJ/cm2 intensity. For example, the Viqua E4 system (when used with a 1.89 liter per minute drinking filter) will provide approximately 1280 mJ/cm2 to 1440 mJ/cm2. Please contact us for more information.
Should I switch to bottled water from filtered water?
In my opinion, the main water concern in China should be about cancer-causing chemicals from agricultural, industrial and water disinfection sources. The activated carbon block technology we recommend for water filtration is recognized as the best possible technology for removing these chemicals. A properly certified carbon technology filter is recommended by the U.S. National Institutes of Health President’s Cancer Panel Report as one of the top ways to reduce environmental cancer risk:
Filtering home tap or well water can decrease exposure to numerous known or suspected carcinogens and endocrine-disrupting chemicals. Unless the home water source is known to be contaminated, it is preferable to use filtered tap water instead of commercially bottled water.
Bottled water, regardless of brand, has the potential to contain many chemicals and contaminants that will be removed by a proper filtration system. NDMA at found levels is not a cancer risk and changing to bottled water based on the news media reports would be a wrong decision.
Resources and References
Wikipedia has a simple summary https://en.wikipedia.org/wiki/N-Nitrosodimethylamine. The WHO is an objective source regarding conservative guideline values (100 ng/L) and general information http://www.who.int/water_sanitation_health/dwq/chemicals/ndmasummary_2ndadd.pdf. Canada, once again a world leader in progressive and aggressive guidelines (40 ng/L) to protect its citizens, has a particularly informative page on NDMA
http://healthycanadians.gc.ca/publications/healthy-living-vie-saine/water-nitrosodimethylamine-eau/index-eng.php Australia has an excellent fact sheet as well www.waterra.com.au/publications/document-search/?download=746 as does the US EPA https://www.epa.gov/sites/production/files/2014-03/documents/ffrrofactsheet_contaminant_ndma_january2014_final.pdf