(File photo)

Traces of a potentially cancer-causing chemical known as NDMA was detected in the drinking water of 44 Chinese cities, while samples from the east and south were found to have relatively high concentrations, according to a new study.

Measured in nanograms per liter, NDMA－short for N-nitrosodimethylamine－is a byproduct of the disinfection process used for chlorinating drinking water and is considered an "emerging contaminant", together with other related chemicals known as nitrosamines.

Samples were taken from 155 sites in 44 cities across 23 provinces, including original sources, finished water from treatment plants and tap water. The average NDMA concentrations－11 ng per liter for finished water, and 13 ng per liter for tap water－is nearly four times what was reported in the United States in 2012.

The two figures reached 27 ng/L and 28.5 ng/L in Yangtze River Delta areas, posing a digestive cancer risk for residents there, lead researcher Chen Chao, an associate professor at Tsinghua University's School of Environment, was quoted as saying by Science and Technology Daily on Friday.

"The NDMA concentration in drinking water is a pressing issue that demands more research and systematic modifications," he said.

NDMA and other nitrosamines cause cancer in laboratory animals, according to the World Health Organization. Currently, the substance is not regulated as a drinking water quality standard in China. No maximum level has been set.

Chen said it may be included soon as a standard for water safety.

The WHO sets a cap on NDMA in drinking water at 100 ng/L. The standard in Canada is 40, while in Australia it's 100.

The states of Massachusetts and California in the US require a stricter standard－10 ng/L.

"Industrial and sewage contamination might be blamed for high levels," Chen said.

Chen Wanqing, director of the National Central Cancer Registry, confirmed a link between nitrosamines and human cancers, but stressed that cancer would be only caused by high levels.

"The trace found in the water cannot lead to cancer via drinking tap water," he said. Also, boiling tap water further lowers the NDMA concentration.

NDMA has also been detected in foods such as pickles, cured fish and tobacco smoke, he said, adding that unlike heavy metals, NDMA is unlikely to accumulate in the body.