Dangerous levels of a cancer-causing agent have been found in the tap water of 44 Chinese cities.

Traces of N-nitrosodimethylamine, or NDMA for short, and related chemicals known as nitrosamines were found in samples taken from 155 sites in 23 provinces across China.

NDMA is a byproduct of the disinfection process used to chlorinate drinking water.

Chen Chao, an associate professor at Tsinghua University’s School of Environment, says that residents of the Yangtze River Delta area face higher than average cancer risks due to elevated levels of NDMA in the region’s treated water and tap water, which average 27 ng/L and 28.5 ng/L, respectively.

“Previous epidemiological studies have shown regional cases of cancer that share the presence of nitrosamines,” said Chen.

Additionally, the World Health Organization (WHO) has found NDMA and related nitrosamines to cause cancer in laboratory animals.

China has no regulations in its national drinking water standards for NDMA and other nitrosamines. By contrast, Canada mandates that its tap water must have a NDMA level below 40 ng/L, whereas the US states of California and Massachusetts have set a maximum tolerance of just 10 ng/L. The WHO recommends a limit of 100 ng/L.

Chen Wanqing, director of the National Central Cancer Registry, confirmed that nitrosamines play a role in causing cancer, but dismissed the idea that the levels in Chinese drinking water pose a threat to public health. “The trace found in the water cannot lead to cancer via drinking tap water,” he said. Chen noted that boiling tap water further reduces the risk of exposure to NDMA.

Another scientific study from last year found that one out of ten bottled drinking water samples from eastern China had a NDMA concentration of at least 4.8 ng/L.

Traces of NDMA is typically found in cured and smoked foods such as pickled vegetables and cured fish. A 1982 study found that 20 of 26 domestic Chinese beers were contaminated with NDMA.

In 2013, a Fudan University student was poisoned to death with NDMA by his roommate, a medical student who had written numerous papers on NDMA.