SINGAPORE - Scientists are studying whether a chemical compound found in minute traces in tap water here could be harmful if ingested in larger amounts.

There is currently no cause for concern, said national water agency PUB, and further research will need to be conducted to ascertain the health risks of such additives.

The synthetic chemical in question is a by-product of diphenyl-guanidine (DPG), which is usually found in rubber products, such as car tyres.  

While Singapore’s water treatment regimen removes any DPG leaching into the catchment, the scientists believe it could be leaking into tap water post-treatment – when it comes into contact with rubber parts in plumbing and reacts with chlorine in the water.

This then turns into a DPG-chlorine by-product.

Researchers from the Nanyang Environment and Water Research Institute (Newri) at Nanyang Technological University (NTU) found that the by-product caused some damage to human DNA in lab tests on human cells.

Professor Shane Snyder, Newri’s executive director, said: “The DPG-chlorine by-product is not an immediate cause for concern in Singapore, as the concentrations found in tap water here were much lower than what would be expected in other countries.  

“However, we do need to consider the long-term chronic exposure scenario as our study evaluated only a single, one-time dose to the human cells. For drinking water, our exposure should be considered daily across our lifetime. Ultimately, it is the dose and duration of exposure to these compounds that will determine one’s carcinogenic risk.”

PUB said the experimental conditions in the laboratory represent a “stress” test.

Dr Elaine Quek, PUB’s chief specialist for treatment and distribution at its water quality department, said: “These do not represent the actual conditions within Singapore’s drinking water distribution system, where the average level of disinfectant is lower, and the contact time with the tap fittings is typically transient.” 

Worldwide, countries are keeping watch on DPG and its potential impact on human health, although authorities such as the World Health Organisation (WHO) have yet to label it as a contaminant of concern.

Further studies will be needed to understand how the body metabolises the compound as it goes through the digestive system, said Prof Snyder. 

Dr Quek said further research is needed to ascertain the health risks of such compounds.

The same approach is taken by international regulatory agencies such as WHO and the United States Environmental Protection Agency (EPA) to address emerging contaminants of concern.

Both WHO and the US EPA do not have any guidelines for DPG or any of the disinfection-related by-products, due to the lack of any credible evidence. 

Explaining how the substance gets into the water system, Prof Snyder said tiny rubber particles from car tyres often wash into rivers and creeks during heavy rainfall, sometimes killing fish in the process. 

Hence, scientists globally are also closely studying the presence of DPG – and 6PPD – another chemical found in rubber products, in air and dust samples, he added. 

He and his team set out to see whether either chemical would be present in Singapore’s water catchments, since reservoirs here also collect urban run-off. For instance, the Republic hosts the Formula One race in the Central Business District every year, increasing the possibility of tyre fragments ending up in surrounding reservoirs.

Replicating PUB’s water treatment processes, the team found that the treatment system was robust enough to remove all traces of DPG and 6PPD, even if they had been present in the water.

Surprisingly though, when the researchers took water samples from taps in 20 buildings across the island, they found small amounts of DPG in all the samples.

The DPG had combined with chlorine in water to form the new by-product.

In comparison, 6PPD was found in a quarter of the samples, without any formation of obvious by-products. 

Chlorine and other types of disinfectants kill harmful bacteria and viruses, and are used to ensure that tap water remains pathogen-free and safe to drink.

The scientists believe that rubber gaskets and seals commonly found in taps had leached these chemical substances after being exposed to the chlorine in tap water when it flowed through them. 

The DPG-chlorine by-product was found to be the most harmful when tested on a human cell, said Prof Snyder. 

Boiling the tap water did not remove these chemicals, the team found. It is unclear if filtering the water will be beneficial. 

Their work was published in scientific journal Environmental Science & Technology Letters in August.

To better understand the impact on human health, the researchers are now looking to test the DPG-chlorine compound in an “artificial stomach” to see how it gets broken down or if it enters the bloodstream. This would mimic the body’s metabolism more closely, Prof Snyder added.  

WHO told The Straits Times that it does not have a guideline value for DPG, nor these disinfection by-products. 

“WHO considers establishing guideline values when there is credible evidence of occurrence of the chemical in drinking water combined with evidence of actual or potential concern for human health,” said its spokesman.

It added that the relevance of the study for actual exposure to DPG and its by-products requires further review, with concentrations of chlorine and chloramine (compounds used to disinfect drinking water) likely to be much lower in water from the tap than the concentrations used in the study.

However, the disinfection by-products are on WHO’s priority list for a future update of its guidelines for drinking-water quality, the spokesman said.

Professor Zhang Xiangru, from the Hong Kong University of Science and Technology’s Department of Civil and Environmental Engineering, said he believes that NTU’s study could stimulate additional research in Hong Kong and other places around the world.

“The exposure to DPG and 6PPD is likely the tip of the iceberg in terms of what we can discover at trace levels in drinking water,” he added. 

PUB’s Dr Quek said the DPG and chlorinated by-products were detected only at extremely minute levels in water samples, and that the likely exposures from other sources to these substances would be more relevant – such as in air and dust samples.