Evidence on Water Pollutants and their Potential Impact on Cancer Risk

What are the main contaminants in drinking water and recreational water (e.g., swimming pools)? Do these contaminants increase the risk of cancer?

Air pollution is an issue that often attracts our attention, and we are generally aware of the health risks it poses. However, water pollution in developed countries is often less visible, and its impact on human health has been less extensively studied. Nonetheless, at ISGlobal, we have compiled evidence on water contaminants and their potential impact on cancer risk.

We should aim to consume between 2 and 3 litres of water per day, totalling around 1000 litres per year. Recognising the importance of this volume is crucial when considering water as a health determinant. Furthermore, water is not only ingested but also used for activities such as showering, bathing, and washing hands. As a result, exposure to certain chemicals present in the water can also occur through skin absorption or inhalation.

Contaminants resulting from human activities

Water serves as a carrier of microbes, evoking memories of John Snow, the cholera outbreak, and the iconic London fountains that marked the beginning of epidemiology. Although microbiological contamination remains a significant problem in many impoverished regions, the primary concern in our society today is the chemical contamination of drinking water resulting from human activities.

• Intensive agriculture and livestock farming contribute to water pollution through pesticides, as well as nitrates and phosphates from fertilisers and animal waste.
• Industry, landfills and incinerators also introduce chemicals and toxic waste into water sources.
• In addition, urban waste, including cleaning products, pharmaceuticals, and microplastics (e.g., from washing synthetic clothes), contributes to the contamination.
• Air pollution, especially from traffic, can also enter water through precipitation.

Water treatment typically reduces the levels of many contaminants in source water. However, new contaminants can be created during water disinfection treatment, and the materials used in distribution pipes can also contribute to contamination. Even in countries with established drinking water regulations, many of these chemicals continue to cause health concerns.

Contaminants unintentionally generated during water disinfection treatment

Water disinfection is essential to prevent infection, and chlorine, the most widely used disinfectant, has been credited with saving the most lives since its introduction in the early 20th century, along with penicillin. However, chlorine's high reactivity causes it to interact with natural organic matter, bromide, and other water precursors to form unintended by-products known as disinfection by-products (DBPs). Trihalomethanes (THMs) stand out as the most common chlorination byproduct, with chloroform and three brominated compounds (bromodichloromethane, chlorodibromomethane, bromoform) being the most common THMs. As THMs are volatile and permeable to the skin, exposure occurs not only by ingestion but also by inhalation and dermal absorption during activities such as showering, bathing, washing dishes, cooking, and swimming pool attendance.

Chloroform and bromodichloromethane are specifically classified by the International Agency for Research on Cancer (IARC) as possibly carcinogenic to humans. The health outcome most consistently associated with human exposure to THM is bladder cancer, followed by low birth weight and reproductive health effects. Importantly, these effects have been observed at THM levels well below the maximum allowable concentration of 100 µg/L.

Water contaminated with nitrates from intensive agriculture and livestock farming

Drinking water is also susceptible to nitrate contamination, mainly due to the extensive use of nitrogen-based fertilisers in intensive agriculture and intensive animal farming. While nitrates themselves are not toxic to human health, their ingestion leads to their reduction to nitrites in saliva. In the acidic environment of the stomach, nitrites can form N-nitroso compounds, such as nitrosamines, which have carcinogenic effects in various organs (based on animal data). Consumption of nitrates from drinking water has primarily been associated with stomach and colon cancer. However, there are only a few studies on other types of cancer, and the results are not yet conclusive.

In our recent research contribution, we investigated the association between exposure to THMs and the risk of chronic lymphocytic leukaemia and prostate cancer in a multicase control study in Spain. Our results suggest that:

1. An association between exposure to brominated THMs and chronic lymphocytic leukaemia
2. An increased risk of leukaemia associated with frequent use of swimming pools with high DBP concentrations
3. A possible association between exposure to nitrate in drinking water and prostate cancer, especially more aggressive tumours
4. The consumption of fruit, vegetables, and certain vitamins may prevent or reduce the risk of cancer associated with nitrate in drinking water

Importantly, these associations were observed at contaminant levels within regulatory limits.

The solution is not to switch from tap water to bottled water, but to minimise the waste that finds its way into groundwater, rivers and the sea

Is bottled water the answer?

The answer is a resounding no. While bottled water is free of THMs, it can contain nitrates that contaminate aquifers and groundwater —the primary sources of bottled water. However, the main drawback of bottled water is the production of plastic waste and the resulting environmental impact. Not only is this practice unsustainable, but from a human health perspective, it's important to recognise that there is a migration of microplastics and plastic additives such as bisphenols, phthalates, and flame retardants from the plastic packaging into the water. This migration becomes more pronounced with prolonged storage, higher temperatures and direct sunlight. The additives in plastic bottles are endocrine disruptors, with the potential to disrupt our endocrine systems at very low doses. A growing number of health problems are being linked to endocrine disruptors, from infertility to immune system disorders and metabolic diseases. So the solution is not to switch from tap water to bottled water, but to minimise the waste that finds its way into groundwater, rivers and the sea.

Future objectives and needs

The use of exposure biomarkers, the measurement of additional water chemicals beyond THMs, including emerging contaminants such as microplastics and PFAS, and the assessment of the mixtures of these contaminants are essential to address real-world scenarios. Equally important is the study of the interaction of diet and its active compounds with water contaminants.