Comparative formation of disinfection byproducts and human health risk under different chlorinated disinfectant types and water matrices

Water treatment relies on a simple, powerful idea: add a little chlorine to kill the germs that make us sick. For decades, this strategy has been the backbone of public health, turning dangerous water into something safe to drink and swim in. But chemistry is rarely one-sided. When chlorine meets the natural organic matter in water, or the sweat and urine swimmers inevitably introduce, it doesn't just vanish. It transforms, creating a new set of chemicals called disinfection by-products. These by-products are the unintended side effects of keeping water clean, and while they are necessary to prevent disease, some of them can pose their own risks to human health. The challenge for scientists and regulators is to understand exactly how these chemicals form, how people are exposed to them, and whether the specific type of chlorine used makes a difference in the final risk.

A recent study by Yeonjeong Ha at Pyeongtaek University in South Korea takes a close look at this complex balance. The research compares two very different ways of adding chlorine to water. On one side are the conventional methods used in most drinking water plants, which rely on chlorine gas or liquid solutions like sodium hypochlorite. These release chlorine immediately and powerfully. On the other side are solid disinfectants, often seen in swimming pools, which contain cyanuric acid. This acid acts as a stabilizer, holding onto the chlorine and releasing it slowly over time, which is useful for maintaining a steady level of protection in a busy pool. The study asks a straightforward question: does the choice between these two methods change the types and amounts of harmful by-products that end up in the water, and does it change the risk to the people using that water?

To answer this, the researchers gathered data from dozens of previous studies, looking at real-world measurements from both swimming pools and tap water. They focused on two main groups of by-products: trihalomethanes and haloacetic acids. These are the most common chemicals formed when chlorine reacts with organic matter, and they are the ones regulators watch most closely. The team also looked at two other substances that are less studied but potentially important: nitrogen trichloride, a gas that gives indoor pools their distinct smell and can irritate the lungs, and cyanuric acid itself, which accumulates in pools over time. By comparing the concentrations of these chemicals across different water types and disinfectant methods, the researchers could build a clearer picture of where the risks lie.

The findings confirm what many experts suspected but needed data to prove: swimming pools are a much more intense environment for chemical reactions than drinking water. Because swimmers constantly add organic matter to the water and the chlorine stays in the pool for long periods, the concentration of by-products in pools is significantly higher than in tap water. However, the type of disinfectant used made a surprising difference. When the researchers looked at pools using the cyanuric acid-stabilized solids, they found that the formation of brominated by-products—chemicals that contain bromine and are often more toxic than their chlorine-only cousins—was substantially reduced. In contrast, pools treated with standard chlorine gas or liquid solutions showed higher levels of these brominated compounds. This suggests that the stabilizing acid might actually change the chemical pathways in a way that suppresses the creation of the most dangerous by-products.

The study also examined how people come into contact with these chemicals. It turns out that the way we get exposed depends heavily on the setting. In swimming pools, the most significant way people absorb these volatile chemicals is by breathing them in. The air above the water contains high levels of these gases, and for swimmers, inhalation accounts for nearly all of their exposure. In contrast, for people using tap water at home, the main route of exposure is drinking the water, though breathing in the steam during a shower also plays a major role for the more volatile chemicals. When the researchers calculated the potential health risks, they found that while most non-cancer risks were low, there was a measurable chance of cancer from long-term exposure to these chemicals, particularly in tap water scenarios where people drink the water daily.

Interestingly, the use of cyanuric acid-stabilized disinfectants was linked to lower estimated cancer risks compared to conventional chlorine. This aligns with the finding that these stabilizers reduce the formation of the more toxic brominated compounds. However, the study also highlighted significant gaps in our knowledge. For nitrogen trichloride, the gas that causes that sharp pool smell, there is very little toxicological data available to determine a safe breathing limit. Similarly, while cyanuric acid is widely used, the scientific community has not yet agreed on a reliable safety limit for how much of it a person can safely ingest over a lifetime. The existing guidelines vary wildly, making it difficult to assess the true risk of this common pool additive.

Ultimately, this research provides a more nuanced view of water safety. It suggests that the choice of disinfectant is not just a matter of logistics or cost, but a factor that can influence the chemical profile of the water and the health risks associated with it. While cyanuric acid-stabilized solids appear to lower the risk of forming certain toxic by-products in pools, the study underscores that we still need better data on the long-term effects of the chemicals we are already using. The work serves as a reminder that keeping water safe is a continuous balancing act, where every chemical choice has a ripple effect that reaches far beyond the pool deck or the kitchen sink.