Pregnant women exposed to low levels of disinfection byproducts in chlorinated drinking water were more likely to give birth to underweight babies, according to a recent peer-reviewed study based in Sweden. The risk increased with the levels of disinfection byproducts in the water.

Disinfectants in water systems are necessary to prevent the spread of microbial diseases like dysentery and cholera. But unintended byproducts form when the disinfectants mix with organic material like plant matter, animal waste and runoff. These disinfection byproducts are associated with cancer and harm to the developing fetus.

Low birth weight babies are more likely to experience health problems, such as difficulty breathing, bleeding in the brain, intestinal complications and increased risk of infections because of weak immune systems. Later in life, people who had low birth weight are more likely to have health problems, including diabetes, heart disease, high blood pressure, intellectual and developmental disabilities, and obesity.

The Swedish study is one of the largest research projects ever to examine the relationship between disinfection byproducts in drinking water and adverse birth outcomes. It looked at trihalomethanes, a group of four disinfection byproducts regulated in both Sweden and the U.S.

The researchers analyzed data from approximately 500,000 births over a 10-year period. They addressed many limitations common in similar studies. For example, they excluded mothers exposed to high levels of trihalomethanes from other sources, and took into account monthly fluctuations in the level of disinfection byproducts in drinking water.

The study linked the risk of low birth weight to low trihalomethane concentrations, starting at just 5 parts per billion, or ppb, equivalent to about five drops of water in an Olympic-sized swimming pool. This is worrisome for people in the U.S., where the legal limit for trihalomethanes is 16 times that amount.

The Environmental Protection Agency has set a national legal limit of 80 ppb for total trihalomethanes, or TTHM. According to utility test data reported in EWG’s Tap Water
Database, 292 million people – about 88 percent of Americans – are served by utilities delivering water contaminated with trihalomethanes. The tap water for more than 850,000 Americans has with trihalomethanes above the legal limit.

Hundreds of disinfection byproducts affect the environment and human health, but only a few, including trihalomethanes, are monitored or regulated.

As in the Swedish study, trihalomethanes are commonly used as a marker for overall exposure to disinfection byproducts, which is an effective strategy for water disinfected with the chemical hypochlorite. But a change in the disinfectant can change the overall mixture of the byproducts in the water and mask the relationship between byproduct exposure and health outcomes, because the unmonitored byproducts also contribute to health risks.

Alternative chemicals, such as chloramine, chlorine dioxide and ozone, have become more attractive for drinking water disinfection, because they tend to create lower concentrations of the regulated disinfection byproducts, including trihalomethanes. However, research shows that the use of these disinfectants actually results in increased concentrations of unregulated disinfection byproducts.

What can parents do?

  • There are a number of disinfection byproducts to watch for. Find out which have been detected in your water by searching for your water system in the EWG Tap Water Database.
  • Granular activated carbon filters are highly effective at reducing disinfection byproduct levels. Reverse osmosis systems typically have built-in carbon filtration, too. Use the EWG Water Filter Guide to choose a water filter type, especially if you’re pregnant or planning to become pregnant.
  • Safe drinking water should be accessible to all, but that requires action at the community level. Reducing runoff and source water pollution can keep disinfection byproducts from forming. Contact your local elected officials to tell them to prioritize public health and source water protection.
  • Drinking water isn’t the only source of disinfection byproduct exposure for parents and children. Poor swimming pool hygiene can result in high levels of byproducts in pool water. Protect yourself and others by practicing good pool hygiene habits.