For years, scientists have described climate change as a slowly emerging public health crisis. But for many, it’s difficult to imagine how a complex planetary phenomenon can impact personal well-being beyond the obvious effects of natural disasters, which climatologists say will happen more frequently and intensely as the world warms. That disconnect is what piqued my interest in a new study on old infrastructure, heavy rainfalls and spikes in human illness.

Drinking water quality is among the many adverse effects that climate change is expected to have on human health. But what exactly does that look like, how does it happen and how concerned should we be? This study, which was published earlier this month in Environmental Health Perspectives, doesn’t answer all those questions. But it does offer persuasive evidence that we should be worried enough to invest in preparations and adaptations that protect the basic foundations of good health, such as clean drinking water. Designed to assess the link between heavy rainfall, gastrointestinal illness and the presence of combined sewer overflows (CSOs), the study found that communities with aging infrastructures may be particularly vulnerable to drinking water contamination during extreme precipitation events. And climate change is only expected to increase the frequency and intensity of extreme rainfall.

“There’s a lot in the literature about precipitation and gastrointestinal illness, but we don’t fully understand the association,” said study co-author Jyotsna Jagai, research assistant professor in the Environmental and Occupational Health Sciences Division at the University of Illinois-Chicago School of Public Health. “Is there something about infrastructure in certain parts of the country that might be modifying the effect?”

Jagai told me that while it’s easier to understand how heavy rainfall can affect drinking water quality in countries where people gather water directly from the source, it’s not entirely clear what’s happening in more developed countries with established water treatment and regulation systems. So, to get a clearer picture, she and her research colleagues analyzed Massachusetts data on heavy rainfalls as well as emergency room visits for gastrointestinal illness between 2003 and 2007. They examined the data across three different regions: those in which CSOs impact drinking water, those where CSOs impact recreational waters and those with no CSOs at all.

But first, a little background. CSOs happen in combined sewer systems that collect rainwater runoff, sewage runoff and industrial runoff together in one pipe for transport to a water treatment facility. That means when rainfall is particularly extreme, the amount of water can overwhelm the system, resulting in untreated wastewater being discharged into bodies of water — this where the name “combined sewer overflows” comes from. More modern systems, on the other hand, have separate pipes for stormwater and sewage. Jagai’s study cites data from the U.S Environmental Protection Agency, which reports that nearly 800 U.S. communities that are home to about 40 million people are served by combined sewer systems. Jagai and co-authors Quanlin Li, Shiliang Wang, Kyle Messier, Timothy Wade and Elizabeth Hilborn write:

Studies have demonstrated that pathogen concentrations in receiving waters are higher following CSO events. A bacterial indicator of fecal contamination, Escherichia coli, was increased in recreational waters in Ontario, Canada and Lake Michigan following CSO events. Increases in concentrations of bacteria associated with sewage, including Streptococcus, Enterococcus, and several pathogenic viruses, were seen following CSO events in the Lower Passaic River. Given the potential for increased CSO events due to changes in precipitation patterns associated with climate change, it is important to better understand impacts on human health.

The researchers found that the three regions studied experienced similar rainfalls over the five-year study period; however, they only found an association between gastrointestinal emergency room visits and heavy rainfalls in regions where CSOs affected drinking water sources. Specifically, in such regions, extreme precipitation was associated with a 13 percent increase in the expected rate of gastrointestinal emergency room visits over an eight-day period for all age groups and a 32 percent increase for people older than 65 about a week after heavy rainfall. The researchers wrote that their study “is the first we know of to demonstrate” such an association.

Jagai told me that while she had a hunch they’d find an association, she was surprised at how strong it was — “I expected the signal would be harder to detect,” she said. She also noted that the recreational water region included in the study is mostly used for boating and canoeing, as opposed to swimming, so her study likely doesn’t represent the health risk of CSO events into recreational waters. Jagai also cautioned that her study used heavy rainfall as a proxy for an overflow event — in other words, she and her colleagues can’t say for sure if an overflow event occurred during the precipitation events included in the study — and so further research is needed.

Obviously, a long-term solution is to begin modernizing old wastewater and sewage collection systems, she told me, but that’s a pretty expensive proposition. In the meantime, she said public health practitioners can use her findings to better understand which communities might be at risk and when to issue water safety advisories. The study also underscores the importance of having clear lines of communication between water authorities and local public health officials.

“It’s definitely a critical area of research, as climate change scientists expect more frequent and heavier rainfall events,” she said. “If these events are going to happen more often, we need to better understand the relationship to human health.”

To read a full copy of the study, visit Environmental Health Perspectives.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.