When we think of climate change, we may consider extreme weather events – record-breaking heatwaves, heavy downpours and devastating floods. But have you considered that these changes could also increase your risk of exposure to certain viruses? We now live in world where extreme weather events are common. With the increasing frequency of prolonged rainstorms and heatwaves, climate change may raise the likelihood of being exposed to sewage-associated viruses in rivers, lakes and coastal waters. Intense rainstorms can also result in rainwater overloading urban sewer systems. As a result, raw untreated sewage is released into rivers, lakes and coastal waters. Newly published research I have collaborated on shows that sewage-associated viruses can persist for days in certain weather conditions, raising health risks for people exposed to untreated outfall. Raw sewage contains human urine and excrement, and carries with it a rich load of dead cells, food waste, pharmaceuticals, bacteria and viruses. Although most viruses shed by humans are relatively harmless, people infected with disease-causing viruses – such as enterovirus and norovirus – can shed billions of viral particles each time they go to the toilet. Even after the disease symptoms have passed, people can still shed the viruses in large amounts when they use the lavatory. These are then released into the sewer system, flowing through the network until they reach the sewage treatment plant. Typical wastewater treatment practices used in the UK are more than 99% effective at removing viruses. But despite this efficiency, treated wastewater discharged into the environment still poses some risk. Consequently, every day of the year, our rivers, lakes and seas receive potentially harmful viruses. However, the release of raw untreated sewage represents a much more severe risk. Health risks from extreme weather While the UK is prolonged periods of rain, climate projections also predict more heatwaves like those in 2022, where temperatures reached record highs. It’s important for scientists to understand how these extreme weather events will influence the sewage-associated viruses discharged into the environment. We already know that recreational water users are exposed to harmful pathogens as a direct result of sewage discharge and agricultural runoff entering bodies of water. What remains unclear is how weather conditions affect a virus’s ability to infect people – and whether climate change could make these risks worse. Understanding this is crucial to tackling the growing health challenges posed by extreme weather and sewage contamination. To explore these questions, scientists need reliable ways to detect potentially infectious viruses in the environment. This isn’t easy. Environmental samples often contain contaminants including various chemicals and bacteria, which can interfere with standard laboratory testing methods. These challenges make it difficult to accurately assess the presence and potential risk of harmful viruses in water sources. To address these challenges, my colleagues and I developed methods that filter out viruses too damaged to cause infection. This approach ensured our data focused solely on the potentially infectious viruses in each sample. Our methods also allowed us to identify multiple types of virus simultaneously, making the process more efficient and comprehensive. We conducted a series of experiments to investigate how climate change affects sewage-associated viruses, and the risks they pose to human health. These experiments were designed to simulate both short-term weather events, such as storms, and long-term changes including rising temperatures. We introduced sewage-associated viruses, such as adenovirus and norovirus, into samples of river, estuary and seawater, and tracked how they degraded over two weeks. In one experiment, we exposed the samples to different temperatures, while in another, we simulated sunlight exposure. At various intervals, we measured the levels of intact, potentially infectious viruses to monitor their decline. From this data, we calculated “T90 decay rates”, which is the time it takes for viral loads to drop by 90%. These rates were measured separately for viruses still capable of causing infections, and for those in all stages of decay. Interestingly, we found that the type of water – river, estuarine or sea – had little effect on how long viruses remained infectious or detectable in our analyses. Enteric viruses – those that cause stomach upsets – stayed infectious in seawater for up to three days at temperatures as high as 30C. At cooler temperatures, they lasted even longer, persisting for up to a week. When exposed to sunlight, viruses in water survived less than 24 hours on a sunny day. But on cloudy days, they remained viable for around 2.5 days. These findings demonstrate the significant health risks posed by sewage-associated viruses in