 How dangerous is the chemical 1,4-dioxane if it gets into your water? It may seem an obscure question, but if you live somewhere where there's dioxane contamination, it's an important one. 1,4-dioxane, let's call it dioxane for short, is a colourless flammable liquid that's used as an industrial stabiliser and solvent. It dissolves readily in water and doesn't easily biodegrade, which is potentially a problem if it gets into the environment. And to make matters worse, it's considered a probable human carcinogen by the US Environmental Protection Agency. All of this is not good news if the stuff somehow gets into your water supply, but just how worried should you be if your supply is affected? According to toxicology studies, dioxane, not to be confused with a similarly named but chemically different dioxane, can damage the liver and kidneys if consumed in large quantities. However, an adult would need to be drinking a couple of microlitres of the stuff every day or around half a thousandth of a teaspoon for serious harm to occur. This is for non-cancer health risks, though. When it comes to cancer, things look somewhat different. Based on animal studies, the US EPA estimates that if an adult spends their life drinking water contaminated with just three and a half micrograms of dioxane for every litre of water, that's less than one millionth of a teaspoon. They have a one in 100,000 chance of developing liver cancer. But what does this mean in reality? The city of Ann Arbor provides a useful case study here for making practical sense of a somewhat academic risk assessment. From the 1960s to the 1980s, tens of thousands of gallons of dioxane were released into the environment to the west of the city. Aquifers underneath the city were contaminated, and some dioxane ended up in private wells. Cleanup operations are still ongoing and there's a groundwater use prohibition zone where contamination levels are excessive. There's also a fear that the contamination could reach the city's major water source, a local river in the future, and over the past few decades, some people will have been exposed to the chemical. The Michigan cleanup limit for dioxane is 85 parts per billion, or just under 20 millionths of a teaspoon per litre of water. This is the theoretical level of contamination people could be exposed to before action is taken. So a useful question is, what's the cancer risk at this level? Following the US EPA analysis, if you were to drink water contaminated with dioxane at 85 parts per billion your whole life, your chances of developing liver cancer would be around 1 in 4000. This sounds high, but it's based on some assumptions that may not be realistic, such as assuming that affected adults will be drinking 2 litres of contaminated water every day of their life for 70 years. So what if we consider a more realistic scenario? Imagine for instance that you've lived in Ann Arbor for 20 years and only drink 1 litre a day of tap water. Your highest dioxane related cancer risk in this case is closer to 1 in 30,000, still high but much lower than the 1 in 4000. Or imagine you're a teenager who drank 10 litres a week of contaminated water for 10 years before leaving for college. According to current research, your cancer risk would be closer to 1 in 35,000. These calculations do need to be treated with some care, there's still uncertainty over how 1,4 dioxane causes cancer in animals and the human risk estimates here do err on the side of caution, but they do show how a detailed scientific assessment can be used to put a number on real-world risks. However, just being able to put a number on the probability of something bad happening does raise some more challenging questions, including what you do with that number once you have it and who decides how much risk is okay, especially if someone else created the risk in the first place. These though are questions for another risk bites and another day. For more information on 1,4 dioxane, check out the links below and as always please do join the conversation in the comments.