E10 gas is a type of fuel that contains 10% ethanol and 90% gasoline. It is approved by the U.S. Environmental Protection Agency (EPA) for use in any conventional, gasoline-powered vehicle. E10 is being touted as a greener alternative to regular unleaded gasoline because it contains bioethanol, which is a renewable fuel source. The use of E10 was spurred by the Clean Air Act Amendments of 1990, which mandated the sale of oxygenated fuels in areas with unhealthy levels of carbon monoxide. Today, E10 is sold in every state and is the most commonly available fuel. However, there are some concerns about the compatibility of E10 with certain vehicles. As of 2021, there could be as many as 600,000 vehicles on the road that are not compatible with E10. One of the areas that E10 can have a negative impact on is fuel system seals and gaskets, which are often made of rubber on older vehicles, a material that ethanol will degrade much faster than petrol. The UK government has published an online tool for checking if a vehicle is compatible with E10. If a vehicle is not compatible with E10, it is best to avoid using it. If E10 is accidentally filled in an incompatible car, it is recommended to not leave the fuel sitting in the tank and fuel system for too long and to dilute the E10 with E5 fuel at around half a tank.