There are a few types of CO2 that can be used for SodaStream, but not all of them. The most common type of CO2 that is used is called "food grade" or "beverage grade" CO2. This type of CO2 has to meet certain purity standards set by the FDA in order to be considered food safe. It's important to use food grade CO2 because if you use a lower quality CO2, it can contaminate your drinks and make them unsafe to consume. There are other types of CO2 that can be used for SodaStream, but they are not as common and may not be as easy to find. One example is "industrial grade" CO2 which is often used in welding and other industrial applications. This type of CO2 is not as pure as food grade CO2 and should only be used if you cannot find any other option.