The common medium used for heat distribution is water, but steam is also used. The advantage of steam is that in addition to heating purposes it can be used in industrial processes due to its higher temperature. The disadvantage of steam is a higher heat loss due to the high temperature. Also, the thermal efficiency of cogeneration plants is significantly lower if the cooling medium is high temperature steam, causing smaller electric power generation. Heat transfer oils are generally not used for district heating, although they have higher heat capacities than water, as they are expensive, and have environmental issues.