Let's start with town gas, which no longer exists. It was produced from coal gasification and contained a fairly high proportion of toxic carbon monoxide, see page 74. Town gas was available until around the end of the 1970s, and in West Berlin until the mid-1990s. It was gradually converted to natural gas, which was not quite so toxic. To do this, the incinerators had to be rebuilt, and different seals and valves were needed. However, the name "town gas" is still present in the population, which is why we still call our sensor for combustible gases the town and natural gas sensor (SE).