Ammonia (NH3) is a compound of nitrogen and hydrogen that is used to produce pharmaceuticals, household cleaning products, and fertiliser. It is a colourless gas with a distinct unpleasant smell. When a character smells ammonia in a story, it is often an indication that some kind of clean-up operation is in progress. However, in this story ammonia might also be one of the smells of the atmosphere in the outside world. Ammonia has been suggested as a possible alternative to fossil fuels, so may have been one of the solutions used during the ‘Alternative Energy Source century’ that seems to have destroyed the environment in Rick’s world (see bookmark for page 87).