Carbon dioxide is one of the earliest substances to have been used as a refrigerant. In the mid-19th century, when refrigeration technology was in its infancy, technicians used readily available substances, such as ether, sulphur dioxide, air, ammonia and carbon dioxide. Each substance had its advantages and disadvantages, and as compressor technology developed, ammonia came to dominate the early refrigeration market. The recent swing away from chlorine-based synthetic refrigerants has renewed interest in these early substances, and particularly in carbon dioxide, which provides exceptionally high refrigeration capacities, is relatively benign and is easy to work with.