Faulty gas appliances such as heaters and boilers can produce carbon monoxide, a deadly but colourless and odourless gas.

Most vessels have hydrogen buildup warning systems, with detectors mounted high up where the lighterthan-air and odourless gas tends to concentrate.

The supply of nitrous oxide for recreational use - where balloons of the colourless, odourless gas are inhaled to give a temporary, dizzying rush - has become increasingly common.