Ammonia (NH3) is a colourless acrid-smelling reactive gas at ambient temperature and pressure and is considered a significant public health hazard (WHO, 1986; PHE, 2019).

Carbon monoxide is a colourless, odourless gas that can be poisonous to humans and is considered a significant public health hazard (WHO, 1999).

Phosphine (PH₃) is a colourless, flammable, and explosive gas at room temperature. The major uses of phosphine are as a rodenticide and fumigant for agricultural products and in the manufacture of semiconductors for the electronics industry. Exposure to low doses causes non-specific symptoms, such as nausea, vomiting, stomach pain, diarrhoea, thirst, muscle pain, difficulty breathing and fluid in the lungs. Exposure to higher doses may cause more severe effects, even death (adapted from PHE, 2017 and CDC, 2019).

Chlorine is a reactive pale green gas with many uses including disinfection of water that is approximately three times heavier than air and has a characteristic odour similar to bleach. Most significant exposures to chlorine result from loss of containment of chlorine during storage and transport. Human exposure can result in symptoms ranging from mild irritation to rapid death related to pulmonary oedema. It is considered a significant public health hazard (adapted from IPCS, 1982 and PHE, 2019).