Gases and vapours produced, under many circumstances, have harmful effects on workers exposed to them by inhalation, being absorbed through the skin, or swallowed. Many toxic substances are dangerous to health in concentrations as little as 1ppm (parts per million). Given that 10,000ppm is equivalent to 1% volume of any space, it can be seen that an extremely low concentration of some toxic gases can present a hazard to health.

Gaseous toxic substances are especially dangerous because they are often invisible and/or odourless, and are physically harder to avoid than liquids or solids. Their physical behaviour is not always predictable: ambient temperature, pressure and ventilation patterns significantly influence the behaviour of a gas leak. Hydrogen sulphide for example is particularly hazardous; although it has a very distinctive ‘bad egg’ odour at concentrations above 0.1ppm, exposure to concentrations of 50ppm or higher will lead to paralysis of the olfactory nerves rendering the sense of smell inactive. This in turn may result in the assumption that the danger has cleared. Prolonged exposure to concentrations above 50ppm can have other symptoms and in extreme cases result in paralysis and death.

Definitions for maximum exposure concentrations of toxic gases vary according to country. Limits are generally time-weighted as exposure effects are cumulative: the limits stipulate the maximum exposure during a normal working day and for shorter periods down to 15 minutes or less.