Toxic gases may be classified as follows:
· Simple Asphyxiants—These gases displace oxygen fromthe ambient air and reduce the partial pressure of avail-able oxygen. Examples include carbon dioxide, nitrogen, aliphatic hydrocarbon gases (butane, ethane, methane, and propane), and noble gases (argon, helium, neon, radon, and xenon).
· Respiratory Irritants—These gases damage the respira-tory tract by destroying the integrity of the mucosal barrier. Examples include acrolein, ammonia, chloramine, chlorine, formaldehyde, hydrogen sulfide, methyl bromide, methyl isocyanate, oxides of nitrogen, osmium tetroxide, ozone, phosgene, and sulfur dioxide. Heavy metal-related gases also come under this category (cadmium fumes, copper fumes, mercury vapour, zinc chloride and zinc oxide).
· Systemic Asphyxiants—These gases produce significantsystemic toxicity by specialised mechanisms. Examples include carbon monoxide, cyanide, and smoke. It must be noted that systemic toxicity may also be observed in the case of some simple asphyxiants and respiratory irritants, though it is not the predominant feature.
Discussion of toxicity of the examples mentioned under the various categories now follows, while pointing out that some of them have been discussed elsewhere (consult Index).