Humidity has a direct effect on the rate of corrosion of metals, complicated by the presence of chloride ion and the nature of the chloride salt. This is of particular interest after fire, where hydrochloric acid has inevitably been generated by combustion of chlorinated plastics such as PVC. Experience and observation suggested that the rate of corrosion after exhaustion of the original hydrochloric acid would be determined by how wet the metal surface is, and by the chloride concentration on the surface; and the wetting of the surface would depend on humidity and the nature of the chloride salt.