Specifies a classification system for corrosivity of atmospheric environments. Defines corrosivity categories of atmospheric environments accounting for ISO 9223; describes the determination of corrosivity assessing the mass loss of standard specimens after the first year of exposure; estimates the corrosivity of an environment based on knowledge of local conditions (or specific data that characterize them), where an experimental determination is not possible. The classification system contributes to the knowledge and comparison of corrosivity of atmospheric environments at different locations; the choice of materials and corrosion protection measures. Cannot determine corrosivity categories from exposure periods less or greater than the first year.