ISO 9223:2012 establishes a classification system for the corrosivity of atmospheric environments. It defines corrosivity categories for the atmospheric environments by the first-year corrosion rate of standard specimens, gives dose-response functions for normative estimation of the corrosivity category based on the calculated first-year corrosion loss of standard metals, and makes possible an informative estimation of the corrosivity category based on knowledge of the local environmental situation. ISO 9223:2012 specifies the key factors in the atmospheric corrosion of metals and alloys. These are the temperature-humidity complex, pollution by sulfur dioxide and airborne salinity. The temperature-humidity complex can be evaluated in terms of time of wetness. Corrosion effects of other pollutants (ozone, nitrogen oxides, particulates) are not considered decisive in the assessment of corrosivity according to ISO 9223:2012. ISO 9223:2012 does not characterize the corrosivity of specific service atmospheres, e.g. atmospheres in chemical or metallurgical industries.