This part of ISO 12944 deals with the classification of the principal environments to which steel structures are exposed, and the corrosivity of these environments. It - defines atmospheric-corrosivity categories, based on mass loss (or thickness loss) by standard specimens, and describes typical natural atmospheric environments to which steel structures are exposed, giving advice on the estimation of the corrosivity; - describes different categories of environment for structures immersed in water or buried in soil; - gives information on some special corrosion stresses that may cause a significant increase in corrosion rate or place higher demands on the performance of the protective paint system. The corrosion stresses associated with a particular environment or corrosivity category represent one essential parameter governing the selection of protective paint systems.