Atmospheric corrosivity information is essential for selection of materials of construction, protective coatings and other measures used to prevent corrosion. Standards ISO 9223 - 9226 elaborated in ISO/TC 156/WG 4 are well accepted, used by corrosion engineers and introduced in other normative documents. Work on adjustment of the atmospheric corrosivity classification system is going on in the WG 4 with the aim to cover some gaps in this system. Statistical treatment of the ISOCORRAG program data and other data makes it possible to obtain a better definition of environmental influencies and specification of standard specimens. More sensitive procedures for corrosivity derivation for low aggressive indoor atmospheres have been recommended.