Abstract: A plastic bag of polyolefin material such as polyethylene, for packaging materials, comprising particles of less than 50 .mu.m, and a closed bag containing such materials and a foil material for such a bag.The foil wall of the bag is provided with venting apertures with smooth edges, obtained by laser radiation, having a maximum size of 50-100 .mu.m; the distance between the venting perforations is such that the tensile strength of the foil is substantially the same as the tensile strength of the similar non-perforated foil.In a low density polyethylene foil of a thickness of 130-190 .mu.m distance between the perforations of 80 .mu.m is more than 20 mm, in a linear low density polyethylene foil of about 50-110 .mu.m the perforation distance is at least 5 mm.The bag may consist of two perforated foil layers, the perforations being staggered with respect to each other.