Radioactive materials, when not properly handled, present risks of irradiation and contamination; their radiations, which are capable in varying degrees of penetrating matter, are not directly perceptible by the human senses. Despite the risks, such materials have to be transported and stored so that precautions are simple and do not require ordinary transport workers to possess any special knowledge or use any special appliances. Accordingly, the regulations make safety depend essentially on the packaging of the materials.