ISO 361:1975

Specifies shape, proportions, application, restrictions on the use of the symbol (possibly accompanied by additional symbols or words). Shall be used to signify the actual or potential presence of ionizing radiation (including gamma and X-rays. alpha and beta particles, high-speed electrons, neutrons, protons and other nuclear particles, but not sound waves and other types of electromagnetic waves). Does not specify the radiation levels at which it is to be used.