29 CFR 570.57: Exposure to radioactive substances and to ionizing radiations (Order 6).

(a) Finding and declaration of fact. The following occupations involving exposure to radioactive substances and to ionizing radiations are particularly hazardous and detrimental to health for minors between 16 and 18 years of age:

(1) Any work in any workroom in which (i) radium is stored or used in the manufacture of self-luminous compound, (ii) self-luminous compound is made, processed, or packaged, (iii) self-luminous compound is stored, used, or worked upon, (iv) incandescent mantles are made from fabric and solutions containing thorium salts, or are processed or packaged, (v) other radioactive substances are present in the air in average concentrations exceeding 10 percent of the maximum permissible concentrations in the air recommended for occupational exposure by the National Committee on Radiation Protection, as set forth in the 40-hour week column of table one of the National Bureau of Standards Handbook No. 69 entitled “Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and in Water for Occupational Exposure,” issued June 5, 1959.

(2) Any other work which involves exposure to ionizing radiations in excess of 0.5 rem per year.

(b) Definitions. As used in this section:

(1) The term self-luminous compound shall mean any mixture of phosphorescent material and radium, mesothorium, or other radioactive element;

(2) The term workroom shall include the entire area bounded by walls of solid material and extending from floor to ceiling;

(3) The term ionizing radiations shall mean alpha and beta particles, electrons, protons, neutrons, gamma and X-ray and all other radiations which produce ionizations directly or indirectly, but does not include electromagnetic radiations other than gamma and X-ray.