41 C.F.R. § 50-204.36 – Radiation standards for mining.

(a) For the purpose of this section, a “working level” is defined as any combination of radon daughters in 1 liter of air which will result in the ultimate emission of 1.3 × 10 ⁵ million electron volts of potential alpha energy. The numerical value of the “working level” is derived from the alpha energy released by the total decay of short-lived radon daughter products in equilibrium with 100 pico-curies of radon 222 per liter of air. A working level month is defined as the exposure received by a worker breathing air at one working level concentration for 4 1/3 weeks of 40 hours each.
(b)
1. (1) Occupational exposure to radon daughters in mines shall be controlled so that no individual will receive an exposure of more than 2 working level months in any calendar quarter and no more than 4 working level months in any calendar year. Actual exposures shall be kept as far below these values as practicable.
2. (2) In enforcing this section, the Director of the Bureau of Labor Standards may at any stage approve variations in individual cases from the limitation set forth in paragraph (b)(1) of this section to comply with the requirements of the Act upon a showing to the satisfaction of the Director by an employer having a mine with conditions resulting in an exposure of more than 4 working level months but not more than 12 working level months in any 12 consecutive months that (i) under the particular facts and circumstances involved the working conditions of the employees so exposed are such that their health and safety are protected, and (ii) the employer has a bona fide plan to reduce the levels of exposure to those specified in paragraph (b)(1) of this section as soon as practicable, but in no event later than January 1, 1971.
3. (3) Whenever a variation under paragraph (b)(2) of this section is sought, a request therefor should be submitted in writing to the Director of the Bureau of Labor Standards, U.S. Department of Labor, Washington, DC 20210, within 90 days following the end of the calendar quarter or year, as the case may be.
(c)
1. (1) For uranium mines, records of environmental concentrations in the occupied parts of the mine, and of the time spent in each area by each person involved in underground work shall be established and maintained. These records shall be in sufficient detail to permit calculations of the exposures, in units of working level months, of the individuals and shall be available for inspection by the Secretary of Labor or his authorized agents.
2. (2) For other than uranium mines and for surface workers in all mines, paragraph (c)(1) of this section will be applicable: Provided, however, That if no environmental sample shows a concentration greater than 0.33 working level in any occupied part of the mine, the maintenance of individual occupancy records and the calculation of individual exposures will not be required.
(d)
1. (1) At the request of an employee (or former employee) a report of the employee's exposure to radiation as shown in records maintained by the employer pursuant to paragraph (c) of this section, shall be furnished to him. The report shall be in writing and contain the following statement:
  This report is furnished to you under the provisions of the U.S. Department of Labor, Radiation Safety and Health Standards (41 CFR 50-204.36). You should preserve this report for future reference.
2. (2) The former employee's request should include appropriate identifying data, such as social security number and dates and locations of employment.