Standard Interpretations - Table of Contents Standard Interpretations - Table of Contents
• Standard Number: 1910.1096; 1910.1096(c)(1); 1926.53

December 28, 2009

Mr. Thomas Van Hooser
6850 NW 2nd Ave., #29
Boca Raton, FL 33487

Dear Mr. Van Hooser:

Thank you for your June 16, 2006, letter to the Occupational Safety and Health Administration (OSHA). Your letter was referred to OSHA's Directorate of Enforcement Programs for a reply to your questions on exposures to radon for persons working in trenches or excavations. We apologize for the delay in our response. This reply letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any question not detailed in your original correspondence. Your question and our reply are below.

Question: You provided us with a copy of a July 2002 fact sheet on radon from the U.S. Environmental Protection Agency (EPA), and you then asked, "What is OSHA's position on radon as a hazard to personnel who work within an excavation or trench? Is radon a recognized excavation hazard to personnel, and, if so, where does OSHA address radon hazard control measures in the excavation standards?"

Reply: Radon is a naturally-occurring, colorless, odorless, tasteless, radioactive gas. It is formed from the natural radioactive decay of uranium and thorium found in rocks, soil, and water. Radon is considered a known human carcinogen. As the EPA radon fact sheet you provided points out, exposure to radon and its decay products increases the risk of lung cancer.

OSHA's Ionizing Radiation standards (29 CFR 1910.1096 and 1926.53) address radioactive materials such as radon. Generally, where radon concentrations disperse into the open environment and are not "artificially enhanced," exposures are likely to be well below the permissible exposure limit (PEL) in the Ionizing Radiation standards.1 However, where radon concentrations are confined, enclosed, or restricted within a space, such as a building, mine2, cave3, or tunnel, their containment artificially enhances or increases the radon concentration level. Because natural ventilation dilutes radon gas levels, construction employees working at open air excavation or trenching work sites are not likely to be exposed to hazardous concentrations of radon except in rare circumstances. However, employees working in underground or enclosed construction worksites may be exposed to radiation where the ground emits radon and the gas concentration accumulates to hazardous levels. When an employee occupies an area with a high concentration of radon, the risk of lung cancer increases in proportion to the amount of time exposed and the level of radiation.

Radiation hazards from radon in such work environments may be addressed, if necessary, within the scope of the provisions for "hazardous atmospheres" in OSHA's Excavations standards at 29 CFR Part 1926, Subpart P. Specifically, §1926.651(g), Hazardous Atmospheres, requires employers to test for hazardous atmospheres before employees enter excavations greater than four feet in depth. Subsection 1926.651(g)(1)(i) provides examples of hazardous atmospheres, including, but not limited to, oxygen deficiency and air contaminants in landfill areas or by storage areas for hazardous materials. In most cases, construction employers would not be expected to test for radon unless excavations are enclosed or made in geographic areas known to contain high radon levels, and the employer, through exercise of reasonable diligence, knows or has reason to know that accumulations of radon are likely to reach hazardous levels.4 If radon exposures are hazardous, then employers must follow the requirements addressing hazardous atmospheres. Control measures are specified in §1926.651(g)(1)(ii), which requires employers to take adequate precautions to prevent employee exposure to hazardous atmospheres, including providing proper respiratory protection or ventilation in accordance with 29 CFR 1926 Subparts D and E, respectively.

To determine whether a given radon level may result in hazardous exposures to radiation, OSHA's standards for ionizing radiation, 29 CFR 1926.53 and 29 CFR 1910.1096, apply. Specifically, §1910.1096(c)(1) of OSHA's Ionizing Radiation standard, which the Agency issued in 1971, requires employers to limit radiation exposures to the levels specified in the Nuclear Regulatory Commission (NRC) radiation standards in effect at the time (10 CFR Part 20, Appendix B, Table I and II). Table 1 of Appendix B to Part 20 sets the maximum permissible concentration for radon-222 at 100 picoCuries per liter (pCi/L).5 The OSHA standard requires employers to ensure employees are not exposed to radon in excess of the average concentration for 40-hours in any workweek of seven consecutive days (§1910.1096(c)(1)). For additional discussion on the permissible radon concentration, see OSHA letter of interpretation to Ms. Connie DeWitte, December 23, 2002.

OSHA also has other standards besides the Excavation standards, including 29 CFR 1926.800, Underground Construction, and 29 CFR 1926.65, Hazardous Waste Operations and Emergency Response, which contain similar provisions to address control of employee exposures to hazardous air contaminants, including radioactive gases such as radon.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA requirements are set by statute, standards, and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at If you have any further questions, please feel free to contact the OSHA Office of Health Enforcement at (202) 693-2190.


Richard E. Fairfax, Director
Directorate of Enforcement Programs

1Memorandum for Harvey Harris, Director, Office of Training and Education, from Patricia K. Clark, Acting Director, Directorate of Compliance Programs, August 16, 1989, concerning interpretation and policy determinations for OSHA's Ionizing Radiationstandard (29 CFR 1910.1096). [ back to text ]

2 The Mine Safety and Health Administration (MSHA) standard, 30 CFR 57.5037, Radon Daughter Exposure Monitoring, provides radon monitoring requirements and radiation exposure limits for miners.[ back to text ]

3OSHA's Region VII created an alliance in 2003 with the Ozark Underground Laboratory in Protem, MO, to assist show cave employers with reducing radon exposures for employees who lead cave tours. [ back to text ]

4See, OSHA letter of interpretation to the Honorable John McCain, April 17, 1991. [ back to text ]

5 The NRC updated 10 CFR Part 20 after OSHA adopted 29 CFR 1910.1096 in 1971. NRC's updated derived air concentration for radon-222 is now 30 pCi/L, however, OSHA has not revised its standard since it was adopted so the enforceable limit remains at 100 pCi/L [ back to text ]

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