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 https://www.osha.gov.

July 25, 2019

[Name and address withheld]

Dear [Name withheld],

Thank you for your letter to the Occupational Safety and Health Administration (OSHA). Your letter was referred to OSHA’s Directorate of Enforcement Programs for a response. You have a question related to entry into a regulated area in manufacturing under OSHA’s Respirable Crystalline Silica (RCS) standard for general industry, 29 CFR 1910.1053. This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any question not delineated within your original correspondence. Your paraphrased question and our response are below.

Background: You explain in your letter that air sampling performed in the mezzanine of your company’s manufacturing area, where mixers are operated, demonstrated that airborne RCS concentrations were above the OSHA permissible exposure limit (PEL) of 50 micrograms per cubic meter (µg/m3) as an 8-hour time-weighted average (TWA). As a result, the mezzanine area has signage that identifies the area as a regulated area. Also, you state that the company has implemented safer work practices for material handling and is now investigating engineering controls to further reduce worker exposures to RCS.

Your question concerns your maintenance workers who need to enter the mezzanine area each morning, approximately eight to ten hours after the mixing operations have ended, to check equipment. The maintenance workers exit the area after performing their checks, leaving before the mixers begin their work.

Question: Does the RCS standard allow maintenance workers to enter a posted regulated area without using appropriate respirators, if performance of the RCS-generating tasks or operations has been suspended for the last eight to ten hours?

Reply: The RCS standard, at 29 CFR 1910.1053(e)(4), requires employers to provide each employee entering a regulated area with an appropriate respirator and to require its use while the employee is in the regulated area. The term “regulated area” is defined as an area where an employee’s silica exposure exceeds, or can reasonably be expected to exceed, the PEL. See 29 CFR 1910.1053(b). Conversely, if employee exposures in an area will not exceed the PEL, or are not expected to exceed the PEL, then the standard does not require the employer to establish and demarcate that area as a regulated area.

Regulated areas can also be associated with intermittent or temporary activities. For example, an area could be a regulated area on days when a particular silica-generating activity causes exposures to exceed the PEL. However, on other days, when that activity is not occurring and exposures do not exceed the PEL, and are not reasonably expected to exceed the PEL, employers do not need to treat the area as a regulated area. Thus, the key determination is whether employee exposure in the area exceeds the PEL or can be expected to exceed the PEL.

Paragraph (d) of the standard (29 CFR 1910.1053(d)(1)) requires employers to assess the exposure of each employee who is, or may reasonably be expected to be, exposed to RCS at or above the action level. An exposure assessment is not required if the employer has made a reasonable determination that employee exposures will remain below the AL, although the reasonable determination must be based on relevant evidence or experience that employee exposures will remain below the AL during the performance of tasks/activities. Please review OSHA’s Frequently Asked Questions (FAQs) for Occupational Exposure to RCS in General Industry on OSHA’s website.1

In your case, if the assessment of the maintenance workers’ exposures establishes that their exposures do not exceed, and are not expected to exceed, the RCS PEL during non-RCS generating maintenance activities, then the maintenance workers need not wear respirators when performing their checks in the mezzanine area—even though the area would be a regulated area during mixing operations.2 In other words, the mezzanine area where the mixers are located would become a “temporary” regulated area, as described above. That is, the mezzanine area would be a regulated area when the mixers are operational and exposures exceed the RCS PEL, but it would not need to be a demarcated, regulated area (and, thus, the standard would not require respirator use) during maintenance checks performed eight to ten hours after the silica-generating work has been completed. However, if maintenance activities are more than just checks and could disturb silica-containing dust that may exceed the RCS PEL (e.g., if the workers are cleaning out the mixers or replacing dust filters), then your maintenance workers must wear appropriate respirators while in the regulated area, as required by 29 CFR 1910.1053(e)(4).

You also state that the company has begun using safer material handling practices and is in the process of identifying suitable engineering controls. Note that the standard requires employers to use engineering and work practice controls to reduce and maintain employee exposure to silica to or below the PEL, unless they can demonstrate that such controls are not feasible. Respiratory protection should be used only when all feasible engineering and work practice controls have been implemented but have proven inadequate to meet the PEL.3 In addition, the company should be aware of paragraph (h)’s housekeeping prohibitions related to dry sweeping, dry brushing, and the use of compressed air, and should ensure the use of proper housekeeping measures to avoid exposing maintenance and other workers to RCS dust that may be re-entrained into the air.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA’s requirements are set by statute, standards, and regulations. Our letters of interpretation do not create new or additional requirements but rather explain these requirements and how they apply to particular circumstances. This letter constitutes OSHA’s interpretation of the requirements discussed. From time to time, letters are affected when the Agency updates a standard, a legal decision impacts a standard, or changes in technology affect the interpretation. To assure that you are using the correct information and guidance, please consult OSHA’s website at www.osha.gov. If you have any further questions, please feel free to contact the Office of Health Enforcement at (202) 693-2190.



Patrick J. Kapust, Acting Director
Directorate of Enforcement Programs


1 Occupational Exposure to Respirable Crystalline Silica, 29 CFR 1910.1053, Frequently Asked Questions for General Industry, January 2019, https://www.osha.gov/dsg/topics/silicacrystalline/generalindustry_info_silica.html.

2 The regulated area requirements would also not apply if your company has made a reasonable determination that the maintenance workers’ exposures are below, and are expected to remain below, the action level. See 29 CFR 1910.1053(d)(1), (e)(4).

3 Respiratory protection is also required where exposures exceed the PEL during periods necessary to install or implement engineering and work practice controls and where exposures exceed the PEL during tasks, such as maintenance and repair tasks, for which engineering and work practice controls are not feasible. See 29 CFR 1910.1053(g)(1)(i)-(ii).