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.

November 15, 1985



Directorate of Field Operations
Acting Director
[Directorate of Science, Technology and Medicine]
SUBJECT: Use of Chemical Cartridge Respirators for Protection Against Mercury Vapor


A chemical cartridge respirator for protection against mercury vapor manufactured by the Mine Safety Appliance Company (MSA) has received approval from the Mine Safety and Health Administration (MSHA) and the National Institute for Occupational Safety and Health (NIOSH), TC-23C-629 (Mersorb).

The respirator cartridge is equipped with a passive end-of-service-life indicator (ESLI) which changes color from orange to black before breakthrough occurs. Under NIOSH policy, the mercury respirator can only be approved when the cartridges are installed at the belt-mounted position so the ESLI is visible to the wearer.

We have received a request from MSA concerning the use of their mercury cartridges on the face-mounted position on a half-mask. MSA claims that the color indicator changes color in a much shorter time than that of the service life of the cartridge, and there is sufficient margin of safety against breakthrough even if the wearer does not perform frequent monitoring of the color change of the cartridge during the work shift. The Mersorb cartridge has also received an approval for use in an environment which contains both mercury vapor and chlorine gas.

We contacted NIOSH to request the certification test data. The test results indicated that when the cartridges were tested at a mercury vapor concentration of 21.5 mg/M3 (saturation concentration at room temperature) no breakthrough (0.05 mg/M) was observed after a test period of 500 to 690 minutes. The color change occurred from 31 to 305 minutes after the onset of the test.

Since the mercury cartridges were tested at concentrations about 210 times the OSHA permissible exposure limit (PEL) for mercury (0.1 mg/M3), the actual service life would be much longer at lower concentrations. It appears that the end-of-the-service-life indicator offers sufficient margin of safety against breakthrough.

In view of the above facts, it is concluded that the use of the MSA Mersorb cartridge on the face-mounted position on the half-mask would be acceptable provided the following conditions are met in addition to the limitations which appear on the approval label:





  1. The maximum use limit is up to 10 times the PEL for mercury vapor and chlorine gas.
  2. A quantitative or qualitative fit test is performed on the three sizes of the Comfo II facepiece.
  3. Monitoring of the condition of the end-of-the-service-life indicator be performed at periods of no more than 30 minutes.

[Corrected 10/22/2004]