From an OSHA Letter of Interpretation:

Scenario: An employee is dry cutting concrete in an outdoor, well-ventilated environment that creates a small amount of dust that never approaches the permissible exposure limit (PEL), and the supervisor advises the employee to put a dust mask on.

Question: Does a supervisor advising an employee to put on a dust mask constitute non-voluntary (required) use even though the generated dust amount is below the PEL?

Reply: Respiratory protection is required when such equipment is necessary to protect the health of the employee or whenever respirators are required by the employer.  If an employer responsible for the safety and health of employees in a company requires a respirator to be worn in a particular area, even when airborne contaminants are determined to be below all OSHA PELs or other recognized exposure limits, any respirator use (including an N-95 dust mask) would not be considered voluntary.

For this instance, a written respirator protection program meeting the requirements of 29 CFR 1910.134 would be required.  Since it was apparently the opinion of the supervisor that the dust generated from cutting concrete apparently is of concern, advising the employee to put on a dust mask would be considered required use.

If respiratory protection is not required and the employer did not advise the employee to use a dust mask, but the employee requested to use a dust mask, it would be considered voluntary use. Under these conditions, there would be no requirement to develop a written respiratory protection program; however, the employer would be responsible for providing the employee with a copy of Appendix D of 1910.134.