Respirator use is a last resort when feasible engineering and work practice methods are not fully effective in controlling harmful airborne exposures to below allowable limits.   Even if respirators will still be necessary:

• use chemical products that produce the lowest hazard levels while meeting specifications;
• use the best available ventilation;
• limit unnecessary access to the contaminated area to the extent feasible;
• avoid entering areas of elevated exposure to materials used by other workers whenever feasible; and
• use respiratory protection to control the remaining exposure when air contaminants are still not adequately controlled even when following these and other engineering and work practice methods.

If you work as an independent contractor you can obtain respirator selection and use information from the manufacturer of the respirator that you are using. Some safety supply companies also offer expert guidance. With regards to selecting the appropriate respirator and filters/cartridges, information is available from NIOSH and all manufacturers.  You must know what you are exposed to and what the level of exposure is to properly select a respirator and appropriate filters or cartridges.

Do not use an air purifying respirator in an Immediately Dangerous to Life or Health (IDLH) environment, including oxygen deficient atmospheres (less than 19.5% ).

The following information applies to the use of an air purifying respirator (APR). If you wear an air purifying respirator at work:

• Use only a model and size for which you have been successfully fitted.  Unfitted respirators may not provide effective protection.
• Ensure that there is no beard or hair in the respirator sealing area. Even a light beard can break the seal and does not act as an effective filter.
• Fit check the respirator after each donning and adjust the straps until you have a reliable seal.
• Make sure that you install the correct cartridge for the exposure. Refer to the product SDS if you are not sure about what's in the materials that you are using. Compare your exposure to the respirator manufacturer's cartridge selection chart or web tool. Respirator bodies and filters/cartridges are certified as a unit - do not mix component from different manufacturers.
• Follow your employer’s cartridge change schedule, but if you smell the contaminant or feel irritation or other symptom of exposure while wearing the respirator, there may be a break-through.  Leave the exposure area and change the cartridges even if it is before the specified change time. 
• If you experience difficulty in breathing, the filters may be overloaded and should be changed.
• If there is evidence of deterioration, the filters or cartridges may not work effectively anymore and should be changed.
• Start the day with a fresh chemical cartridge. Do not reuse a chemical cartridge on a second day.
• When you finish for the day, clean and properly store the respirator. Remove and discard the cartridges or filters.

Filters remove solid particles and aerosols but not vapors or gases. They are rated for three levels of oil resistance and three levels of filtration efficiency.  The classification of "N" means the filter is not resistant to oil; "R" means the filter can resist oil and "P" means that the filter is oil-proof. A "P" or "R" filter must be used when oil mists are present.  Filters are rated at 95%, 99% or 99.97% efficiency.  99.97% efficient filters are labeled at 100. An N95 filter (commonly used in general construction) is acceptable if you only need to remove 95% of particles in an oil free environment.  Higher rated filters are needed in more extreme situations.  Filter efficiency must also be factored when calculating the maximum use level of a respirator.  For a half-face respirator with an N95 filter, the total leakage is calculated as 10% (around the face piece) + 5% (through the filter).

For workers exposed to engineered nanomaterials, NIOSH has concluded that N95 and P100 particulate filters provide the expected filtration efficiency according to their efficiency ratings (Rengasamy, et al 2009). However, both OSHA and NIOSH recommend that N100 or P100 filters be used when workers are exposed to high levels of airborne engineered nanomaterials (OSHA Fact Sheet FS-3634; NIOSH blog post December 7, 2011). Surveys have shown that the most common respirators that are in use against engineered nanomaterials are P100 filters on elastomeric half mask or filtering facepiece respirators (Conti, et al 2008).

For chemical cartridges, a qualified person must establish a cartridge change schedule. It is essential to know the nature and level of all contaminants that will or may be in the workplace air to select a respirator and filter and to establish a cartridge change schedule.  Work level, humidity and altitude will also effect respirator selection and performance.  For painting tasks, a combination particle and organic vapor cartridge may be required and a pre-filter may also be used to limit filter overloading.

E-tools are available that can provide expert guidance in determining medical clearance to wear a respirator, selecting respirators and cartridges, and in establishing cartridge change schedules.

OSHA provides a compliance guide for small employers with references to applicable standards and Cal OSHA provides an extensive guide to respirator selection and use, including fit checking, cleaning and storage. For work with lead, asbestos, or methylene chloride workers and their employer must adhere to additional specific requirements for respirator selection.  If you elect to voluntarily wear a respirator, even though you are not legally overexposed, you may do so, but your employer must include you in the respiratory protection program unless the respirator that you select is a filtering face piece.  If you voluntarily wear a filtering face piece your employer only needs to provide you with a copy of Appendix D of the respiratory protection standard.  Surgical masks, bandanas or other non-certified filters do not provide reliable protection and they are not permitted by OSHA to be used to protect workers from inhalation hazards. For reliable protection, OSHA requires the use of a NIOSH-certified filtering face piece.