Protecting workers from inhaling hazardous airborne substances requires selecting the correct equipment, such as a respirator, to reduce the concentration of hazards like dusts, fibers, gases, or vapors. The system used to quantify the expected performance of this equipment is the Assigned Protection Factor (APF). This standardized metric provides the basis for selecting the appropriate respirator for a given environment and its specific concentration of contaminants.
Defining Assigned Protection Factor
The Assigned Protection Factor is a numerical value assigned by regulatory bodies to a class of respirators. This number represents the level of protection a user can expect the respirator to provide in a compliant workplace setting. For instance, a respirator with an APF of 10 is designed to reduce the concentration of a hazardous substance inside the mask to at least one-tenth of the concentration found outside. This determination is an official, documented minimum level of performance.
The APF is applied only when the respirator is used as part of a complete respiratory protection program, which includes proper fit-testing, maintenance, and user training. A higher APF indicates a greater reduction in the concentration of airborne contaminants reaching the user’s breathing zone. This factor serves as the definitive multiplier used by safety professionals to ensure the selected equipment offers adequate protection against known hazards.
APF Versus Related Protection Metrics
The Assigned Protection Factor is often confused with other metrics used to describe a respirator’s performance, particularly the Nominal Protection Factor (NPF). The NPF, sometimes called the Laboratory Protection Factor, is a theoretical value derived from standardized testing under ideal conditions in a controlled laboratory setting. It represents the maximum potential performance a respirator can achieve without considering the variables of real-world use.
In contrast, the APF is deliberately a more conservative value derived from studies of real-world use, known as Workplace Protection Factors (WPF). The APF accounts for practical variables like the wearer’s facial movements, minor imperfections in the seal, and lapses in maintenance, which reduce the respirator’s efficiency outside of a perfect lab environment. Regulators assign the APF to ensure a margin of safety, making it the legally recognized standard for selecting equipment in operational environments.
Calculating the Maximum Exposure Limit
The practical application of the APF is in calculating the Maximum Use Concentration (MUC) for a specific hazardous substance. The MUC is the highest concentration of an airborne contaminant in which a particular respirator can be safely used. This calculation is performed by multiplying the respirator’s APF by the Occupational Exposure Limit (OEL) for the substance, such as the Permissible Exposure Limit (PEL).
The MUC formula is expressed as: MUC = APF x OEL. For example, if a substance has an OEL of 10 parts per million (ppm), and the respirator has an APF of 50, the MUC is 500 ppm. This figure determines the concentration threshold above which a higher-rated respirator must be selected.
The MUC must never exceed the concentration designated as Immediately Dangerous to Life or Health (IDLH) for that substance. If the calculated MUC is higher than the IDLH level, the IDLH concentration becomes the maximum allowable limit for that class of respirator. This limitation ensures that air-purifying respirators are not used in atmospheres where equipment failure could instantly threaten the user’s life or health.
APF Values for Common Respirators
Different types of respirators are assigned specific APF values based on their design and ability to maintain a seal against the face. Tight-fitting half-mask respirators, including disposable filtering facepieces and reusable elastomeric half-masks, are typically assigned an APF of 10. This means they are suitable for environments where the contaminant concentration is up to ten times the OEL.
Full facepiece elastomeric respirators, which cover the eyes and provide a larger sealing area, offer greater protection with an APF of 50. Powered Air-Purifying Respirators (PAPRs) use a battery-powered blower to create positive pressure inside the facepiece, which reduces inward leakage. Loose-fitting PAPRs, such as those with hoods or helmets, are commonly assigned an APF of 25, while tight-fitting PAPRs can achieve an APF of 1,000 or higher.
Self-Contained Breathing Apparatus (SCBA) devices, which supply air from a portable tank, generally offer the highest level of protection. SCBAs operating in a positive-pressure mode are often assigned an APF of 10,000, reflecting their ability to protect the user in nearly all hazardous atmospheres, including those that are IDLH.