An Air Purifying Respirator (APR) is a type of personal protective equipment designed to safeguard the wearer against inhaling airborne contaminants. APRs draw ambient air through a purifying element, such as a filter or a chemical cartridge, before the air reaches the user’s respiratory system. They are fundamentally dependent on the surrounding atmosphere because they do not supply oxygen or a separate source of breathing air. APRs are specifically engineered to remove hazardous substances like dust, mists, fumes, gases, and vapors. Their selection and use are often governed by standards set by regulatory bodies, such as the Occupational Safety and Health Administration (OSHA) in the United States, to ensure worker safety.
How Air Purifying Respirators Clean the Air
APRs utilize two distinct mechanisms to remove hazards, depending on whether the contaminant is a physical particle or a molecular gas or vapor. The first mechanism involves mechanical filtration, which is employed to capture solid or liquid particles suspended in the air, such as dust, aerosols, and fumes. These particulate filters operate by trapping contaminants within a fibrous material as air is inhaled, preventing them from passing through to the user.
Particulate filters are classified based on their efficiency and resistance to oil using a system developed by the National Institute for Occupational Safety and Health (NIOSH). Letter designations (N, R, P) indicate oil resistance, while numbers (95, 99, 100) reflect the percentage of airborne particles removed. For instance, an N95 filter removes at least 95% of particles and is not resistant to oil. A P100 filter is strongly oil-proof and removes a minimum of 99.97% of particles, including those as small as 0.3 micrometers.
The second purification method targets molecular contaminants like gases and vapors. This process relies on chemical adsorption or absorption, which takes place within specialized cartridges or canisters. These cartridges are filled with a highly porous material, typically activated charcoal, which chemically attracts and holds the gas or vapor molecules to its surface.
Unlike filters, chemical cartridges have a finite capacity and are specific to certain contaminants. For example, a cartridge designed for organic vapors will not provide protection against an acid gas. For environments containing both particles and molecular hazards, combination cartridges are available, which incorporate both a particulate filter and a chemical sorbent material.
Categorizing Air Purifying Respirators
Air Purifying Respirators are broadly categorized based on how the air is moved through the filter or cartridge, distinguishing between non-powered and powered systems. Non-powered respirators, also known as negative pressure respirators, rely entirely on the wearer’s inhalation effort. As the user breathes in, they create a negative pressure inside the facepiece, which pulls the contaminated air through the purifying element.
This negative pressure system requires the facepiece to form a tight seal against the user’s face. The degree of breathing resistance increases as the filter material becomes loaded with trapped particles, requiring more physical effort from the user over time. Examples include disposable filtering facepiece respirators and reusable half-mask or full-facepiece elastomeric respirators.
In contrast, a Powered Air Purifying Respirator (PAPR) uses a battery-operated blower to actively draw air through the filter or cartridge. This mechanism delivers filtered air into the facepiece, creating a positive pressure environment for the user. The positive pressure significantly reduces breathing resistance, making the device more comfortable for extended use or during periods of heavy exertion.
The positive pressure within a PAPR offers a higher level of protection, achieving greater Assigned Protection Factors (APFs) compared to non-powered models. While a typical half-mask negative pressure respirator may have an APF of 10, a PAPR can provide an APF ranging from 25 to 1,000, depending on the facepiece design. PAPRs can use a tight-fitting facepiece or a loose-fitting hood, helmet, or face shield, where the constant outward airflow mitigates the need for a perfect facial seal.
Essential Safety Limitations
Air Purifying Respirators are not universally suitable and have several strict limitations that must be understood to ensure user safety. APRs cannot be used in atmospheres that are deficient in oxygen. These devices only purify the air already present and do not generate or supply oxygen, making them unsafe if the oxygen concentration falls below 19.5% by volume.
APRs are also strictly prohibited for use in environments designated as Immediately Dangerous to Life or Health (IDLH). An IDLH atmosphere poses an immediate threat to life, could cause irreversible adverse health effects, or impair a person’s ability to escape. If the concentration of a hazardous contaminant is unknown or exceeds the maximum use concentration of the purifying element, the atmosphere must be treated as IDLH.
The contaminant-specific nature of the cartridges represents another major limitation. Using a particulate filter against a toxic gas or using a vapor cartridge designed for a different chemical provides no protection whatsoever and is extremely dangerous. Before use, a thorough hazard assessment must be conducted to match the specific contaminant with the appropriate filter or cartridge type.
The service life of purifying elements is a constraint. While particulate filters are replaced when breathing resistance becomes noticeably high, chemical cartridges have a limited adsorption capacity that can be exhausted without a noticeable change in breathing effort. Cartridges must be replaced according to a set schedule or when the wearer detects the contaminant.