A respirator is a device worn over the mouth and nose, or the entire face, engineered to protect the wearer from inhaling hazardous airborne material. Unlike simple barrier masks, which primarily catch droplets expelled by the wearer, the respirator purifies the air from particulates, gases, or vapors before it reaches the lungs. This protective mechanism relies on a sophisticated combination of physical filtration methods and a perfect seal against the face.
The Science of Particle Capture
Respirator filters do not act like a simple sieve. Instead, the filter media, composed of a dense web of non-woven fibers, utilizes four distinct physical mechanisms to trap particles of all sizes. The most straightforward mechanism is Inertial Impaction, where larger, heavier particles traveling in the airstream cannot follow the air as it curves around a filter fiber. Due to their momentum, they collide directly with the fiber and become stuck, which is most effective for particles larger than about 1 micrometer.
For slightly smaller, medium-sized particles, Interception is the dominant mechanism. These particles are light enough to follow the air’s path, but if the particle’s edge comes within one radius of a fiber, it is intercepted and adheres to the fiber’s surface. The smallest particles, typically less than 0.1 micrometers, are captured primarily through Diffusion, also known as Brownian motion. These tiny particles move randomly, colliding with air molecules, causing them to deviate from the airstream and impact a filter fiber.
The most challenging particle size to capture, known as the Most Penetrating Particle Size (MPPS), typically falls between 0.1 and 0.3 micrometers, where none of the mechanical mechanisms are fully dominant. To boost efficiency across all sizes, many modern respirators, such as N95s, incorporate Electrostatic Attraction. The filter fibers are electrically charged during manufacturing, which draws charged particles toward the fiber surfaces, enhancing particle capture without significantly increasing breathing resistance.
Essential Differences in Respirator Design
Respirators are fundamentally categorized by their air source. Air-Purifying Respirators (APRs) are the most common type, working by drawing ambient air through a filter, cartridge, or canister to remove contaminants. This category includes disposable Filtering Facepiece Respirators (FFRs), such as N95s, and reusable Elastomeric Respirators with replaceable cartridges. APRs are limited because they can only be used when the surrounding air contains enough oxygen to breathe.
In contrast, Atmosphere-Supplying Respirators (ASRs) provide clean, breathable air from an independent source. This makes them suitable for environments immediately dangerous to life or health (IDLH) or those lacking sufficient oxygen. ASRs include Self-Contained Breathing Apparatus (SCBA), which feature a portable air cylinder carried by the user, providing unrestricted mobility for a limited time, commonly used by firefighters. The other main type is the Supplied-Air Respirator (SAR), which delivers air through a hose connected to a remote source, offering a continuous supply but restricting the user’s range of movement.
Why the Seal is Critical to Protection
The high-efficiency performance of a respirator’s filter material depends on forming a perfect seal with the wearer’s face. Any gap allows unfiltered, contaminated air to bypass the filter media, rendering the device ineffective. The effectiveness of this seal is measured by the Fit Factor, a quantitative value representing the ratio of the contaminant concentration outside the mask to the concentration measured inside.
A successful fit test demonstrates that the seal is adequate for the user, but the real-world safety margin is defined by the Assigned Protection Factor (APF). The APF is a regulatory number that indicates the level of protection a specific type of respirator is expected to provide. Fit testing is performed using two methods: Qualitative Fit Testing (QLFT), which relies on the wearer’s ability to detect a test agent like a bitter taste or smoke, and Quantitative Fit Testing (QNFT), which uses an instrument to measure leakage and calculate the numerical fit factor.
Decoding Respirator Classification Ratings
In the United States, the National Institute for Occupational Safety and Health (NIOSH) sets the performance standards for respirators, using a classification system that combines a letter and a number. The letter indicates the filter’s resistance to oil, which can degrade the performance of the electrostatic charge in some filters. The three categories are:
- N (Not resistant to oil).
- R (Resistant to oil, usable for a single shift).
- P (Oil Proof, for environments where oil aerosols are present).
The number that follows the letter indicates the minimum filtration efficiency against the Most Penetrating Particle Size (MPPS). The three numbers are:
- 95, meaning the filter is at least 95% efficient.
- 99, indicating at least 99% efficiency.
- 100, which denotes at least 99.97% efficiency.
For example, an N95 respirator is at least 95% efficient at removing non-oil-based particles, while a P100 is 99.97% efficient against both oil and non-oil particles.