A negative pressure respirator (NPR) is an air-purifying device worn over the mouth and nose that operates solely on the power of the wearer’s lungs. The device creates a seal against the face, and when the user inhales, a slight vacuum is generated within the mask. This internal pressure drop, or “negative pressure,” pulls ambient air through the filtering material. The respirator’s function is dependent on the user’s breathing effort to draw contaminated air through the filter media.
The Core Mechanism of Operation
The function of a negative pressure respirator is based on the pressure differential created by the wearer’s inhalation. When a person breathes in, the volume inside the mask expands, causing the pressure to momentarily fall below the atmospheric pressure outside the mask. This lower internal pressure forces the surrounding air to flow through the filter element into the facepiece, ensuring all inhaled air is purified.
The filter itself, often made of non-woven fibers, removes particulates through a combination of physical mechanisms. The largest particles are captured by inertial impaction, where their momentum prevents them from following the air stream around a fiber, causing them to collide and stick. Slightly smaller particles are caught by interception, which occurs when a particle follows a streamline but comes within one particle radius of a fiber and is physically trapped.
The smallest particles, typically less than 0.1 micrometers, are captured primarily by diffusion. These tiny particles move randomly due to collisions with gas molecules, a phenomenon called Brownian motion, which increases the likelihood of them contacting and adhering to a fiber. The overall efficiency of the filter is a result of these mechanisms working together across a range of particle sizes.
A defining characteristic of these devices is the breathing resistance they impose. The physical effort required to pull air through the densely packed filter media is noticeable, especially over long periods of use or during heavy exertion. This resistance is a direct consequence of the filter’s high efficiency.
Categorization of Negative Pressure Devices
Negative pressure respirators can be categorized into two primary types based on their construction and reusability. The first type is the Filtering Facepiece Respirator (FFR), which is a disposable mask consisting entirely of filtering material. These single-use respirators, such as the widely known N95 model, are designed to cover the nose and mouth and are discarded when they become dirty, damaged, or difficult to breathe through.
The second major type is the Elastomeric Respirator, which is a reusable device made of a soft, tight-fitting material like silicone or rubber. These devices feature replaceable filters, cartridges, or canisters that attach to the facepiece. Elastomeric respirators are more durable and can be used for extended periods, requiring only the replacement of the used filter elements.
Both types of respirators use filter designations that indicate their efficiency and resistance to oil aerosols. The efficiency is rated at 95%, 99%, or 100% against the most penetrating particle size, which is typically around 0.3 micrometers. The letter designation indicates resistance to oil: N (Not resistant to oil), R (Resistant to oil), or P (Oil Proof).
For instance, an N95 respirator filters at least 95% of airborne particles but is not resistant to oil aerosols, while a P100 filter is at least 99.97% efficient and strongly resistant to oil. These ratings apply to both the built-in filters of FFRs and the exchangeable cartridges used with elastomeric models. Selecting the correct rating depends on the specific contaminants present in the environment.
The Importance of Proper Seal and Fit Testing
The effectiveness of any negative pressure respirator is fundamentally dependent on achieving and maintaining a perfect seal with the wearer’s face. If the mask leaks, the pressure differential created during inhalation will cause unfiltered air to bypass the filter material entirely. This air follows the path of least resistance, flowing directly through the gap in the seal and into the breathing zone.
To ensure a device fits a specific individual’s facial contours, a formal procedure called Fit Testing is required. This professional assessment uses either qualitative or quantitative methods to confirm that a particular make, model, and size of respirator forms an adequate seal. Qualitative testing is a pass/fail method that relies on the wearer’s ability to detect a test agent, such as a sweet or bitter taste.
Quantitative fit testing uses a specialized machine to measure the actual amount of leakage into the mask, providing a numerical fit factor. This testing must be completed before the initial use of a respirator and typically on an annual basis to account for any changes in the wearer’s face shape. This process ensures the assigned device is physically capable of forming a tight seal.
In contrast to the formal fit test, a User Seal Check is a quick procedure performed by the wearer every time the respirator is put on. This check confirms the device is seated correctly for that specific use, often involving blocking the air intake and gently inhaling to ensure the facepiece slightly collapses. Common factors that prevent a proper seal, such as facial hair or improper donning, must be identified and corrected immediately to prevent contaminant exposure.