With the introduction of the controlled negative pressure (CNP) fit test method and the CNP Redon fit test protocol, it is now possible to complete an OSHA-accepted fit test of a worker’s actual respirator in three minutes or less.

The CNP Redon fit test protocol places a strong emphasis on mask re-donning. Mask donning plays a critical role in determining how much protection a respirator will actually provide. The CNP Redon protocol assesses three separate mask donnings during the fit test.

Basics and basis
The basic question every fit test operator tries to answer during a fit test is, “How much does the respirator leak?” The answer to that question will judge whether the respirator will likely provide an acceptable level of protection when worn in a specific work environment.

Systems based on CNP answer the leakage question by measuring respirator leakage directly during a fit test. There is always a sufficient challenge agent concentration because, during a CNP fit test, the difference between challenge and in-mask concentrations is about a million billion air molecules/cc.

Using air as the CNP challenge agent enables faster detection of respirator leakage. Because it is based on air pressure that equilibrates by wave propagation at the speed of sound, a CNP system detects air molecules the moment they enter a respirator leak site. CNP’s pressure and leak measurements can readily be traced to primary measurement standards.

The objective of the respirator faceseal is to block air leakage into the respirator. CNP quickly measures how well that objective is met by the test respirator. Since air is the medium that transports both particulate and gaseous contaminants through respirator leak sites, it is the most conservative challenge agent possible. If air cannot leak into a respirator, neither can a particulate or gaseous contaminant.

Most current fit test pass-fail criteria rely on aerosol-based assessments of respirator leakage and fit. Systems based on CNP use air as the challenge agent. Differences in system measurements can be traced to the basic differences in each method’s approach to answering the question of how much does a respirator leak. A comparison metric to allow CNP and aerosol fit test results to be assessed using the same pass-fail criterion is available, and numerous scientific studies can be reviewed to discern the positive difference CNP systems can make in detecting respirator leakage.