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The Optimal Environment for Respirator Fit Testing

This Month’s Tip: Climate Change and its Effects on your RFT Program.

It’s Spring again. The grass is greening, leaves are appearing, and fruit trees are in full bloom. Summer is right around the corner. Optimism fills the air, but potentially troublesome conditions can appear as well. The arrival of Summer can mean problems for those of us involved in Ambient Aerosol Concentration /Condensation Nuclei Counting – (AAC/CNC) Fit Testing.

Let’s Review The Key Factors:

AAC/CNC Respirator Fit Testing relies on the fact that ultra-fine aerosol particulates tend to act like molecules of a gas in terms of their ability to penetrate the respirator-facial seal interface. These particulates are absorbed in the tissues of the lungs at about 98-99% efficiency. (Don’t worry; you have been absorbing these aerosols all your life as part of living on planet Earth) If you breathe air that has been filtered by a HEPA (High-Efficiency Particulate Aerosol) filter, after a minute or so the concentration of these particulates asymptotically approaches zero.

AAC/CNC fit testing measures the concentration of these particulates in the breathing zone while the test subject is performing a series of exercises designed to stress or breach the seal of the respirator as they are wearing it. The ratio of this concentration in the breathing zone to the ambient particulate concentration is the “Fit Factor”.

NOTE – The key word in the above paragraph is “Ratio”.

Over the past couple of decades, Heating, Ventilation, and Air Conditioning (HVAC) technology has vastly improved, resulting in an Indoor Air Quality (IAQ) environment that is considerably better than it was in the past. Automatic control of make-up air is now predicated on CO2 content and temperature set points which result in considerably fewer “stale air” complaints. Extremely efficient filtration of incoming air has eliminated many of the pollens and other irritants introduced from outside air. All this is very good for workers in the buildings which have this much-improved HVAC capability. However, it can also be problematic for those of us involved in CPC-AAC fit testing.

In Arizona, where I live, the amount of make-up air and re-circulated, cooled air which the HVAC system requires in the summer can be as much as five times greater than in winter. This also means that this make-up air is being filtered at a rate five times greater. If the indoor ambient concentration of these ultra-fine particulates is (for example) 2000 particulates per cubic centimeter (2000 p/cc) in the winter, it would not be surprising to see fewer than 500 p/cc when the HVAC system is operating at maximum output.

How To Ensure a Great Outcome:

Under these conditions, in order to be able to perform a valid respirator fit test, we need more particulates. The easiest way to generate more particulates is to use an ultrasonic humidifier as a particle generator whose reservoir is filled with tap water (which contains calcium carbonate) or distilled water prepared by dissolving approximately 0.6 gram of NaCl per liter of water. Tap water works well, but there are two caveats;

  1. Because the water droplets expelled from the ultrasonic particle generator are all about the same size regardless of the concentration of ionic compounds (in drinking water calcium carbonate or CaCO3), the size of the anhydrous crystal which remains after the water has evaporated is a function of the concentration of the CaCO3 in the water used in the ultrasonic particle generator. If the concentration of CaCO3 is too great, the particles produced may be larger than the optimal size for AAC/CNC Respirator Fit Testing (Particularly in the case of N95 testing) and,
  2. Dr. Roy McKay of the University of Cincinnati has pointed out that tap water may contain bacteria which grow in the residual moisture at the tap, and which could then be dispersed into the atmosphere where the ultrasonic particle generator is operating. His suggestion is to use inexpensive bottled water which can be purchased at any supermarket. A further benefit is that since the concentration of CaCO3 is fairly constant in any brand of bottled water the resultant particle size distribution will be more consistent.

The particle generator uses an ultrasonic transducer which “blasts” small droplets of water into a cloud which is expelled from the device. The water evaporates leaving the dissolved salt (CaCO3 or NaCl) as the challenge particulate. Two very important considerations are:

  1. Ensuring that the ultrasonic humidifier is sufficiently distant from the AccuFIT9000 to allow the water to completely evaporate (otherwise you can form ‘stalactites’ on some of the internal parts of the optics system which would require non-warranty service work.
  2. Understanding the HVAC airflow in the room where the testing is to occur. Obviously if the particulate generator is placed downstream of the AccuFIT9000 the particulates would be swept out of the room via the return vent before they would be of any value in the fit test. If the supply vent is between the particle generator and the AccuFIT9000 the generated aerosol would be diluted by the incoming filtered air. Ideally the air flow sequence would be: Air Supply louvre, particle generator, the AccuFIT9000, and Return Air Vent. This would provide the maximum useable particle concentration for conducting AAC/CNC Fit Testing.

Bringing it all together:

When you are busy and in the middle of performing multiple respirator fit tests, it may not be readily apparent when conditions are not optimal in regards to the amount of particles present, and those conditions may change from early morning, when the air conditioner is on “Low”,  to the afternoon when it is running on “High”.. Being aware of what is needed for getting and keeping the right concentration of particles available is critical.

The design of the AccuFIT9000 has been optimized to be as sensitive as possible for low concentrations of particulates, but even so, it is important to recognize it is necessary to have a minimum of 1000 p/cc available in ambient air for a meaningful ratio and consequently meaningful Fit Factor to be achieved.

Armed with this comprehensive knowledge of how environmental factors can affect the outcome, you’ll rest easy knowing that all your fit tests are compliant, accurate and won’t need to be repeated.

With the advent of Spring a new baseball season also begins – and in Fit Testing, sometimes Mother Nature throws us a curve ball…but with a little knowledge and a few ‘practice swings’ under your belt, you’ll be aiming for the fences like a Pro!

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