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Wednesday
Oct012014

APR canister life expectancy? 

APR”s or Air Purifying Respirators are used throughout the hazardous materials response arena. Most frequently associated with level “C” responses, they are also used in conjunction with level “B” suits in an area that deals with liquid contaminants that “CAN” identify a constant supply of breathable air in a non-confined area. With the benefit on not having to carry an SCBA (Self Contained Breathing Apparatus) increases responder ease and length of operations while on-site. By not having to replenish a contained air supply, the question now becomes, “How long shall a set of canisters/filters be used and still offer the proper respiratory protection?” This month, we shall focus on this topic.

The most common form of APR is the negative pressure system. In this function, breathable air is negatively drawn into the mask by the users’ lung respiration. Since there is no powered supply, (unless a PAPR system is used) the responder will note considerable exhaustion the first two weeks while adapting to this system. This is normal as respiration is a muscular function on the part of inhalation. Exhalation is merely a release of pressure from within the lungs and not a muscular requirement on the part of the responder. The benefit of the APR over PAPR is the reduction of battery and blower motor weight carried by the responder.

Many different protective canisters are available for all hazards that are designed to be protected by a negative pressure system. The interesting issue is length of use. While some canisters define the useful life in terms of time, they may only do so at 100% concentration of the contaminant they are exposed to. What happens at less than 100% concentrations? Could they realistically be tested at all concentrations? The obvious answer is that canister length-of-use increases with reduced concentration. But by how much and for how long? What happens to a cartridge once outside of the hot zone? Is saturation/contamination still affecting the cartridge particulate? These questions taunt Emergency Managers concern for their employee’s safety. Let’s look closer at some of these and try to come up with a workable solution for our operations.

Since every scene and environment is different, we need a reasonable scale to apply to insure worker safety. The first concern of defined time is contaminant concentration. While certification is done at 100%, real world conditions are frequently less. This builds in an automatic “fudge factor” of protection for our responders. We can reasonably assume that saturation will in fact continue at a reduced level INSIDE the canisters/filters layers after the cartridge has been removed from the hot zone. Since this is immeasurable, we can assume continued deterioration of this filtration over time. How much time? This time must be added back to the total exposure time, but at what rate? So, once the canister is used in the hot zone, we can deduce that at some point it will reduce safe filtering levels. Therefore, the canister should be replaced at the end of the TOTAL exposure time. This time begins from initial exposure, to hazard encapsulation. This will reduce the possibility of personnel contamination from the picture.

If, the canister/filters are not limited by time, the standard rule of thumb should be employed. Since the total time, inside and out of the hot zone should be used for suggested canister life, a start to finish time can be used. Industry generally configures an 8 hour point for this value. This time length has been shown to be far shorter than actual perceived and tested laboratory limits. This builds in another "automatic" safety factor for the employee/responder. So, if you do not have a rated “length of time” for your various canisters you should replace them after each incident or every 8 hour shift whichever comes first.

The general danger of not having any format is not in the best interest of the worker/responder. Since we cannot measure reduced rates of exposure locked inside canisters/filters, we cannot assume the level of saturation and contamination “leach-ating” towards the lungs of the responder. Before studies existed the rule of thumb was to instruct wearers to replace canisters when breathing became difficult. This, at the time, was the only way to demonstrate saturation of filter paper and possible contamination. Now that respiratory research has demonstrated how sensitive the human lung is to contamination exposure, it is safe to assume that this “rule of thumb” is liberal and may endanger the responder.

Seeing as how difficult it is to determine the molecular movement of volatile liquids across the template of absorption factors within the medium of different filtration systems for various canisters at varying rates and concentrations, these practices will assist Emergency Managers calculating an effective tool for the protection of responders. Consider these factors when choosing canister length of use at your incident.

                 Haz Mat Mike      

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