XCAPER FILTER – HOW IT WORKS
SOME NOTES ON THE OPERATION OF THE XCAPER™ SMOKE FILTER AND HOW IT CAMPARES TO OTHER AIR PURIFYING FILTERS OF DIFFERING CONSTRUCTION
The XCAPER™ Smoke Mask works by trapping gaseous and particulate contaminants (often referred to as aerosols by smoke researchers) within a filtering medium similar to the “packed bed” filters commonly used in the chemical processing industry. Many such so-called mechanical filters, which do not rely on chemically reactive or catalytic components, are in widespread use in hazardous chemical and industrial environments; their efficiencies can approach 100%. They are often used to purify air by filtering out not only dusts and mists but also organic and acid gases such as carbon monoxide, carbon dioxide, hydrogen chloride, and many others. Many varieties of mechanical filter type purifying masks are available through industrial and laboratory safety supply houses. Several have been tested and investigated by NIOSH and other organizations for both filtering efficiency and tightness of fit. Vendors who market such personal respiratory devices through established industrial safety supply outlets publish performance data that suggests that these marks can be very effective filtering devices, indeed, when used within their prescribed limits.
Claims have been made that higher filtering efficiencies can often be achieved with “chemical” filters, those that rely on transformation of harmful contaminants by involving them in a chemical reaction as they pass through the filter medium. The reaction changes the chemical nature of the contaminant to something harmless (usually a relatively inert solid which remains trapped within the filter). This type of filter, however, must be chosen for a specific toxic species or a very limited range of species, as the reaction mechanism is tailored to a particular type of substance.
Since smoke from fires is a complex mix of particles, liquid droplets, gases, and sticky agglomerated mixtures of all three, chemically reactive filtering media, regardless of sophistication, cannot be expected to provide sufficient protection for all of the products of combustion which may be a threat to life-safety. As reactive media fill up with trapped particulates and the residues of reacted products, they become markedly less efficient. Breathing through them also becomes more difficult as the partially blocked air passageways produce a large pressure drop, which the wearer must overcome by breathing through the filter with more force. Chemically reactive filters also generate considerable heat through the chemical reaction mechanism, causing the breathable air to become in many cases too hot. The purported advantages of high capture and retention efficiencies of such filters, therefore, would not be borne out in most real fire situations.
Unlike chemical filters, the Xcaper filter can act effectively on a wide range of substances and actually become more efficient with use, as the trapped contaminates act to further obstruct the path of newly entering contaminants. The air passageways remain relatively free of obstruction since the filter acts primarily by adsorbing contaminants onto electrochemically active surfaces of otherwise inert filter material. No additional reaction products are created, and the adsorbed material is effectively removed from further interference with the passing stream. Adsorption continues even with particulate build-up because the micro scale electrochemical activity continues unabated. The growing quantity of adsorbed material acts only to further slow the passage of the coagulation aerosols, providing more “residence” time for the capture to take place. The filter, thus, becomes more efficient over time. Saturation will be approached but only very slowly and, as field tests have shown, the time to saturation is much greater than the time of anticipated use.
EXAMINATION OF THE POSSIBLE MICROSCALE PHYSICAL/CHEMICAL MECHANISMS RESPONSIBLE FOR THE OBSERVED PERFORMANCE OF THE XCAPER™ SMOKE FILTER
The XCAPER™ is a new type of smoke filtering device claiming a very high level of efficiency in filtering out the toxins and particulate products of combustion from common fires. A professional fire protection device called WHIFFS™ Wildfire Hazardous Inhalation Firefighters Filtration System holds an XCAPER™ filter in a protective NOMEX™ shroud that is worn over the nose and mouth and allows the wearer to breathe normally, maintain a clear line of vision, and keep both hands free.
The XCAPER™ filter has undergone rigorous laboratory testing through an independent laboratory and it has also been informally field tested under a variety of conditions. Since the filter eliminates a significant percentage of the gas phase products of combustion, a question arises as to how the observed effectiveness of the filter in preventing toxic substance build-up in the body for extended periods of up to 4 hours or more can be explained. The purpose of this paper is to identify several possible physical and chemical mechanisms by which the filter’s performance may be explained and is the result of extensive discussions with individuals highly educated in fire science.
The functional aspects of the filtering process are complex. They are dependent on details of the filter’s construction as well as on the physical and environmental conditions of use. Among the most readily identifiable mechanisms are the following:
(1) As a rather densely packed collection of small plastic beads in a contained natural gel bath, the filter behaves most fundamentally as a simple physical adsorption device, utilizing a large enclosed volume for “storage” of breath-entrained combustion particulate matter.
(2) The active thickness of the filter offers substantial interference with the path of smoke particulates drawn in through the normal breathing process. Given the internal packing structure of fine beads and relatively small void fraction it is clear, even without quantification of the effect, that the mean travel distance (mean path length) of smoke particle from outer to inner surface of the filter would be several times the direct linear thickness of the filter. As interstitial spaces (between the beads) fill with trapped particulates, mean path lengths would increase far more, delaying arrival of particulates at the inner surface.
(3) Over the course of active travel of combustion particulates, well known “aging” effects alter both their character and behavior. This widely observed phenomenon, generally termed coagulation, describes the agglomeration and coalescing behavior of gaseous, liquid, small solid and aerosol products into much larger masses whose number and size vary in a complex manner with both time and ambient temperature. Coagulation traps large numbers of toxic gas molecules within the clotted mass, the effect being enhanced by longer residence times and cooler temperatures.
(4) The physical structure of a smoke particulate may be thought of as characterized by a large surface area and a variable surface electric charge resulting from the polar structures of its constituent molecules. Polar charges of the far smaller molecules of the remaining free gaseous combustion products will eventually cause electrochemical adsorption of the gases onto the surfaces of the particulates. Due to the very large-scale difference between a gas molecule and even the smallest particulates, it is likely that thousands of gas molecules can adhere to the surface of a single particulate given sufficient residence time for absorption to occur.
(5) Most common gases, especially those typically found as combustion by-products, are soluble in water and other solvents and natural gels. Free gases not yet trapped by aerosol formation or by adsorption/coagulation processes are subject to secondary entrapment through dissociation in a solvent gel.
Still other mechanisms may be of importance, particularly as they effect the movement of ionic or surface charged species through the filter. Among these are electrolytic solution tension effects (i.e., Helmholtz double layer effects), electrophoretic effect, and potential equilibrium altering gas-solvent reactions. Such “secondary” mechanisms could be considered insignificant, but the exceedingly long reaction residence times characteristic of this type of filter may lead to enhanced effects which would not normally be expected.
In view of the major roles played by such processes as adsorption, aerosol formation and coagulation in typical fire and smoke aging processes, it is likely that these mechanisms, along with the added effects of absorption, interference and solvent-induced dissociation, are also important in accounting for the observed performance of the Xcaper filter. It is also possible that other mechanisms related to microscale details of the electrochemical environment within the filter can be identified as possible contributors.