|
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 masks 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 contaminants
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.
|
