Worker Exertion Level: a worker
breathing twice as fast as another will draw twice the amount of contaminant through the
respirator cartridgeThe service life of a cartridge or canister respirator depends
upon the total amount of contaminant captured by the adsorbant. The total amount of
captured contaminant is directly related to the work rate or breathing rate; i.e., a
worker breathing twice as fast as another will draw twice the amount of contaminant
through the respirator cartridge. Most cartridge studies have used a breathing rate, 50-60
liters per minute, that approximates a high end of moderate workrate. For workrates that
exceed this level (e.g., heavy shoveling, running)you may need to apply or take into
account a correction factor when determining a service life.
Respirator Cartridge Variability: some cartridges
contain more activated charcoal than others
The service life of a respirator cartridge is directly related to the amount of
active material in the cartridge. For instance, most dual cartridge organic vapor
respirators contain between 35-50 grams of activated charcoal in each cartridge. If the
specific cartridge being evaluated can be reproducibly determined to have a certain amount
of active material, then modifications to the service life may be justified. You can
obtain information on cartridge specifications from manufacturers.
Temperature: the hotter it is, the shorter the
service life
High tempertaures can adversely affect the adsorptive capacity of respirator
cartridges and canisters. The high temperature may act by thermally loosening the
attractive forces that make adsorption happen or may act in concert with humidity by
increasing the moisture carrying capacity of air. This latter mechanism may represent the
greatest likely effect on service lives of cartridges. Temperature effects alone have been
reported to reduce the service life 1-10% for every 10 degrees Celsius rise depending on
the specific solvent (Nelson, et. al., 1976). Corrections to cartridge estimated service
life for this effect alone are probably not necessary under normal working temperatures.
Relative Humidity: water vapor will compete with the
organic vapors for active sites on the adsorbent
Relative Humidity is a measure of the amount of water vapor the air will hold at a
specified temperature and is expressed in percentage values. Since warmer air will hold
more water than colder air, the same relative humidity at a higher temperature represents
a significantly greater amount of moisture. High relative humidity is a significant
negative factor in the capacity of organic vapor cartridges since the large quantity of
water vapor will compete with the organic vapors for active sites on the adsorbent. Most
of the laboratory work determining adsorbent capacity has been performed at a low relative
humidity of 50% at approximately 70 degrees F.
If the actual use of the organic vapor respirators will take place in a significantly
more humid environment, then you may need to apply or take into account a safety factor
when determining a service life. The exact magnitude of the humidity effect is complex,
dependent in part upon chemical characteristics and concentrations of both the contaminant
and the water vapor. Based upon relatively few studies, a reduction by a factor of 2 in
the cartridge service life originally estimated based upon 50 % relative humidity, may be
made when the relative humidity reaches 65% (Nelson, et. al., 1976; Werner, 1985). If the
relative humidity exceeds 85%, you should consider experimental testing or another method
to more specifically determine the service life. Mathematical modeling may be an
appropriate, albeit complex, approach to predict the effect of humidity at various
chemical concentrations (Wood, 1987; Underhill, 1987).
Multiple Contaminants: predictions should be
derived from the least well adsorbed compound
Multiple contaminants introduce a great deal of variability into the prediction of
service life for respirator cartridges. Much of the laboratory testing and the
mathematical models have utilized a single contaminant to determine service lives. Only a
limited number of multiple contaminant situations have been studied and reported in the
literature (e.g.Yoon, 1996; Jonas et. al., 1986). Cartridge service life for mixtures of
compounds with significantly different chemical characteristics is probably best
determined by experimental methods. Predictions based upon models without experimental
data should probably be very conservative and ascribe the service life derived from the
least well adsorbed compound to the total mixture concentration in terms of parts per
million. The displacement of a less well adsorbed compound by a more highly adsorbed one
may alter the actual service life from the estimated one in some cases.
