Guidelines for care & Use of Dry Solvent Stills [Example]

The PELs are listed in OSHA's so called "Subpart Z list", referenced as 29 CFR 1910.1000,
Subpart Z, Toxic and Hazardous Substances. Tables Z-1 through Z-3 are best seen at . TWAs, or "action
levels", are listed for each particular chemical. In the absence of TWAs, CLs are listed. Chemicals which
have STELs are referenced as Substance Specific Standard chemicals, located in 29 CFR 1910. 1001-1050.
Units
Caution should be used in determining whether or not the various regulatory limits for air contaminants
have been exceeded. The majority of regulatory limits (as presented in the “Z” tables) are listed in two
different sets of units: ppm-volume and mg/m 3 . The units are different, but the concentrations are the
same! From the ideal gas law, PV=nRT, it can be seen that at atmospheric pressure and room temperature
the volume of a gas, V, will be proportional only to the number of moles of the gas, n. Therefore when
concentrations of an air contaminant are expressed as ppm(volume), what is being expressed is the volume
of contaminant per million volumes of air which has nothing to do with the weight of the contaminant or the
air. When contaminant concentrations are expressed as mg/m 3 , the molecular weight of the contaminant is
important. To convert ppm(volume) to mg/m 3 , the following equation can be used. Note that this equation
is valid at 1 atmosphere pressure and 25 degrees Centigrade. If you are measuring concentrations at high
altitudes where pressure is lower or in rooms where the temperature is much warmer or cooler than 25
degrees, the number of liters of volume occupied by one mole of a gas may be recalculated from the ideal
gas law.
mg Liters of contaminant grams 1 mole 1000 Liters
� � � � �
3 3
m 1,000,000
Liters of air mole 24. 45 Liters 1 meter
1000
1
Note that the million from the ppm(volume) cancels the 1000 L/m 3 x 1000 mg/g so the equation can be
simplified to
2. Air Monitoring
mg
� ppm � molecular weight �
3
m
1
24. 45
mole
Liters
OSHA requires two types of air monitoring when substances for which PELs have been adopted
are in use. The first type is "initial monitoring". As indicated in 29CFR1910.1450(d)(1), the employer
must "measure the employee's exposure to any substance regulated by a standard which requires monitoring
if there is reason to believe that exposure levels for that substance routinely exceed the action level (or in
the absence of an action level, the PEL)". The key to this is whether or not a standard exists for that
substance and whether or not there is reason to believe that exposure levels routinely exceed the standard.
Because of the way OSHA defines action level, it is appropriate to interpret TWA as "action level". For
instance, if you use something in a lab only once, there's no point in taking air samples to determine an
exposure level because by the time you've analyzed the samples the exposure will have ended. On the other
hand, if you routinely perform a particular procedure involving a substance for which a PEL has been
established, the concentration to which you are exposed during that procedure should be measured.
The second type of air monitoring is "periodic monitoring". As indicated in 29CFR1910.1450(d),
the employer must "comply with exposure monitoring provisions of the relevant standard" if the initial
monitoring discloses employee exposure over the action level (or in the absence of an action level, the
PEL). This essentially means that if initial monitoring shows that any of the concentration limits are
exceeded, then the air concentration of that particular chemical should be measured periodically on a
regular schedule. The frequency of monitoring varies according to the specific substance being monitored
and how often the procedure that results in exposure is used in the lab.
mg
g
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