CONCENTRATION UNITS
AND VOLUME UNITS
Yves Alarie, Ph.D
Professor Emeritus
University of
Pittsburgh,USA
3
A. mg/m
Milligrams of pollutant per cubic meter of air.
Can be used for gas, vapor or aerosol. Always
correct to use. You will also encounter μg/L.
Now with SI nomenclature you may encounter
g/m3. Please do not use g/m3, nonsense.
B. ppm
Parts per million = volume of gas or
vapor to volume of air relationship, i.e.
one ml of benzene vapor in 1,000,000 ml
of air. Since we use a volume of gas or
vapor, this unit cannot be used for
aerosols.
1
C. VOLUME PERCENT
Same volume/volume relationship
as ppm. Used for high concentration,
i.e., 1% or 0.1% instead of 10,000 or
1,000 ppm respectively.
D. mg/m3 to ppm
E.
ppm to mg/m3
F. FIBERS/CC
Number of fibers/cubic centimeter of air,
example: asbestos.
G. mppcf or mp/ft3
Million of particles per cubic foot,
example: silica.
4
H. INDUSTRIAL AND TOXICOLOGICAL
APPLICATIONS
Dilution to TLV or PEL
Assume 1 ml of toluene diisocyanate (TDI), (specific
gravity 1.2) evaporates completely:
Threshold limit value (TLV), 1979, was 0.02 ppm or
0.14 mg/m3 and permissible exposure limit (PEL)
was the same.
Common Industrial Operation: Need for
Engineering Control. Assume TDI is stored at about
100oF. When a 55 gallon drum is filled,  55
gallons of TDI vapor are liberated during filling.
55 gallons = 200L or 200,000 ml
Dilution to 1 ppm: need 200,000 m3
Dilution to 0.02 ppm: need 107 m3, a huge
building!
Disasters Even With Huge Dilution
A
large amount, even with huge air volume
dilution goes a long way. The episode in
Bhopal in 1984 illustrates the point. With 10
to 25 tons of evaporating and reacting methyl
isocyanate (MIC), even with large air dilution
the results can be devastating.
The Solution to Pollution Cannot be Only Dilution
It is important to understand that when very low
concentrations are needed for protection against a toxic
effect a small amount of material will need a lot of air
dilution. It is not normally possible to rely on dilution in
these instances. Engineering controls such as hoods, etc.
must be instituted. This is also happening in Los Angeles.
Every time a gasoline tank is filled, out comes gasoline
vapors. Since the level of hydrocarbons must be maintained
very low to avoid formation of photochemical smog and
ozone they have no choice but to capture these vapors, they
no longer can rely on simple air dilution.
Saturation Concentration (CS)
This is the maximum amount that can
exist as vapor above a liquid, or
“saturation concentration”.
Example: for TDI,
-2
p = 2  10 mm Hg at
o
22 C and 760 mm Hg and
MW is 174.2, then CS is:
If toxic level or TLV is 0.02 ppm
If toxic level or TLV is 2 ppm
If toxic level or TLV is 20 ppm
Hazard level or chance of exceeding TLV
follows if there is a spill.
Example: for MIC, CS = 350,000 ppm.
Since the 4-hour LC50 with deaths observed
within 7 days is only around 10 to 30
ppm, it is obvious that if there is a spill
the situation will become very hazardous
very quickly, much more so than with TDI.
The vapor pressure of a chemical is
therefore very important in assessing the
hazard of a spill since it is the primary
factor in determining how high the
concentration in the air can be, given a
fixed volume (and given surface area for
evaporation) of chemicals being spilled.
The toxic hazard of chemicals is not only
due to their potency but also due to their
vapor pressure.
Toxicologists seldom take into account the
vapor pressure of chemicals in their evaluation
of toxic hazard. They only look at potency, i.e.
how much to produce a given effect. From this,
lists of “most hazardous” chemicals are
produced. This is nonsense. They will go one
step further, multiplying the potency by how
many pounds are produced/year. This is
nonsense. And even further by multiplying by
the number of individuals potentially exposed.
This is nonsense. Fortunately such practices
are going away.
A recent article on the influence of vapor
pressure for inhalation toxicology
practice should be consulted. Perez, C.
and Solderholm, S. C. Some chemicals
requiring special consideration when
deciding whether to sample the particle,
vapor or both phases of an atmosphere.
Appl. Occ. Environ. Hyg. 6, 859-864,
1991.