CONCENTRATION UNITS AND VOLUME UNITS

1
CONCENTRATION UNITS AND VOLUME UNITS
Yves Alarie, Ph.D Professor Emeritus
University of Pittsburgh,USA
2
A. mg/m3 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.
3
B. ppmParts 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
4

• 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

5
E. ppm to mg/m3F. FIBERS/CCNumber of
fibers/cubic centimeter of air, example
asbestos.G. mppcf or mp/ft3Million of
particles per cubic foot, example silica.
4
6

• 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.

7

• 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!

8

• 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.

9

• 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.

10

• Saturation Concentration (CS)
• This is the maximum amount that can exist as
  vapor above a liquid, or saturation
  concentration.

11
Example for TDI, p 2 10-2 mm Hg at 22oC and
760 mm Hg and MW is 174.2, then CS is
12
If toxic level or TLV is 0.02 ppmIf toxic level
or TLV is 2 ppmIf toxic level or TLV is 20
ppmHazard level or chance of exceeding TLV
follows if there is a spill.
13
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.
14
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.
15
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.
16
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.