CONCENTRATION UNITS AND VOLUME UNITS Yves Alarie, Ph.D Professor Emeritus University of Pittsburgh,USA A. mg/m
3 Milligrams of pollutant per cubic meter of air. Can be used for gas, vapor or aerosol. Always correct to use. You will also encounter g/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. p = 2 10 mm o Hg at 22 C and 760 mm Hg and MW is 174.2, then
CS is: -2 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.
350,000 ppm. Since the 4hour 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. 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,
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