CONCENTRATION UNITS AND VOLUME UNITS

Yves Alarie, Ph.D Professor Emeritus University ofPittsburgh,USA

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.

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.

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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/CCNumber of fibers/cubic centimeter of air, example: asbestos.

G. mppcf or mp/ft3

Million of particles per cubic foot, example: silica.

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H. INDUSTRIAL AND TOXICOLOGICAL APPLICATIONSDilution 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 DilutionA 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 DilutionIt 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,

p = 2 10-2 mm Hg at 22oC and 760 mm Hg and MW is 174.2, then CS is:

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.

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.