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B.PARTICLE SIZE AND DISTRIBUTION Size or mass are given as count median diameter (CMD) and mass median diameter (MMD) with the geometric standard deviation ( σ g ). Since toxicity is more likely to be related to mass, the MMD is the expression of choice. We also like to define it as an “aerodynamic diameter” (MMAD) which takes into account shape and density of the particles which influence their behavior and deposition sites, as given above.
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CHARACTERISTICS OF AEROSOLS INFLUENCING THEIR TOXICITY
Yves Alarie, Ph.D Professor Emeritus University ofPittsburgh,USA
A. MASS CONCENTRATION
mg/m3, direct sampling, gravimetric or other analysis.
B. PARTICLE SIZE AND DISTRIBUTION
Size or mass are given as count median diameter (CMD) and mass median diameter (MMD) with the geometric standard deviation (σg). Since toxicity is more likely to be related to mass, the MMD is the expression of choice. We also like to define it as an “aerodynamic diameter” (MMAD) which takes into account shape and density of the particles which influence their behavior and deposition sites, as given above.
C. SURFACE AREA AND LENGTH
While mass deposited may be the primary factor for toxicity, other factors may play a role. Surface area of particles may be an important factor. Certainly an important factor for fibers is their length, which obviously is coupled to surface area. However length itself seems to be a factor. Short asbestos fibers seems to be much less potent than long asbestos fibers. Still for a given effective length, toxicity increases with the number of fibers or mass of fibers.
D. SOLUBILITY - TRANSLOCATION How long the particles are retained in the respiratory tract can
obviously have an effect on the degree and/or type of toxic effect.
a) Solubility: lipid or water soluble, non reactive. If a particle is soluble in lipid or water it will be absorbed rapidly into
blood, more so if lipid soluble than water soluble. If absorbed from the pulmonary compartment, it is equivalent to an intraarterial injection.
b) Translocation This is a more general term and is more
appropriate than solubility. It includes absorption into blood via solubility as given above, but also includes a variety of other mechanisms by which particles are cleared from the respiratory tract. These include:
c) Categories of “solubility” or “translocation”.
Three categories have been established.
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i) Category Y, Very Insoluble, Clearance in year,(avidly retained). ii) Category W, Somewhat Soluble, Clearance in weeks,
(moderately retained).iii)Category D, Very Soluble, Clearance in mins/days, (minimally retained).
