Document 2qOda5j9pYj3EZw1rVNy3brjg

162 CHAPTER 8 1958 Guid'e^S Table 4. Limits for Toxic Dusts, Fumes and Mists (Concluded) Substance A.S.A. Standards Threshold Limit M.A.C, Values, A.C.GJJB. g/cu m* 1957 g/cu m* Zirconium compounds (as Zr).................................................................... 0.00005 0.00025 0.0005 0.0001 0.0005 0.015 0.005 -h't a Grams of dust, fume, or mist per cubic meter of air. (x) These values appeared on the A.C.<7./.f. tentative list, for 1956. , _ i^r.2 Radioactivity: For permissible concentrations of radioisotopes in air, see ``Maximum Permissible Amounts.^:': of Radioisotopes in the Human Body and Maximum Permissible Concentrations in Air and Water," Hand- 4^x5, book 52, U. S. Department of Commerce, National Bureau of Standards, March, 1953. In addition, see ^^n "Permissible Dose from External Sources of Ionising Radiation," Handbook 59, V. S. Department of ComZ' meree. National Bureau of Standards, September 24, 1954. Table 4 values reprinted by permission from AJ&.A. Archives of Industrial Health, September 1957, VoL 16, pp. 261-265. . ... ^ temperature scale. It may be defined as the temperature to which a com-jVf bustible liquid must be heated to produce a flash when a small flame is W-'; passed across the surface of the liquid. The higher the flash point, the-^y more safely can the liquid be handled. Liquids with flash points underils 70 F should be regarded as highly flammable. 1 Upper and lower limits of flammability of gases and vapors, and the flash |g| points of the corresponding liquids are given in Table 6. '.ilj Methods for estimating the flammable limits of mixtures of gases of/ylf vapors must be applied with caution; the reader is referred to other publi-fll cations for this information.12 ls Design of equipment for the control of combustible anesthetics is lined in Chapter 7. Construction of equipment for handling air contain- sg, ing flammable substances, or operating in atmospheres, so contaminated, Jp is discussed in Chapter 45. (_ ||| It is customary to report the concentrations of flammable gases or vapors r ` in percent by volume, or volume percent. Comparison with concentre-1 tions on the part per million scale used in chemical, medical or industrial j hygiene literature is readily made by the conversion: 1 percent = 10,000 ppm (parts of contaminant per million parts of air, by volume, or in other | Table 5. Limits fob Mineral Dusts Threshold Limit Values A.C.G.I.H. 1956 mppcf* Aluminum oxide........................ Asbestos...................................... Carborundum............................. Cement, Portland........... ...... Dust (nuisance, no free silica). Mica (below 5% free silica)--........... Silica--high (above 50% free SiOi) . . Silica--medium (5 to 50% free SiOi). Silica--low (below 5% free SiOi)........ Slate (below 5% free SiOi)................. Silicon carbide..................... Soapstone (below 5% free SiOi). Talc....... ......................................... 50 5 50 50 50 20 5 20 50 50 50 20 20 a mppcf--million particles per cubic foot of air, standard light field count. . Table 5 values reprinted by permission fropi A-M.A. Archives of Industrial Health, September 1957, VoL J6, pp. 261-265. Air Contaminants 163 words, cubic feet of contaminant per million cubic, feet of air). . It will be noted in Table 6 that nearly all of the substances listed haVe lower explosive limits above 1.0 percent, while the maximum allowable concentrations for gases and vapors in Table 3 are'below 1000. ppm or 0.1 percent in most cases. Therefore, control of toxic or injurious vapors to levels below their maximum allowable concentrations for health usually requires much more effective ventilation than for the prevention of a fire hazard. COMBUSTIBLE DUSTS A dust explosion is essentially a sudden pressure rise caused by the very rapid burning of airborne dust. The primary explosion often originates from a small amount of dust in suspension exposed to a source of ignition and the pressure and vibration it creates may be sufficient to dislodge large accumulations of dust on horizontal ledges or surfaces of the building and equipment,- thereby creating a secondary explosion of great force. Thus the air conditioning engineer is involved for two reasons: (1) .to obtain a movement of dust-laden air into exhaust hoods or openings, and through ventilating or pneumatic conveying ducts, in a manner that will prevent accumulation of highly flammable dust at points where it could ignite inside the equipment; and (2) to so design process ventilation as to prevent the escape of dust which might settle on horizontal surfaces and become a potential source of disaster at some distance from the dusty operation. (See Chapter 45). Intensity of a dust explosion depends upon: chemical and thermal properties of the dust; particle size and shape; concentration in air; propor tion of inert dust in the air; moisture content and composition of the air; size and temperature of the ignition source; and degree of dispersion of- the dust cloud. Investigations on the explosibility of dusts require determina tion of the maximum pressure developed during explosion of a known air concentration, as well as determination of the rate of pressure rise. In vestigators frequently experience difficulty in obtaining dust suspensions of uniform dispersion, and this should be kept in mind when comparing results from several sources.14 Minimum explosive concentrations of airborne dusts already tested range from 0.01 to 0.5 oz per cubic foot, or 10 to 500 grams per cubic meter 01 -airi u ^axlmum pressures generated have been reported as high as 500 psi, although they are more likely to be of the order of 50 psi. Investiga te^11 hammable characteristics of dusts are currently made at 0.1 ana 0.5 oz per cubic foot.16-21 ATMOSPHERIC POLLEN re-mft16? f. Pollen grains discharged by weeds, grasses and, trees and pmVLnS1 . , "ay fever, are of special interest to designers of air cleaning ter ^m*enuukef A%Sic Disorders in Chapter 7, and Air Cleaning, Chap- between in !f J'ra'n.s an<^ fragments transported by the air range chiefly as 5 mi an<* j microns ln size, but some have been measured as small CT,,- cron,s>. an<i others over 100 microns in diameter. Ragweed pollen Pollen ^re- Mr y ooiform in size within the range of 15 to 25 microns, of dust riainSlCan I)e remove<f from the air more readily than the particles latter ,,?r?Va ent ln outdoor air or produced by dusty processes, since the Most mmate in the Fange f 01 t0 10 microns in size- humiditv^Tn t hygroscopic and therefore vary in weight with the y- Uustrations and data on individual pollen grains are available