Document rwnw8R14ENk4m8nGrNXnVXMV

358 0. A. SANDER without an extensive background in engineering or physics, and by engineers an industrial hygienists without medical training. In the later sections an attempt i' made to review the pneumoconioses in sufficient detail to aid the physician calle upon to examine employees whose work exposes them to dust. A. PROPERTIES OF DUST The important physical and chemical properties of dust have been sum; marized by Drinker and Hatch,2 and much of the material presented here was obtained from that source. 1. Settling rates. Suspended microscopic dust particles, like any other frel body, are attracted to the earth by gravity. However, because of their small mas and the relatively great resistance of the air, they do not fall according to th usual laws of gravity. Settling rates are much greater for large than for smal particles. A settled dust, accordingly, may contain a proportionately greater per centage of large particles than the contaminated air from which it settled and if which the fines predominate. Ore as it is found in mines may contain a certai amount of quartz, which, is frequently harder than the other minerals presen- Because of this relative hardness the quartz may be ground less finely in c operations. Much of this coarser quartz dust may settle out from the air in th working area with comparative rapidity, leaving a smaller percentage of quart in the atmospheric dust than may be present either in the parent material or 5 the settled rafter dust. ** ....#. Flocculation. Dust produced by crushing or grinding a material such .a' quartz differs from fume such as magnesium oxide in respect to its tendency 1 flocculate. Freshly formed fume particles are usually well below 0.3 p and accor ingly exhibit marked Brownian movement. Because of this and perhaps other fa tors, they tend to collide and form floes which, because of their larger size, sett out relatively quickly. Such masses tend to adhere to vertical walls and projectin surfaces. True dust is much less apt to flocculate than is fume, and any floes formed "a!f much more loosely bound together. Settling of dust clouds is therefore much Ie pronounced than is settling of fume clouds. As a dust cloud clears, the size of tf. particles remaining in suspension decreases and the degree of dispersion increases that is, the ratio of flocculated to discrete particles becomes less. In a metal fume, on the other hand, both the percentage of floes and their size increase w the age of the cloud. * Turbulence in the air increases floe formation in fume clouds by increasing thd- collision frequency, but it does not improve the settling rate of dust. :---Humiclity-.-ber6w^saturation-i9-saicl-to-'have-Iittle-efTect-:on-flocculation-tiii However, humidity to the point of supersaturation is an important factor, as th.. dust particles function as nuclei on which water may condense, thus increasing, particulate size. p. Drinker and T. F. Hatch, Industrial Dusts. 2nd ed., McGraw-Hill, New York, 1954,.|" PULMONARY DUST DISEASES 359 8. Wetting. Wetting is primarily an adsorption phenomenon in which the .faces of the particles become covered with a film of water. Most liquids tend to Had on plane surfaces, but great force must often be used to wet dust, probably flause the particles are already surrounded by a film of air. Wetting is of impor- nfe.in dust sampling and in control of dust. 'Three factors have been described by Drinker and Hatch as of importance in ^wetting of dust. First, it is necessary to maintain intimate and vigorous contact Jong duration between the dust and liquid. Second, it is advantageous to apply Illiquid immediately at the dust source, because the heat usually generated in ducing dust by drilling or grinding hampers air adsorption and because continuusj;flooding at the source excludes air. Third, wetting agents may be used to in4pe the wetting power of water. '.4. Electrical properties. Dust particles are electrically charged and accord-iy are attracted to oppositely charged particles. If, therefore, particles in a dust cipension are given an electrical charge, the tendency for floe formation should be leased. Attempts have been made to utilize electrically induced flocculation as a Jhod of air purification but without great success. Electrostatic precipitation, on t'other hand, has been used quite successfully for collection of dust and fume, but ucttprecipitation depends upon the attraction of the electrically charged particles ifii oppositely charged plate rather than upon flocculation. `5. Optical properties. Dust or moisture particles reflect light and it is this t) reflected from dust particles otherwise too small to be seen, that makes them bln"W-hen-a-beam-of-light-enters-a-darkened room. The scattering of a beam of %by dust or mist is known as the Tyndall phenomenon. The optical properties suspended particles in air vary with their shape, their transparency, and espe|y their size. With particles larger than the wave length of light (about 0.7 p), Strength of the Tyndall beam varies directly with particle surface area and conntration; thus, for a given weight concentration it varies inversely with the par'lefsize. Attempts have been made to utilize these optical properties in the measeraent of dust concentration, but variation in the size of 'the particles makes such fminations unreliable. B. ATMOSPHERIC DUST CONCENTRATIONS AND PARTICLE SIZE ^enever-a-solid-or-liquidd'S'ground'-Or'brdken-intb-pAiticles as-srila'll as dust, "SSSfirface area is greatly increased. Thus, if a cube of mineral 1 cm. in each of its intensions is ground into small cubes 1 cu. p in size, there will'be 1012 particles with Jal surface area of 6 sq. meters as compared with 6 sq. cm. for the original cube. __ __ ___ ^ increase in surface area is largely responsible for the, increased chemical *Sfe5tjr of finely ground materials and may be an important factor in the produc- '^''of silicosis. ^ Industrial dust concentrations may range from the amounts present in "pure in operations where no dust is produced or where there is adequate dust coni#, to as high as 2 billion particles per cubic foot of air, in rock drilling. Dalla-