Document M4KyJovZo7ejmbMqaRD7LdQNz

198 FRANK A. FATTY or blue filters must be tempered with caution, else a grossly erroneous cdjgff lying between the true light-field and dark-field counts may be obtained.*?* usual microscopic arrangement includes a 16-mm. objective and a 7.5*! o^fi1 containing a Whipple disk, the tube length being adjusted so that the Whil _ disk exactly subtends 1 sq. mm. of the floor of the counting cell. This adjustn|b; is made by the use of a stage micrometer, Method oj Making and Computing Counts. The recommended practice! counting is to prepare two cells from each dust sample and to count one fourth the ruled field of the Whipple disk in five well-separated representative locaftf of each cell. The ten counts are averaged and from this is subtracted a Bimilp! obtained count of a control sample of the collecting liquid through which nil has been drawn. The control count compensates for dust acquired in manipulfi|iS as well as dust in the sampling medium or the microscopic system. This av|f net count per fourth of the field is then computed to million particles per c foot in the sampled atmosphere, taking into account air volume sampled^ volume of impinger liquid. rarticies per c..u.. Citl. _---- count per fourth of field.. X ,4 X 1000 X ml. of sample liql) cu. ft. of air sampled There is considerable variation in the manner of making dilutions and filli cells, the length of settling time, the choice of a counting cell, and the miscrosp system employed; therefore to expect greater correlation than 10 per cerffe tween two independent counts is to be overly optimistic, and variations oil 50-per-cent-are-not-uncommon--Despite-all-oHhese*objections'Our-presen' of evaluation is not to be condemned, but we should encourage research into;Jje methods and be mindful of the folly of evaluations based upon a single dustc| We should avoid the idea that the threshold limits of dustiness now accepts) absolute figures rigidly separating safe atmospheres from dangerous ones, iff they should be regarded as bench marks of desirable control. {J Dark-Field versus Light-Field Counting. Dark-field illumination obtaii by means of a Zeiss "Dissecting Condenser" and counts made with a microti! system employing a 16-mm. achromatic objective with N.A. = 0.2 (8*j.;~!| ocular, and a tube length adjusted to give a magnification of 250* have-;] shown to yield results from 2 times to more than 50 times as high as thqsg^O proportion of. particles less than 6.7 p in diameter and is an indication of pj| size. This, does not, necessarily indicate that the dark-field count is a better pr;ey.qa,ihakiA^9iil4i.o-.ait^.th.edest.abljshment_pi.su&cient..coirelatiomib|' lung damage and dustiness determined by such dark-field counts. Althouf pinger samples are more easily counted by the dark-field technique, an3|pi much higher results, the collection efficiency of the impinger as now used .drop^';. "T. Hatch and C. L. Pool, J. Tnd. Hyg. Toxicol., 16, 177 (1934). If SAMPLING AND ANALYSIS OF CONTAMINANTS 199 diminishing particle size, for particles below 3/< p, that the advantage (ibe more than offset by decreasing efficiency of collection. Our evaluation is largely limited to those particles above 0.7 p in Ih^ier this gives rise to serious errors remains to be proved. It has }K`l|hat particle size may be a factor in explaining the relatively low 7l?vi^mts found in the silica brick industry in work atmospheres where *b^ft|wn to occur. It would seem desirable to collect samples in this and Iplifia exposures and, after evaluation of all particles in the range !13,,'5.0 p, make a statistical study in an effort to illuminate the effect n\! ljj||jparticles as well as possibly to explain the incidence of silicosis in manufacture.68 'iifiW^FTojector. The microprojector64 furnishes a useful variation in JWtMisI ifels for evaluating impinger samples, whereby the dust image is i.,VJl.1<^.`,.|ually at a magnification of 1000 diameters, onto a ruled screen. A `rf^jnV^ciij'j,i||tiye is used and the increased magnification is obtained by employ- iLW'il^"jfver in the ocular and by projection. Two cells are prepared and ky counting all the particles in five well-separated representative 8m|l)$5fcu. mm. each (10 X 50 cm. strip on the screen, at 1000*). A similar ^o.i^Sontrol is then, subtracted from the average of the counts, on the two uMllIPbd the final computation is as follows: . ft. of air = @KB8U* count in 0.25 cu. mm. X 4 X 1000 X ml. of dust suspension cu. ft. of air sampled iRf.'gi.iiLcjOpyjl'ctor can be used for both counts and particle-size determinations, '''feiof^'ausSlexcessive eye strain, and, at least among the inexperienced, tends to "^iJi^Sthgreement and reproducibility of counts. It is probably not a time &pmits are being made by one individual; With direct counting *Kwr$vl^0Pe, ana]yst can arrange all his material within arm's length ^.trvj'c^laye opening and closing the viewing booth door and walking from Imo6u?;!]]srjscope to adjust the light source and to fill cells and place them on gained with the microproj ector in avoiding secondary dilution ^n|iS|ftaaydoe8 not compensate for this loss. The microprojector is, however, ^^^Rfi-^p^'|uiiti.ng-as-well-as-for.1particlesize..deter-m,ihationry-ields-depend- results, and is an interesting and valuable demonstration counterpart66 requires less space and obviates the necessity for manipulate focusing and mechanical-stage controls. "ErBatters, A. E. Dooley, F, B. Kopperihaver, JTLrMatkhes, and R. * ** jf Silicosis in the Silica Brick Industry. Bur. of Ind. Hyg., Dept, of Health, L. A. Baum, W. P. Yant, and H. H. Schrenk, U. S. Bur. Mines Rept. Invest. -"rt^JBrjown, U. S. Bur. Mines Rept. Invest. No. 3780, 1944. '