Document QkY2nwbYwDyp0XjaL6K38xy5

40 Txcrnty-sreond Congress--National Safety Council a (ole cratainaq; pamiUtcd ar. whkh filter* om the suspended dost At the lahorautry. the sngar is dissolved hi vsttr and the nuniba' of dost partidcs arc deter* muted hy counting those in a small volume of the sugar solution under a microscope. This method of dost sampling had certain disadvantages, the chief of which was the fact that the purest commercial suga:~ contained a certain quantity of dust, which introduced considerable errors, especially in those sample* containing a lour dots concentration. To overcome the limhatiom of the sugar tube mahod various other devices were developed, one of wind) was the Palmer apparatus. This apparatus consisted essentially of a pear-shaped glass both, the lower end of which terminated m a U-tnbe or trap. Suction was applied to the class bulb by means of an electric exhaust fan and the volume of air measured by means of a Print tube. Dust-free dis tilled water was put into the U-tube. The air drawn through the water in the trap broke it up into a spray within the larger portion of the sampling bulb, which washed out and retained the dust. The dust so obtained was analyzed by the ustal methods. The Palmer apparatus also had certain disadvantage*, the chief of which was that its efficiency as a dust catcher was low. In the search for a more portable type of in strument. which at the same time would yield rapid results, the South African in vestigators introduced in 1916 a new irnli ument known as the Kctie konimeter. The komroetcr sample* dust by impinging a small volume of air against the surface of a latdhectalcd glass plate, the vaseline serving to catch the dust particles. The dust retained cm the glass plate was then counted under the microscope at a suitable magni fication. The chief disadvantage> of the kooimetrr are that in atmospheres containing a high concentration of dost, the spots are too dense to allow counting the individual particles and {he method can. therefore, not he used m such cases, also many samples lad to be taken to secure a correct average determinalion in places where the dust crmcentratioa was variable. As a result os the different methods of dust determination being used by the differ ent governmental bureaus and others, it was soon found that the various dust-sampluig methods did not yield absolute results, consequently some confusion arose m inter preting the various dust studies. In order to arrive at some basts of comparison the IT. S. Bureau of Mines conducted a laboratory study of dust-sampling instruments. The results of tins study were published in Public Health Bulletin No. 144 entitled "Comparative Tests of Instruments for Determining Atmospheric Dusts.** (1923) During the course of this study two of the investigators. Dr. L. Giecnburg and G VY. Smith, devised a new apparatus oiled the Rnpmgcr. In the comparative study the dust collecting efficiency of the inqnnger was found to be high, and in addition the method offered several advantages over previous methods. Afto- several modifica tion* to meet special requiictnmls. this instrument was adopted by the Federal Public Htahh Service and Bureau of Mines as the standard technique for the sampling of dost in air. The instrument has been used in the study of a number of dust trades, and i* the commonly accepted method m use today. Size of Dust Particles Taken into dm Lungs It has hern observed from a study of silicotic lungs that most of the dust particles that have penetrated into the arr sacs arc less dan 10 microns in longest dimension (1 micron is 1/1000th of a millimeter, or 1/2500th of an inch). It is generally agreed, therefore, that we need not bother with particles larger than 10 microns m measuring the concentration of a dangerous dusL So that you may visualize the size of these small particles consider a 280 mesh Dust Problem in Industry 41 screen in which the screen openings are 50 micron*. In the absence of a beam wf light, an atmosphere containing these tiny particles will awear clear to the naked eye, and therefore it extremely deceptive. Only a powerful nncnw>)e can make such particles directly visible. It is necessary, therefore, to noke a microscopical examataltmi of the atmosphere to learn how badly polluted it may be. larger particle-* such as are visible to the naked eye are esscoitally harmless in the causation <{ silicitsis. as these arc caught by the membranes provided by nature in the nasal and otlwr respiratory passages ami are eventually coughed up or eliminated before any danage is done. When small dust particles are dispersed m air. they arc carried about tike smoke and settle out very slowly. How long such particle* will remain suspended in quiet air can be roughly determined from Stake's law. Assuming a round particle to be one micron in diameter, h can be calculated hr mean* this law that such a particle trill fall tally 2(13 feel in 24 hours. It can he seen, therefor^ tint any fine dust dis. persed into the air is mn only a momentary hazard but remains in the air fur an indefinite period. This fact is often overlooked hy wiwtancu, as tt is usually assumed that as soon as the dust cfouri t* no longer visilde all dust ha* settled nut. The term "dusty working place" is only a relative one. Must air. both inside and outside of factories, contains some du>t. It is, therefore, necessary in adopt a standard content for the dividing line between slightly dusty ami dusty working place*. The Wisconsin dust code prescribes a "tentative figure of IS millinn countable dust par ticles under 10 micums in longest duhenskat with free silica ctsilent of 35 per cent in a cubic foot of air as determined by Public Health Service tcriiniquc. Variations m free riltca content wilt make proportional inverse change* in this standard.* In ac cordance with this standard the permissible count for practically pore silica would le 5.250^000 pantries. How Dust Samples are Collected As mealtented previously, the instrument nsed in collecting the dost sample* is that knoun as the "fmpingtr." With this instrument the sur to be sampled is drawn through a glass tube and impinged at a high velocity on a gL?s plate which is hnmerred bocatli a suitable liquid contained in the coQcctsag dk. Tae unjringcr, therefor, combines the principle of collecting dust by impingement with a water-washing or bubbling method and so* possesses the advantages of both principles. The dust b thus trapped and remains in the collecting liquid. Suclit*-* is applied to die fanpmger by means of a steam ejector operated by compressed air. Where compressed air is not avathble. a small electrically-operated vacuum pump may be used. The rate of air flow is measured chlter hy means of a snail vacuum cage or an orifice-type Sow meter. The entire apparatus H oltbratcd before use against a standard gas meter, so that a known volume of air may be sampled. The rate of sampling is approximately one cubic foot of air per minute. The dust-collecting device is placed in close proximity to the breathing level of the workmen involved while pwfouling their respective duties, so that air samples representing actual working conditions may he obtained. After a sufficient volume of <ta*t4aden air ha* Iwen sampled, the colledmg liquid idaced in a suhahSc omtainrr and removed to the lataratory fiir the neccssry analysis. When the dust sample readies the laboratory, the entire sample is filtered into a clean graduated flask through a 280-mesh screen so that only particles snaltcr than 50 iracrcns are permitted to pass; partidc* larger than 50 microns arc not considered