Document zQkoyY0yknjmbNQEr75OVJBQm

FILE NAME: Trade Publications (TR) DATE: 1942 Nov DOC#: TR017 DOCUMENT DESCRIPTION: Trade Journal Article - Silicosis - Occurrence and Control Silicosis: Occurrence ana voniroi WILLIAM M. PIERCE Supervising Chemical Engineer. Employers' Liability A ssurance Corp., Boston. Mass. - - ... -- <- Cfiem. & Met. I N T E R P R E T A T I O N - - In this article the author, dealing with the chemical engineer's role in controlling fibrosis-producing dusts, presents the third of his install ments on combating chronic poisoning in chemical operations. Next month's article, which will deal specifically with industrial toxic solvents, will conclude the series.--Editors. S il ic o s is should be of particular In the second stage, although the interest to engineers because the man may be healthy in appearance, control is chiefly an engineeringhis ability to work is affected. His problem. The chemist also has an chest expansion is decreased and he interest since the problem of detect becomes short of breath on exertion. ing the hazard is partly a chemical The X-ray shows mottling due to one. Medical science has accomplished slightly larger nodules than was the much in the diagnosis and descrip case with the first stage. It is also tion of the various stages of silicosis. possible that there may be some The medical profession can give joining of the nodules. logical explanations of the action of All of the symptoms of first and silica in the lungs, but at the present second stages are further developed the physician can do little for the in the third stage. On slight exertion silicotic with tuberculosis except to there will be a decided shortness of estimate the number of months he breath. The ability to work is de may live. Inasmuch as silicosis is a creased, there is a loss of flesh and a progressive incurable condition, the frequent cough. The X-ray shows a engineer is the one person who can more intense mottling as the nodules accomplish much in its control, since are larger and combine to show dense dust production is generally the re areas. sult of poor engineering practice. Silicosis as ordinarily encountered Silicosis is a condition of the lungs is an attempt of the body to counter caused by the breathing over a period act the inhaled silica. It is not, as of years dust containing silica. This some believe, a gradual filling up of condition results in: (1) lessened the lungs with dust. The stage of capacity for work; (2) shortness of silicosis which may develop depends breath; (3) increased susceptibility on: (1) concentration of dust; (2) to tuberculosis; (4) characteristic composition of dust; (3) size of dust; early X-ray findings; (5) a progres (4) length of exposure; (5) date of sive condition not dependent on con exposure; (6) individual suscepti tinued exposure; (6) inability to bility. combat tubercular infection once ac In any exposure to dust, the quired. amount in the air at the breathing STAGES OF SILICOSIS zone of the worker is important. Generally speaking, a concentration Silicosis may be divided arbitrarily of five million particles per cu.ft. of into three stages for purposes of dis silica dust will not cause silicosis cussion. These do not represent defi even if inhaled over a long period of nite divisions, but are more in the time. A concentration of 50 million nature of an expression of the grad particles per en.ft. is considered ex ual development of disability. cessive even if the exposure is to one In first-stage silicosis the man may of the so-called innocuous dusts. appear and feel quite well and show only a slight shortness of breath. DUST COMPOSITION This early type is generally detect able only by X-ray. The chest plates Percentage of silica in the dust has a bearing on the rate at which sili show characteristic mottling due to cosis develops. Dusts containing un fine nodules of fibrous tissue. der five percent free siliea are con sidered harmless in concentrations under 50 million particles per cu.ft. Dust containing 35 percent free silica, such as granite, has a safe limit of 15 million particles per cu.ft. It is sometimes considered that the allow able dust concentration varies in versely with the free silica content. Considerable investigation has been made into the modifying effect of other dusts on the effect of silica with the hope of finding a preventa tive for silicosis. The investigations were based at first on the examina tion of dusts where the silicosis rate was low in spite of a high exposure to silica. It was found that aluminum dust had a very marked effect on the solubility of silica in laboratory ex periments and on the silicosis rate in animals. The method has been tried on men and offers much encourage ment. Small quantities of metallic alumi num dust almost completely prevent a solution of silica. For example, one gram of 325 mesh quartz when al lowed to stand in 100 c.c. of water, showed 50 p.p.m. of dissolved silica as compared to 1.9 p.p.m. when one milligram of fine aluminum dust was added. Animals dusted with quartz to which less than one percent alumi num had been added showed prac tically no fibrosis, while control ani mals, dusted with quartz only de veloped silicosis. Large particles are harmless since they tend to settle out of the air or are caught in the upper part of the respiratory tract. It is sometimes considered that the harmful dusts are between 0.5 and 3 microns in size. LENGTH OF EXPOSURE In an exposure to granite dust hav ing a silica content of 35 percent, men developed first-stage silicosis in four years and second-stage silicosis in nine years when the concentration was 60 million particles per cu.ft. Perhaps one of the shortest exposures to develop silicosis was one in the manufacture of a hand abrasive soap. In one year the man developed thirdstage silicosis and died in a short time. It is possible that the other ingredients of the soap may have been instrumental in hastening the action in this particular case. 110-- I f NOVEMBER 1942 . CHEMICAL METALLURGICAL ENGINEERING Unfortunately, silicosis may con Such exposures approach the point considered that rafter, or ledge sam tinue to progress after the worker where the worker might be nearly ples are sufficiently accurate if the has left the exposure. We have drowned by the dust in the air. particle sizes are uniform. An excep record of a case which developed Six men out? of eight working for a tion of this type is the type of dust thirty years after the exposure ter contractor doing tile laying were encountered in a foundry. Molding minated. The workman, beginning at found to have developed silicosis in sand is composed of coarse grains of the age of fifteen, worked for ten five years. It was found that they silica surrounded by many fine grains years at an extremely dusty grinding did some dry polishing using a polish of silicate. The fine particles con operation using sandstone wheels. ing material high in silica. An in tribute much dust because of their The sandstone wheels were eliminated vestigation, however, of the methods small size, number, and slow settling and the work was done for thirty used by contractors in the field re rate. However, an analysis of the years with exhausted wheels con vealed that this was one of a very few original sand or the rafter sample is taining no free silica. At the age of who did dry polishing work. much higher in free silica than the fifty-five the man was found to have In the control of the silicosis haz dust in the air. This is because of the third-stage silicosis although he had ard, it is necessary to recognize the larger size of the silica grains which not been exposed for thirty years. existence of the exposure and also to show up in the analysis but do not These and similar eases indicate evaluate its severity. Unfortunately, contribute as much in dustiness as the that the condition produced by the this is not easy to do and it is neces more numerous, smaller silicate inhalation of silica dust is progres sary to employ techniques not ordi particles. sive. Hill reports on a group of 278 narily used in the chemical labora Actual analysis is complicated by cases of very early cases of silicosis. tory. It is first necessary to know the the difficulty of differentiating be Six years after they were removed amount of free silica in the dust. In tween free silica and silicates by from exposure 171 had died. Seventy- obtaining a sample, it is not sufficient chemical methods. It is necessary to five percent of the deaths were caused to take a sample of the material be use optical methods of petrography by silicosis and tuberculosis. It has ing handled or processed as the dust to obtain accurate information about also been stated that life expectancy being carried into the air may vary the free silica. The refractive index of silicosis associated with tubercu considerably from the original ma can be determined on very small losis is eighteen months. Death may terial due to difference in particle particles by the use of immersion occur within a year and it is rare the sizes or densities of the various in liquids. This, with the information patient lives over three years. gredients. Rafter samples are more obtained under the microscope and a OCCURRENCE likely to be representative of the dust exposure, but even these may be mis total silica obtained by fusion, gives a reasonably accurate picture of the Silicosis generally results from op leading due to a difference in settling free silica content. erations involving grinding, polish rates. Samples taken from the air It is also necessary to know the ing, mixing, cleaning, pulverizing or over a long period by means of a amount of dust in the air. Standard drilling. The operations all tend vacuum cleaner would give repre for the amount of dust in the air is either to produce fine particles or to sentative samples. It is generally generally based on the number of disseminate in the air fine dust pro duced on other operations. The haz ard, of course, will depend on the nature of the materials being han dled and the amount of silica. Quarry ing and finishing of granite for In drilling exhausted operations, dust can be controlled b y m eans of dust traps, as shown here. Impinger dust sampling flasks are also shown G. A. Douglas from Gendreau, N. Y monumental stone produces a serious occupational disease hazard unless the greatest of precautions are taken. Similar operations with marble or limestone are considered non-hazard- ous from a health standpoint because of the much lower silica content. Dry grinding operations when sandstone wheels were more generally used were especially hazardous. A : number of metal polishing compounds contain appreciable amounts of silica. i Unless the buffing or polishing wheels are exhausted the operation has a dangerous exposure. Typical of the exposure in mixing operations would be the manufacture of certain abra sive soaps, or wood fillers. The prep aration of molding sand after the shakeout may be one of the severest i% exposures in foundry operation. In a foundry where used sand was worked over with a sand cutter with out adequate wetting of the dry sand, we have obtained concentrations as high as 300 million particles per eu.ft. is necessary to use this equipment to determine whether or not an exposure exists, in operations such as foundry or granite cutting, it is not necessary to run dust counts to determine the hazard because it is perfectly obvi ous. It is sometimes necessary to run dust counts to check on the efficiency of the equipment used to control the hazard. The instrument on which most of the American Standards are based is the Greenburg-Smith Impinger. The air to be sampled is drawn through a tube and impinged at a high velocity on the bottom of the sampling tube. This tube contains water which wets the dust and traps it. After a known amount of air has been sampled, a portion of the liquid is removed to a suitable counting cell for microscopic count to determine the number of particles. The cell is so calibrated that it is possible by means of an eye-piece micrometer to count the dust particles in a known volume of water. The instrument is bulky be cause of the air pump which is neces sary, but it is the nearest thing to a standard and it is possible to com pare results with a considerable amount of similar work. The Konimeter and the B&L Dust Counter are two of the more common type instruments which are also used for dust determination. They are small portable instruments which are well adapted to plant control grab sam ples. It is possible with these instru ments to take large numbers of sam ples and obtain information on the effect of any particular operation on dust production. CONTROL METHODS Control methods vary depending on the operations. One fundamental is to limit the exposure to as few peo ple as possible. The necessity of this is emphasized by the silicosis rate of moulders in foundries. These men are exposed to dust and have a poor record for silicosis although the ac tual moulding operation is not dusty. The dust comes from sand condition ing work, sand blasting tumbling or from poor housekeeping. The shake out operation is sometimes started before the molders have finished pour ing and is one of the worst dust generators. Seriousness of the dust hazard dur ing the shake-out can be appreciated by the amount of ventilation re quired when large castings are shaken out. In one case reported, the castings are as large as 350,000 out section each with a capacity of 83,000 eu.ft. of air per min. were provided to take off the dust. This particular ease was unusual, but on any shake-out ventilation installation large amounts of air have to be moved. Where continuous molding and pouring is carried out, the molds are conveyed to a shake-out station and it is comparatively easy to in stall adequate exhaust ventilation. It is important to provide exhaust ventilation at central shake-out sta tions due to the large tonnage of sand handled at these points and the proximity of molders who would be otherwise unnecessarily exposed. It is possible for some foundries to have a centralized shake-out even though the mold-handling system is not utilized. This arrangement is only suitable where there is some provi sion for transporting* of portmolds to the central station. In many cases the control is carried out by a night gang that does the work after the molders have left the plant. It is necessary to equip these men with respirators. In drilling operations, it is possible to control the dust by means of ex hausted dust traps or by using wet drilling methods. The wet method has the disadvantage of introducing a serious slipping hazard, particu larly in northern stone quarries dur ing the colder months. It has been found that the exhausted type of dust trap makes for more efficient drilling. The trap merely consists of an ex hausted cone which fits over the drill and removes the dust. Tests indicate faster drilling and less sharpening of the tool due to the cuttings being quickly removed from the hole and not being ground up to fine sizes. NECESSARY PRECAUTIONS Sandblasting is a particularly hazardous operation because of the high concentration of dust to which the operator is exposed and also to the fact that the -air is used under high pressure. The contamination is likely to stretch for some distance. The dust exposure may be as high as 241 million particles per eu.ft., although the accepted safe limit for this exposure would be about five million particles per eu.ft. This in dicates the necessity for the use of positive pressure helmets and it has been found that with this type of pro tection (giving the worker a positive supply of six eu.ft. of dust-free air a minute) the exposure of the work men will be reduced to around two million particles per eu.ft. air is discharged through a cyclone in an isolated location or is filtered if there is any chance of the air re entering the plant. Exhausts are de sirable to maintain a negative pres sure in the sandblast chamber to pre vent the dust from leaking out. In addition, the operator should wear a positive pressure helmet. On some jobs the control is carried still further with steel shot being substituted for sand. On operations such as stone crushing, the hazard is best controlled by making the work as nearly auto matic as possible. Dust producers are enclosed so as to prevent the escape of dust into the air. Any workmen who enter the dusty location are equipped with respirators. Respira tors are only intended for emergency use or for short exposures. It is much better practice, if a man is re quired to work continuously in one area, to control the dust hazard by enclosure, exhaust ventilation or by. substituting some non-hazardous ma terial, if possible, rather than to re quire use of a respirator continu ously. In addition to silica, asbestos has been found to produce a fibrotic con dition of the lung. The action is dif ferent from silica in that apparently there is no tendency toward tubercu losis. The condition developed by asbestos is a heart condition which is not curable and is likely to result fatally. Exposure to asbestos dust is limited to a much smaller group than those exposed to silica. In the ease of silica, an exposure may exist without the operators be ing aware of the hazard. It is for this reason that we have emphasized the serious results of the exposure and the steps necessary for its con trol. Silicosis can be best prevented by the removal of dust at the point of origin and control measures should be based on this principle. Bibliography 1. " Review of L ite ra tu re on Effects of B reathing D usts," Inform ation Circulars 6835, 6840, 6848, 6857, D. H arrington and S a ra D avenport. U. S. B ureau of Mines (1935). 2. "Prevention of Silicosis by Metallic A lum inum ," Can. Med. Assoc. Jour. 37, 1-11 (1937). 3. "Anthraco-Silicosis," Special B ulletin 41, D ept, of L abor an d In d u stry , Com m onwealth of Pennsylvania. , 4. "Silicosis in the Foundry Industry, Greenburg, Siegal and Smith, The M onitor, May 1938 . 5. " P ulm onary D iseases in th e M ining In d u s try ," R. R. Sayers, /. O. 111,6, U. S. B ureau of Mines. _ , 6. " Studies in Control of the Dust H azard," H atch, W arren, Drinker, Dalla- valle, H arvard Engineering School, Bulle tin 86 (1932-1933). . ,, 7. " C onstruction an d Use of Im pinger D ust Sampling A pparatus," Greenburg and Bloomfield, Public H ealth Reports, M arch 18, 1932. 112--11 NOVEMBER 191,2 CHEMICAL & METALLURGICAL ENGINEERING