Document wg31N725oGZ94dMwggmJ4rMz6

Reprinted from the Archives of Environmental Health January 1971. Volume 22 Copyright 1971, American Medical Association VC Occupational Acroosteolysis II. An Industrial Hygiene Study Warren A. Cook; Paul M. Giever, MPH; Bertram D. Dinman, MD, ScD; and Harold J. Magnuson. MD, MPH, Ann Arbor, Mich UCC 020846 74 Occupational Acroosteolysis II. An Industrial Hygiene Study Warren A. Cook; Paul M. Giever. MPH; Bertram D. Dinman, A/D, ScD; and Harold J. Magnuson. AID. MPH. Ann Arbor, Mich An industrial hygiene survey was conducted in respect to occupational acroosteolysis (AOL) in 32 plants engaged in production and compound ing of polyvinyl chloride, litis survey was limited to inspection of facilities, interrogation of tech nical personnel, and statistical analyses. All manufacturing processes and chemicals utilized' were explored. A major objective was the iden tification of variables in processes and chemicals that were peculiar to thoee plant* affording known caeoe of AOL No such processes or chemical* were found. The survoy brought to light a num ber of situations suspected of relationship to the precise cause of AOL some of which require additional exploration. Most positive of these was the finding that essentially all of the persons who developed AOL had served ee reactor cleaners using manual methods. Polyvinyl chloride < PVC) has been in production and widespread use for more than 30 years. An extensive account of the chemistry, methods of production, materials utilized, properties, and uses of the PVC resins was recently published in a series of technical articles by Albright.16 Uses in clude such diverse products as door tiles, films for food wrappings, sheeting for show er curtains, phonographic records, and coat ings for cables. As an indication of the magnitude of the PVC industry, the average operating capaci ty in this country for the year 1969 was about 3 billion lb. Much PVC is produced in other countries. Even with this extensive production involving thousands of operators, the occurrence of occupational acroosteolysis (AOL) among these workers was not recog nized until about 1961.7 Submitted for publication Feb 9, 1970; accepted April 18, 1970. From the Institute of Environmental and Induetrial Health, University of Michigan, Ann Arbor. Read before the American Industrial Hygiene Conference, Denver, May IS, 1969. Reprint request* to School of Public Health, University of Michigan, Ann Arbor, Mich 48104 iDr. Dinman). The clinical and epidemiological aspects of this condition are discussed in separate papers in this issue of the akchiyes.81' Industrial Hygiene Survey Surveys were made of 32 plant* by the authors of this paper, most of the plants being visited by a team of one physician and one industrial hygienist. Twenty-six of these were polymer production plants. Vinyl chloride mon omer production plants and plants engaged in compounding and fabricating of the polymer ized resin were a part of the manufacturing complex at a number of these locations. Five plants conducted compounding and fabricating operations only, and one plant produced only the vinyl chloride monomer. These plants were located throughout the United States and one in Canada. This survey was designed to obtain informa tion on equipment used, procedures followed, materials involved, extent of exposure to these, and names and descriptions of jobs in order to compare practices in the plants where cases of AOL occurred with those in plants experienc ing no cases. The extent of exposures was estimated on the basis of general observation, a knowledge of the physical, chemical, and physiological prop- ,4 rcA Environ Health--Vol 22, Jan 1971 ucc 020847 OCCUPATIONAL ACBOOSTEOLYSIS--COOK ET AL 75 Fig 1.--Flow diagram tor production of PVC from vinyl chlonda monomer. ertiea of the materials, and discussion with technically informed persons at the plants. By setting up three broad categories of duration and of relative severity of the exposure, semiquantitative estimates were made. Since mate rials responsible for causing AOL might be absorbed either through the respiratory tract or through the skin or both, such estimates were made for each of these avenues of absorption. Production Operations Vinyl Chloride Production.--The two processes for production of vinyl chloride described by Albright1-2 were found to be in current use. One of these involves the reac tion of hydrogen chloride with acetylene in the presence of a mercuric chloride catalyst. In the other process, I, 2-dichloroethane is pyrolyzed in the presence of chlorine as a catalyst for the production of vinyl chloride and HCI. In some plants, both methods were used, with the HCI produced in the latter of these processes being used in the former. Polyvinyl Chloride Production.--Poly vinyl chloride can be produced in accord ance with four processes, each with many variations among different plants. The final polymer may have a molecular weight vary ing from 50,000 to 150,000, depending on conditions under which it is produced. The production processes are designated as sus pension, emulsion, bulk (or nonsolvent), and solvent. A typical procedure is present ed in the flow-sheet (Fig 1), The essential steps of these operations are outlined in this paper. For more complete details, reference may be made to the papers by Albright.'1* In the widely used suspension process, deion ized water is first introduced into a large auto clave. usually glass-lined, followed by a wide variety of additives. These include suspending agents such as polyvinyl alcohol, methyl cellu lose. and gelatin, emulsifying agents such as sulfonated oils, surfactants such as sorbitan monolaurate. and buffers such as sodium bicar bonate. Trichloroethylene may be added to pro duce the lower molecular weight polymers. Cat alysts are added to initiate the polymerization. These are nearly all organic peroxides, with lauroyl peroxide and diisopropylperoxydicarbonate (IPP) being used at the greater number of plants. The reactor is then closed, evacuated to remove oxygen, and vinyl chlo ride, normally a gas, is piped in as a liquid under pressure from the tank farm or the vinyl chloride plant. In some plants, the catalyst is added to a charge tank from which it is intro duced into the closed reactor following addition of the vinyl chloride. The mixture is agitated with rotating propel ler blades, and steam initially is directed into the reactor jacket to raise the temperature to 50 or 55 C. Since the polymerization reaction is exothermic, cooling water must be run into the jacket after the reaction has begun to avoid excesaive heating. After ten to 16 hours, the polymerization is essentially complete, and the reactor is brought down to atmospheric pres sure. In most plants, it is placed under vacuum for removal of unpolymerized vinyl chloride monomer and transfer to the monomer recov ery area. The suspension of PVC is then dis charged into a blowdown tank below the reac tor. This tank is placed under vacuum for transfer of remaining vinyl chloride to the recovery area. In some plants, the blowdown tank is eliminated. The general appearance of the charging floor of a imlymer plant is shown Arch Environ Health--Vol 22. Jan 1971 76 OCCUPATIONAL ACROOSTEOLYSIS--COOK ET AL in Fig 2. The reactors extend tor ten feet or so down to the floor below. The polymer slurry is pumped to blend tanks, then to centrifuges for removal of much of the water. The dewatered polymer is passed through a rotary dryer with heated air, the temperature of the polymer being kept below 140 F to avoid degradation. The dried granular polymer is pneumatically conveyed into a cy clone separator, with finer particles being re moved from the air stream by means of a filter-type collector. The polymer particles are sized by vibrating screens, then bagged for transfer to compounding and fabricating plants or shipped in bulk. In some plants, the polymer is stored in bins prior to being shipped. In the emulsion process, the equipment and process is essentially the same as in the suspen sion process up to the drying operations. In order to produce the finer polymer particles characteristic of the emulsion process, a wide variety of emulsifying agents is used. These may be soaps such as ammonium or sodium laurate, surfactants such as salts of naphthalene sulfonic acid, or various synthetic detergents. Water-soluble initiators such as ammonium persulfate and hydrogen peroxide are used ae catalysts. Since the fine polymer particles can not be readily separated from the emulsion by centrifuging or filtering, a spray dryer is em ployed in this process. The bulk process is similar to suspension polymerization but is characterized by the pre cipitation of the polymer from die liquid phase, as about 10% of the vinyl chloride becomes polymerized. Solution polymerization is much the same as suspension polymerization, but no water is in troduced. Instead, the polymerization is con ducted in such solvents as cyclohexane, acetone, or n-butane. Copolymer and Terpdymer Production. --In addition to the production of die homo- polymer PVC, many of the plants also produce copolymer and terpolymers, includ ing vinyl chloride. The greater percentage of the monomer tends to be the vinyl chloride in all of these. The copolymer vinyl chlor ide-vinyl acetate is in highest production Arch Environ Health--Vnl 22. Jan 1971 OCCUPATIONAL ACROOSTEOLYS1S--COOK ET AL 77 among this group but is a Tabla t.--Absolut* Rrassura-TIma Valums minor product in compari son to the homopolymer. Absolute Prossure^Time Values Below 10 Absolute Pressure'Tuno Values of 10 or Above Other copolymers are vinyl chloride-vinylidene chloride and vinyl chloride-lauryl- Plant Cod* No. of No. of Reactor Expected Cleaners AOL Cases Plant Code No. of No. of Reactor Expected Cleaners AOL Cases vinyl ether. An example of a terpolymer is vinyl chloride-n-butyl maleate-dibutyl maleste. EE Plants With AOL Cases 31 0.8 B E F R 216 16S 16 39 5.8 4.5 0.4 1.1 Processes, additives, and Plants With Possible AOL Cases CC 32 0-9 catalysts essentially identi Totals 63 1.7 436 11.8 cal to those used for the homopolymer are employed in the production of the co polymers and terpolymere with the addition to the re action mixture of the addi tional monomers. Vinyl Chloride Recovery.-- The unpolymerized vinyl chloride removed from the P Q V w Totals DO Totals Plants With No AOL Cases 30 0.8 D 22 0.6 H 91 2.5 L 50 1.4 S T Z AA 193 5.3 27 26 28 17 9 17 8 132 Plants Started tn Operation in 1965 or 1966 21 0.6 C 54 J 13 21 0.6 67 0.7 0.7 0.8 0.5 0.2 0.5 0.2 3.6 1.5 0.4 . 1.9 batch following completion of polymerization is conveyed to a vinyl chloride recovery area Tabl* 2.--Classification of Standard Job Tlttas by the suction pump that Job Title Code Job Title Coda places the reactor and the Additive makeup (other 1 Monomer recovery operator 14 stripping tank under reduced pressure. Here the vinyl chlo ride gas that is removed is placed under pressure to con dense it together with water vapor to a liquid. The water and the vinyl chloride are im miscible, permitting their sep aration. In some plants, an inhibitor, usually phenol, is added to the vinyl chloride than catalyst) Bagging and shipping oparator 2 Catalyst makeup man 3 Clerical, other office, staff 4 Compounding and fabricating 5 Dryer operator 6 General laborer, custodial 7 Laboratory supervisor 8 Laboratory technician 9 Maintenance, electrical 10 Maintanance, mechanical 11 Monomer maintenance operator 12 Monomer operator 13 Monomer supervisor Plant guard, security officer Reactor cleaner Reactor cleaner, solvent Reactor operator Slurry blender Supervisor, polymer Tank farm oparator Utilities area operator (water treatment, etc) Warehouse operator Transfer operator Other than PVC 15 16 17 18 19 20 21 22 23 24 25 30 to prevent spontaneous poly merization during its distillation, the method due on the agitator blades may require hammer used for its purification. The recovered vinyl and chisel for removal and sometimes even chloride, with the higher boiling phenol remain ing in the still, is then pumped to storage for reuse. Reactor Cleaning Operations.--After the axes. Four plants use water jets with pressures of from 300 to 3,000 lb/sq in for the cleaning operation. One of these used water jets only in polymer has been discharged from the reactor one of two production buildings, as the struc- and the pressure brought to atmospheric, the tional features interfered with movement of the reactor is aerated. This is accomplished by portable high-pressure water unit about the dropping a flexible duct connected to a portable building. Three other plants have been using blower to the bottom of the reactoT, with dis solvents for reactor cleaning. One of these charge normally through the open manhole at started to use this method late in 1964, a the top of the reactor into the charging-floor second in 1966, and the third from the begin area. The loose residue is washed out with ning of plant operation in 1966. The reactor is water, after which one or two men enter the filled with the solvent which must be heated reactor to remove the skin of polymer remain and agitated for a prolonged period, as the ing on the reactor surfaces. The usual method PVC is difficulty soluble. is by manual scraping with putty knives. Resi Polyvinyl Chloride Compounding.--Com- Arch Environ Health--Vol 22, Jan 1971 78 OCCUPATIONAL ACROOSTEOLYSIS--COOK ET AL Table 3.--Magnitude of Ooaago Expr*tt*d a* Sing/m Concept at a weekly, monthly, or Severity/Duration Exposure Parameters! other period. A classification of 26 job Magnitude* of Oosage Slight 1 2 3 4 5 6 Intermediate 7 8 9 10 11 12 Less Than 1 hr/Week Negligible Moderate Moderate Moderate Appreciable Appreciable Appreciable 18 hr/Week None Negligible None Negligible None Negligible Moderate Moderate More Than 8 hr/Week None None Negligible None None Negligible None None Negligible None Negligible Moderate titles for the statistical pur poses of this study was pre pared (Table 1). These job titles corresponded to com parable operations in all the plants and to a common group of types of exposures to production materials. Ev ery job in each of the plants was given its fraction of time related to the job title as defined for the study. De Major 13 14 15 16 17 18 Appreciable Appreciable Moderate Appreciable Appreciable Appreciable None Moderate None Negligible Moderate Appreciable Expressed in order of increasing dosage received from exposure, which is the summation of the three columns to the right. t Severity of exposure is estimated as none, negligible, moderate, or appreciable for indicated durations. tails of the manner in which this was done are discussed in the accompanying paper on epidemiological aspects of AOL.8 Job Progression.--Often the first job assignment of the new employee is that of reactor cleaner. Unless there pounding consists of mixing PVC with a multi plicity of plasticizers, antioxidants, pigments, emulsifiers, stabilizers, and other special com pounds in ribbon blenders and mills. The mix tures are then passed through roll mills or mixers, such as are used in the rubber industry, where the temperature may rise sufficiently to vaporize some of the plasticizer and other vola tile constituents. The compounded polymer is then extruded through rolls or dies to form sheets, films, or pellets. These are used in are many reactors, this work does not require the entire time of the employee. Part of his time may be spent in bagging or shipping finished polymer or in the handling of cata lysts and other additives. As job openings arise and with accretion of seniority, this employee may progress to higher skilled jobs such as reactor, monomer recovery or dryer operator, or any combina tion of these at any one time. fabricating operations. In plants producing other products in ad Job Clarification dition to vinyl chloride polymers, employees may transfer between these product areas Many variables were introduced in the either as full time for a period of months or plants surveyed and even in different units dividing their time within a given week or of the same plant in regard to nearly every even day. The latter occurs particularly phase of the operations. Investigation of among maintenance personnel. unit jobs in the plants disclosed a wide Where production of vinyl chloride is part assortment of job titles. In fact, a job title as of the operation, operators may rotate be used by the technical supervisors often dif tween jobe in the production of the mono fered from that for the same job in the mer and those associated with the produc same plant as listed by the personnel de tion of the polymer. partment. The length of time spent at various jobs is Furthermore, a job assignment in one a function of production needs, economic plant might include a different group of unit conditions, plant growth rates, and activity operations than in another plant. Also, the of other product lines. proportion of time devoted to the several unit operations often differed. To compound Occupational Exposures to Materials the difficulties of classification, workers Extent of Exposures.--Information was would frequently rotate through several jobs obtained not only on each of the materials ArcA Environ Health--Vol 22, Jan 1971 OCCUPATIONAL ACROOSTEOLYSIS--COOK ET AL 79 used or produced but also on the extent of exposure to each. Since this is a function of both duration and severity, each of those factors was estimated. The duration of expo sure was recorded as less than one hour a week, from one to eight hours a week, or more than eight hours a week; the severity as negligible, moderate, or appreciable. Since it was not known whether the materi als might be absorbed through the re spiratory tract, through the skin, or both, the extent of exposure as related to each of these avenues of absorption was recorded. For statistical purposes, it was desirable to combine these two factors of exposure into a single dose value. Although admitted ly the effect of exposure to materials with differing physiological responses would vary somewhat with different combinations of de grees of duration and severity, the 18 possi ble combinations of duration and severity were listed in increasing order of dosage and classified in three groups of dose values des ignated as slight, intermediate, or major. These combinations are presented in Table 2. Materials Involved.--A total of 227 different materials were used or produced in the forma tion of PVC, the copolymer vinyl chloride-vinyl acetate, and other copolymers and terpolymers incorporating vinyl chloride. These materials included monomers, polymers, catalysts, disper sants, inorganic compounds for coalescence reg ulation, emulsifying agents, surfactants, and or ganic solvents (including hydrocarbons, both aliphatic and aromatic, ketones, eaters, and chlorinated hydrocarbons). This total of 227 materials does not include the many more used in the compounding and fabricating plants. A computer analysis of exposures to these materials was undertaken, but up to the pres ent time, this has disclosed no constellation of materials introduced into the batch at the plants exhibiting cases of AOL that was not used in plants without cases. However, special consideration waa given to certain suspect ma terials and also to certain details of procedure. Vinyl Chloride Expoeuree--In evaluating the role of vinyl chloride as a possible in fluence in the causation of AOL, the esti mates of the extent of the exposure of re actor cleaners and reactor operators were reviewed. Since the authors conducted no air analyses, they had to judge the presence of vinyl chloride in the charging floor area principally on the basis of general ventila- Tsbls 4.--Use of Inhibitor* With Vinyl Chtorlrto Use of Inhibitors Plants with AOL ca$es or possible cases Plants without AOL cases In Virgin Vinyl Chloride In Vinyl Chloride Recovery 22 34 tion provided and odor observed. The odor of vinyl chloride could be detected in the immediate vicinity of reactors as they were first opened and also when they were first being vented into the charging room area. Air analyses conducted by others have shown vinyl chloride concentrations in the reactor prior to ventilating to be in the order of 3,000 ppm. The lower limit of detection of vinyl chloride by odor is accept ed as 400 ppm. Except under the foregoing conditions, the odor of vinyl chloride was not observed in the general room air. The reactor cleaner enters the reactor af ter the aeration has reduced the vinyl chlor ide exposure to what is considered satisfac tory limits, normally 15 or 20 minutes. The test is either by sniffing at the manhole opening or by use of a flammable vapor indicator. With the lower explosive limit of vinyl chloride at 4%, it requires 400 ppm for the usual type of flammable vapor indi cator to give a positive reading. Where air analyses have been made in a small number of plants with more sensitive instruments, such as the gas chromatograph, it has been found that the vinyl chloride concentration inside the reactor during scraping operations tends to be below 100 ppm and usually about 50 ppm. With the residue containing some unpolymerized vi nyl chloride, small amounts of this gas are released as scraping is carried oa Air anal yses have shown vinyl chloride concentra tions close to the hand during scraping in the range of 600 to 1,000 ppm. Although this study provided no evidence to suggest that vinyl chloride per se is the etiological agent, the measurement of vinyl chloride concentrations may serve as a use ful index to the adequacy of reactor ventila tion. Inhibitors In Vinyl Chloride.---An inhibi tor, usually phenol, may be added to the Arch Environ Health--Voi 22, Jan 1971 ucc 020852 > 80 OCCUPATIONAL ACROOSTEOLYS1S--COOK ET AL Table 5.--Reactor Cleaning Procadures as a possible factor in the H*ikI Scraping Plant After Every Code Batch Plants with AOL esses BX EX Less than Every Cycle Cleaning by Water Jet Cleaning by Solvent causation of AOL. Contact with these inorganic and or ganic peroxides occurs as they are handled prior to their introduction into the reactor and also during re FX KX (through 1964) From 1965 actor cleaning if any unre acted catalyst remains in the residue. Twenty-two of the RX U Every 5-12 cycles From 1962 (building A) (building B) EE X 26 plants used lauroyl perox ide. Eight of these were plants with AOL cases or Plants with possible AOl cases B8 Every "several'' cycles On glass-lined, intermittently in 1966. entirely possible AOL cases, and 14 were plants without AOL cases. The next most com Plants with no AOL cases AX by December 1966 From September mon organic peroxide was the IPP, and it, too, was in use in both the positive and (to September 1964) CX D From 1962 HX J Every 2V& cycles L Every 4-8 days N P Weekly From 1965 through 1964, every 2 wetXs 1964 the negative plants, in four of the former and in eight of the latter. No catalyst or group of catalysts was used in all positive plants and in no negative rlants. Constituents of Scrapings. --Other suspect materials 1965-1966 Qx SX T X Every 1-3 cycles (emulsion) (suspension) V Every 10 cycles ZX AA Every 4 or more cycles DO GG From start of are constituents of scrapings removed from the reactors during manual cleaning. These are believed to include some partially polymerized resins, along with some un reacted catalysts and addi tives, and unpolymerized operations vinyl chloride. Little specif vinyl chloride to prevent spontaneous poly merization in storage. An inhibitor may also be added to the vinyl chloride recovered from the reactor. The phenol is removed by ic information is currently available concerning these constituents. It has been reported that lauroyl peroxide is definite ly associated with residual PVC particles, whereas little of the IPP remains unreacted treatment with caustic soda prior to its in the polymer.4 transfer to the reactor. The practice over the Further investigation of the scrapings years covered by the study is presented in from the reactor may be expected to furnish Table 3. The use of the inhibitor in the information that may lead to a better under virgin vinyl chloride at two of the positive standing of the factors involved in causation plants and three of the negative plants, with of AOL. A difficulty in such investigation is inhibitor being added in the vinyl chloride that the composition of the scrapings is recovery operation in two positive plants continually changing from the time they are and four negative plants, leads to the con removed from the surfaces of the reactor. It clusion that inhibitors probably are not a factor in AOL causation. Catalyst Exposures.--Since the catalysts are all active compounds, they were suspect is obvious that comparatively small mole cules of partially polymerized vinyl chloride are increasing in size with passage of time, and active constituents such as residual cat- Ar<h Environ HeaHh^Vol 22, Jan 1971 ucc 020853 I OCCUPATIONAL ACROOSTEOLYSIS--COOK ET AL 81 Table S.--Reactor Claming Procadum Method Manual scraping after every reector cycle Manual scraping after 2-3 cyclas Manual scraping after A-12 cycles Water-jet cleaning Organic-solvent cleaning Positive Plants 7 0 2 2 1 Negative Plants 9 2 A 2 2 alyst and vinyl chloride monomer are de creasing or may no longer be present. Procedures Affecting Exposures of Reactor Cleaners Since all but one of the 25 operators diagnosed as positive for AOL had worked as reactor cleaners and the one exception had cleaned laboratory reactors, additional inquiry was made of procedures that might affect the exposures of this group of workers. Reactor Cleaning Procedure.--The number of man hours required to clean the reactors depended not only on the number of reactors but also on the frequency of cleaning. In 16 of the 26 production plants, reactors were manual ly cleaned after every batch. This was true of six of the seven positive plants and one of two plants with possible AOL cases. In only nine of the 17 plants with no AOL cases were reactors manually scraped after every batch. The procedures followed in each of these plants appears in Table 4 and an analysis of the types of procedure by positive and nega tive plants in Table 5. Procedures Following Completion of Poly merization.--Since vinyl chloride and other vol atile components of the residue remaining on the reactor surfaces following discharge of the batch might be factors in AOL causation, infor mation was obtained concerning the degree of vacuum applied to the reactor prior to opening it and the length of time that the reactor was under this reduced pressure. The amount of volatile materials remaining in the residue on the reactor surfaces was considered to be roughly proportional to the absolute pressure and inversely proportional to some function of the time. Since the rate of removal of volatile material would tend to decrease with time, the logarithm of this factor was used in calculating an absolute pressure-time (AP-T) value. It was hypothesized that the relative amount of vola tile materials remaining in the residue follow ing application of vacuum would approximate a nondimensional value obtained by dividing the absolute pressure in inches of mercury by the logarithm of the time in minutes during which the vacuum was applied. Thus, the lower the AP-T value, the less is the amount of volatile materials remaining in the residue. Another factor affecting removal of vola tile materials is the temperature range over the period that the reactor is under vacuum. The variables in temperature control were so diverse that this factor defied classi fication and could not be incorporated in the parameter under consideration. Actually, variations in degree of vacuum and timing of its application were such that meaningful calculation of an AP-T value could be ac complished only for five of the seven posi tive plants, one of the two with possible cases, and 14 of the 17 negative plants. In the subsequent discussion of these val ues, the three plants that started in opera tion as recently as 1965 and 1966, all with no AOL cases, are not included as it is not known whether sufficient time had elapsed for cases to have developed by the terminal date of the study period. These plants are listed separately in Table 6. The AP-T values ranged from 0.5 to 21. The AP-T values were 10 or above fan entirely arbitrary division line") for four of the five positive plants and for one of the two plants with possible AOL cases. Four of the negative plants gave AP-T values below 10, and seven gave values of 10 or above. Consideration is directed to the number of reactor cleaners and the corresponding num ber of expected AOL cases in the two groups of positive plants as compared with those in the negative plants. The total number of reactor cleaners at risk in the positive or possibly positive plants with an AP-T value of less than 10 is 63; that in such plants with an AP-T value of 10 or more is 436. The relation of these two totals is to be compared with that of the negative plants in which 193 reactor clean ers at risk are in the four plants with an AP-T of less than 10 and 132 are in the seven plants with an AP-T above that value. The number of expected cases of AOL for each plant is also listed in Table 6. These values were obtained by dividing the num ber of reactor cleaners at risk by 37 This Anh Environ Health--Vol 22, Jan 1971 ucc 020854 \ 83 OCCUPATIONAL ACROOSTEOLYS1S--COOK ET AL figure derives from the paper on epidemio logical aspects of AOL8 which reports the prevalence of AOL or possible AOL as one case per 37 reactor cleaners at risk. Of the negative plants with an AP-T val ue of 10 or more, none of those in operation earlier than 1965 had enough reactor clean ers at risk to expect an AOL case, whereas two of the four plants with an AP-T value under 10 have enough workers to expect cases. Cases would be expected from the total population of reactor cleaners in the entire group of the seven negative plants with AP-T values of 10 or more, but the total number would be 3.6, as compared with 5.3 expected cases from the entire group of four negative plants with an AP-T value of less than 10. This contrasts with expected cases in the group of positive plants with an AP-T value of less than 10 as 1.7, compared with 11.8 in such plants with an AP-T value of 10 or more. The proportion of the number of reactor cleaners in the negative plants with an AP-T value of less than 10 to the total number in such plants was 193 to 325. This propor tion for the positive plants was 63 to 499. A x2 test demonstrated that the probability of this difference between the negative and the positive plants occurring by chance is less than 0.001. In order to make this comparison between the positive and negative plants with AP-T values below and above 10, it was necessary to oversimplify a multiplicity of varying de tails of operational procedures from the time the polymerization is completed to the time the cleaner enters the reactor. Accordingly, any interpretation of the findings that re duction of the AP-T value below 10 by reducing the absolute pressure or keeping the reactor at reduced pressure for a longer time or both may results in reduction in development of AOL cases should be ap proached with great caution. Preventive Measures The present knowledge as to AOL is such that categoric statements as to preventive measures are not justified. However, on a somewhat speculative basis, some measures are indicated that may be conducive to AOL prevention: 1. Although some manual removal of resi due from reactor surfaces may be required when solvents or high-pressure water jets are used for cleaning, the more extensive employment of these methods would greatly reduce the number of man hours required for conducting this operation by manual methods. 2. The adequacy of ventilation of reactors prior to entry should be checked with flammable vapor indicator equipped with a scale of greater sensitivity than 1% of the lower explosive limit of vinyl chloride or with other instruments of such sensitivity. 3. Further investigation of the constituen cy of the scrapings from the reactor surfaces should be considered, as such information might disclose the factor responsible for the causation of the disease. References 1. Albright I.F: Vinyl chloride processes ('hem Eng 74:123-130. 1967. 2. Albright LF: Manufacture of vinyl chloride. Chem Eng 74:219-228, 1967. 3. Albright LF: Polymerization of vinyl chloride. Chem Eng 74:151-158, 1967. 4. Albright LF: Vinyl chloride polymerization by suspension proraaaea yields polyvinyl chloride res in*. Chem Eng 74:146-152. 1967. 5. Albright LF: Vinyl chloride polymerization by emulsion, bulk and solution processes. Chem Eng 74:85-92, 1967. 6 PVC producer*: Chem Eng Neale 47:18-19, 1969. 7. Wilson RH, McCormick WE, Tatum CF, t *1: Occupational acroosteolysis: Report of 31 cases JAMA 201:577-581, 1967. 8. Dinman BD, Cook WA. Whitshouae WM, at al: Occupational acrooataolysis: X. An epidamjologitnl study. Arch Environ Health 22:61-73, 1971. 9. Dodson VN, Dinman BD, Whitahousa WM, et al: Occupational acroogteolysis: III. A clinical study. Arch Environ Health 22:83-91. 1971. Arch. Environ Health--Vol 22, Jan 1971 Printed and Published m the United States of America ucc 020855