Document 6540ka39zqmjGJ8jeR5RO2EY6

i .-tmcnVon The Application of Computer Science to Industrial Hygiene J. E, PETERSON, H. R. HOYLE, and E. J. SCHNEIDER Tht Bioehtmieai Research Laboratory, The Dow Ckemital Company, Midland, Michigan ( Automatic saniplinc and anaij-jit of environmental aimopheres can result In voluminous amounts of data describing exposures to chemicals, The use of a digital computer to process such data and the advantages and disadvantages of the technique are presented. Introduction supervision to control exposures, full uv- t.` AUTOMATIC AIR monitoring equipment the data to estimate the average conrrnu.t. is expensive. Nevertheless, if it is used to tion to which men were exposed was n. : signal the need for action by operating perfeasible. To illustrate: The instrument in- sonnel to eliminate leaks and to effect needed orded air concentrations at the rate of repairs of equipment, its cost can easily be every six seconds; this is 432,000 times y justified in terms of reduced exposures, re month. duced hazards, and reduced loss of process Because the first continuous air inoiiii- - material. Furthcimore, if sample locations was a success at day-to-day control. oti> are properly chosen, it provides data on the were installed in several plants dtiring i: concentrations of air contaminants to which 1950`s. As each monitor was put into un men are exposed. That information, handled even more information on workmen'* properly, can be used to show long-term posures was "going to waste" simply lirr.u:* trends of expostire which, correlated with plant operations, can result in still lower ex there was loo much of it. The advent digital computers suggested a solution. T.' - posures. In addition, such data can be used ing advantage of the fact that the Computa to' determine the exposures of workmen in tions Research Laboratory had a Burumc: what is, in effect, a continuing industrial B220 computer, the Environmental Rescan* hygiene survey. These data, in turn, could be correlated with medical information on Laboratory in 1961 acquired a machine fm translating air concentration data from effects of the exposures to confirm or deny recorder to punched paper tape. Continu->: present standards or to suggest new ones. air concentration data could then lie eon At The Dow Chemical Company, auto bined with computer analysis to yield a urn* matic air sampling and analysis were first used in 1950 to monitor air concentrations of more complete description of inhalation < * posures than had heretofore been posable. carbon tetrachloride in a production plant. Tin's paper recounts some of the problem- At that time no method was available for involved, from the installation of air jw- handling the mass of data generated by the toring equipment to the interpretation of t: air monitoring equipment. Even though the data as summarized by the computer. Arm.' instrument was successfully employed by plant data are used, but only for illustrative i'1-'poses, so plant, process, and even the air taminant are immaterial. This is not a rcy" d>i an environmental survey: it is a pnm-i Pretested st the Annual M'ttfne of the Americas Confermc of Government*) Isdutuio) livrfenuu. Howtos, Texas, Slxj 1965, the application of computer sciemr to in; * trial hygiene. 180 Obtaining An aut< hr sensitiv of signific , range fro: ' mediately or no sig: peated ox; selective; i lial(s) of -hould be major) va vibration, . must be ca relatively ! tion by the an instrum ance testhy operation. <*rd of air eoncentrati corded. An these spcci; have used strumcnls . spcctromett able for coi Single-po plant is st analyzer is \ the.plant, l tinuously tc Tubing usei with, or sori Preparing D. There is j reading rest; point cliart means that . tize" the daminimum, t analog data on the slide tal form (for handled by t Because d; minimize pro includes a di, j AP00000107 I \rican Industrial Hygiene Association Journal 181 Obtaining Air Sample* day number, hour, and- minute. A switch selects a "type of data" digit (from 0 to 9) 1 m automatic air monitoring system must which is incorporated into the digitizer out ifc sensitive enough to detect concentrations put. An extra "type o[ data" (for example, ef significance to health. Sensitivity must from an infrared spectrometer or from a com r ge from concentrations that may be im- bustion-conductivity analyzer) digit can allow t diately hazardous to those that have little the computer to reject data that obviously do no significance even for prolonged, re- not belong with that being processed. To p><ited exposures. The instrument should be minimize costs, punched paper tape is used , ective; it must respond'only to the mate- to transfer data from the digitizer to the -!{*') f interest. It must be stable; it computer. should be unaffected by minor (or even I ijor) variations of temperature, humidity, For each datum the digitizer punches two | nation, and line voltage. The instrument "words" on the tape. The first "word" con must be capable of operating unattended for tains the day number, hour, and minute, as (datively long periods of time. Daily atten- well as the "type of data" identification num in by the operator, minor service weekly by ber. The second "word" contains die probe i instrument man. and occasional perform number and the actual datum which, in this ance testing should be adequate to keep it in case, is a number from 0 to 999, propor teration. Finally, because a permanent icc- tional to the concentration of the air con id of air concentrations can be important, taminant. This information is obtained at a concentration data must be automatically re rate that may vary' from two times a minute corded. Any continuous analyzer that satisfies to once in 2.5 minutes. Because collecting IiCsc specifications can be used. So far we each datum generated is not always neces ave used only combustion-conductivity in sary, the digitizer can be programed to skip struments and long-path gas eel! infrared the collection of some data. In every ease, . pectromrters; both types produce data suit- once data from probe No. I ave punched onto I ble for computer analysis. the tape, data from the other probes are obtained in serial order, but the digitizer can Single-point monitoring in a production ignore all but every second, fifth, or tenth tilani is seldom economical. Usually the sots of analyses. I inalyzer is positioned at a central location in l .he plant, and air samples are brought con tinuously to it through probes of some sort, Programing Ihe Computer j Tubing used for these probes must not react We decided that the minimum time in the I with, or sorb, the air contaminant of interest. data summary should be an eight-hour shift. For this period the Dow Computations Re Pieparing Data for Analysis search Laboratory programed die computer to calculate the mean concentration at each 1 There is no method presently available for location, the standard deviation of these data, reading results automatically from multiple- the percentage of time that the concentration ' point chart paper into a computer. This was above several preselected levels, and die 1 means that a device must be used to "digi appropriate time-weighted averages. tize" the data as they arc obtained. At die . minimum, the digitizer must translate the \ analog data (usually the position of a wiper ' on the slide wire of a potentiometer! to digi tal form (for example, numbers) that can be handled by the-computer. Computers are versatile. For instance, if one probe is located to sample air outside the building, the computer can be programed to correct automatically for deviations in the "background." It can take into account peculiarities in the calibration curve of the } Because data on the time of exposure can instrument used for air analysis and can re minimize programing difficulties, the digitizer cognize several kinds of errors in the data --includes a digital clock with an output of the and use only "good" data for calculations. B f. 1 AP00000108 182 March-Afiril, 1W\ Computer Output The basic computer output consists of the mean concentration, standard deviation, num ber of analyses recorded, and percentage of lime the concentration was above preselected levels, all at each location for each shift dur ing the survey. Shift or daily averages can also be obtained over any selected time inter val such as a week or a month. These datat further identified as to department, air con taminant, etc., are permanently stored on magnetic tape. The computer has been programed to re ject poor data such as that caused by a stick ing punch or by a faulty encoder. If poor data are being obtained, this fact is often signaled first by an unexplained decrease in Furthermore, assume that, over the iim<period of interest, concentrations at ih-v data locations behaved in this manner: % of Time Concentration Exceeds Sehrtni Values at Specified Locations Concentration Data Location Number greater than 1 74 12 25 ppm 50 100 26.63% 28.68% 93.44% 31.55' 16.39 22.13 20.08 23.77 6.55 10.65 17.62 11.47 250 1.63 2.04 16.39 3.68 500 0.00 0.00 0.00 0.00 The time-weighted percentage of time spent by operators in concentrations above 25 ppm will then be: (12.3) (26.G3) 4 (31.31 <28.68) + (18.8) (93.J I) + (23.0) (31.53) + (12.4) (0) 100 the number of analyses used by the computer This means that an the average, during tin- to obtain the shift averages. This has become time period of interest, men in the open- the normal signal for nonroutinc maintenance of the digitizing equipment. In addition to the basic output, the com puter calculates time-weighted averages of two kinds. The first kind is the "usual" time- weighted average concentration to which men arc exposed. It is obtained by combining the air analysis data.with ``job analysis" infor mation on the percentage of time spent by men in the vicinity of specific analyzer probes. Job analysis information is also combined with the data on the percentage of time dur ing which concentrations exceed the prese lected levels. This results in a second kind of average which is the "time-weighted percen tage of time" exposures exceeded the prese lected levels. An example of how the time-weighted per centage of time is calculated is given below. Assume that the following information is true for an operator in a plant: Data Location Percentage of Time Spent Number 1 at That Location 12.5 7 31.3 4 18.8 12 Unexposed 25.0 12.4 100.0 tors classification encountered concentrationabove 25 ppm 38.929c of the time. Similar calculations show that these nu-ts encountered concentrations above 50 pp:u 28.24% of the time; above 100 ppm 10.9.V-' of the time: above 250 ppm 4.00% of iJ lime: and above 500 ppm 0.00% of the Data Analysis Automatic air monitoring reveals tliat uan industrial situation the variation of con centration with time can be large despite v use of a rather large time base. Figure 1 * -J plot of daily time-weighted average concvi.trations to which operators were exposed <*:. each shift over one week. Each point on di graph is the mean of several hundred drt< :minations spaced equally over an eiht-h,MH period. These arc actual plant data. On the second shift there was little varia tion during the first four days, but <wer th-last three dqys of this ]>criod the shift avrt.w-varied by a factor of almost four. If d" threshold limit value (TLV) for this imu>rial were 50 ppm. an industrial hyeicnwi l;,k` ing air samples during the first three daytiie second shift would probably lime de clared that the hazard to health was ku i'i nonexistent. On the other hand, if he h^ American IneiuUr 170 160 - TIME-w; cones ISO (20 110 (00 - 90 80 - 70 - 60 80 40 - 30 20 L i I ' Fietn30 to 1\ ppm. sampled on the to second shift! lu strous inclination Variation of o ran also be striki: parison of the t! wcond shifts duri Figure 2 places text. Data for Fig average concent! were exposed. T! riiown in Figure 1 ``an be repeated 1 The information kised upon hum I vidual air sample : Average concci average concenlr: i uuntnaries provii. . conccntralioi 'Undard deviatic 'Uindaicl deviatio *tc two different Vet know how v riently. AP00000109 American Industrial Hygiene Association Journal 183 DATE Fiouke i- Daffy timr-welfchted avetaae exposures for each shift from January SO to February 5. Time-weighted average for the week (all shifts) was 82.32 ppm. sampled on the last day of this period (on the second shift) he could have experienced a strong-inclination to "push the panic button.1' Variation of concentration between shifts can also be .striking, as illustrated bv a com parison of the .third shift with the first or second shifts during the first three days. Figure 2 places Figure 1 in a broader con text. Data for Figure 2 are also time-weighted average concentrations to which operators "We exposed. The rather extreme variation shown in Figure 1 for a time basis of one shift ran be repeated for a time basis of one week. The information contained in Figure 2 is based upon hundreds of thousands of indi vidual air samples. Average concentrations and tiine.weightcd average concentrations arc not the only data summaries provided by the computer; aver, age concentrations arc accompanied by the standard deviation of the data. Having the standard deviation and using it quantitatively are two different things, however. Wc do not yet know hotv to use such information efit* tfently. On the other hand, the spread or varia bility of the data is indicated in a more mean ingful manner by the percentage of time the concentration exceeded certain levels. Figure 3 is a plot of the time-weighted percentage of time above these levels on log-probability paper. The interval during which data were gathered and the operational classification arc identical to those in Figure 2. This graph shows that the median concentration to which men on ail shifts in this classification wore exposed was 40 ppm and that they were exposed to 500 ppm or higher 2fc of the time. The conventional plot (Figure 2) shows a maximum concentration'of about 135 ppm, but it is one of seven-day overages, whereas Figure 3 is a summation of instantaneous values. Both kinds of graph have advantages. A conventional plot illustrates better how exposunrs vary with time, and any trends become readily apparent. With this kind of graph, however, the only usable index of ex|xuro is the time-weighted average, a num ber of limited utility because it cannot reflect 184 Alareh-Aftril, Figure 2. Weekly time-weichted average exposure* for each shift from January 16 to April 10. Time-Weighted average for the whole period (all shifts) was 79.09 ppm. * These points are expanded in Figure 1. concentration variations. A log-probability plot such as that shown in Figure 3 is not very useful ns a trend indi cator, but the fact that time-weighted expos ure data plot as a straight line on this graph paper offers exciting possibilities. In our limited experience all time-weighted percent age of time data have been best fitted by a straight line on log-probability paper. That Figure 3. Data for the period of time shown in .Figure 2 [all shifts). The air monitor was not boinx used by operating personnel to assist in the control of leaks. . straight line is a complete summary of ex posure information', including measures boil: of the average concentration and of the con centration variability.1 The data in Figure 3 were obtained by - survey using the equipment described. He" arc these data to be interpreted? If we M: assume a TLV of 50 ppm, is the cxpoiv represented by Figure 3 a hazardous otic not? These data are among the most cvir. plete ever gathered for men industrially ' posed to a potentially hazardous vapor; wtwt kind of animal experiments should be de ducted to interpret these exposures? \'*c can* not yet answer these questions, but we kn<-' that they must be answered eventually. Despite this void in our knowledge, th* data from this type of survey have sv\i.uses. Any information about the intcnsit' exposures helps to interpret medical find in--* Even without complete interpretation. t.u>` ing of exposures is possible. Front iti-1!*1*' such as Figure 3 one can judge if the posurc is obviously hazardous or nonlu/-11' dous. For Instance, in Figure 4 1;-' concentrations to which the men writ* \posed are shown to exceed 15 ppm -W,* 1,1 American 7n/luilrioi . the time. They excm 50 ppm only about 1; out any further know! resented by this nm judged nonha/ardnuo gicnists. provided tit. concentrations were : distributed, of short d no consequence in tin- The data in Figure same plant, for the s;> by the analyzer using as for Figure 3. Dm when the informaiio: tained. the air anah i ignored by operating On the other hand, i: obtained when the * " by the air analyzer w rectivc action. Even ber of leaks and spilipotentially hazardous without hazard by t analyzer as an opera wav an automatic air it will not justify it- used. Advantages of the Tc The advantages c coupled with a com; arc many. First, air picture of the mantu dons of the eomai Coupled with a gnot age of lime men sulions, this technic] m mate presently ohta Insures encountered, enables a convlatu with varying plant mrrelation can poim readily and, when u dramatic reduction* men are exposed an [ lion in materials It* monitoring is the o vised that truly jlm variation between d pl'mg is done only afternoon and m APOOOOOf11 '] I res both 3 roll' in- u -d. }|<m f 'I Still CtJ 3SIHV is one o{ ioy comrii y rv >ol, wli.i: 1 be conV" can. vi kncr Hy. edeo, ills\vf event 1 \ti shy <: 1 fmdilli:' ic*- rank* rr< graphif1 die <*xnonha/ai e j l the i j ire ex* 'in 50# ot American Industrial Hygiene Association Journal (),r. lime. They exceed the assumed TLV of j 50 ppm only about 14# of the lime. With' jiyt any further-knowledge, the c\-p -ure rep` rfsented by this curve would probably be judged nonhazardous by most industrial hyjicnists, provided that contacts with high i concentrations were more or less randomly . distributed, of short duration, and of little or ' no consequence in themselves. The data in Figure 4 were obtained 5n the | tame plant, for the same group of men, and : bv tire analyzer using the same sample points a for Figure 3. During the period of time when the information in Figure 2 was ob* . uined, the air analyzer was being virtually ignored by operating personnel in the plant. : On the other hand, the data in Figure 4 were 1 obtained when the concentrations recorded by the air analyzer were used to prompt cor rective action. Even plants with a large num ber of leaks and spills can be brought from a potentially hazardous situation to one that is without hazard by using the automatic air analyzer as an operational tool. This is the way an automatic air monitor should be used; it will not justify its expense unless it is so used. Fiqche 4. Exposures for the same people as shown in Figures 2 and 3, bus for the period Jan uary 5 to June 16 when the air monitor was used to signal the need for maintenance. The time* weighted average concentration for this period was 28.83 ppm. ignored or assumed to be similar to the day shift. Finally, the records obtained can be stored on magnetic tape for indefinite per iods of time in a minimum of storage space. Access to records on magnetic tape is simple and easy. These are important considera tions, because proof of past accomplishments can be of considerable value. Advantages of the Technique The advantages of automatic air analysis coupled with a computer analysis of the data are many. First, air monitoring civcs a true picture of the manner in which air concentra tions of. the contaminant vaiy with tune. Coupled with a good analysis of the percent age of time men spend in the various loca tions, this technique can give the best esti mate presently obtainable of the actual ex posures encountered. Second, this technique enables a correlation of air concentrations with varying plant activities. This type of correlation can point out "trouble sjxms'' finite readily and, when used properly, ran lead to dramatic reduction of concentrations to which trim are exposed and a corresponding reduc tion in materials lost. Third, automatic air . monitoring is the only technique so far de vised that truly shows the air concentration variation between shifts. Too often, air sam* pling is done only on the day shift, and the afternoon and midnight shifts are either Conclusions Use of automatic air sampling and analysis equipment, coupled with n computer analysts of the data so obtained, can be an extremely potent weapon in the fight of industrial hy gienists against inhalation hazards. Mere installation of an air monitor, however, does nor end the baulir. Data obtained by the monitor must be used by operating jx-reonnel in much die same way that monitors of other operating variables are used, namely, to assure control. Computer analysis of the data obtained by an air monitor can give the best estimate presently obtainable of the con centrations of air contaminants to which men arc exposed. In fact, the environmental an alysis is so thorough that it poses a challenge to toxicologists to devise animal experiments that will provide a better basis for judgment than does the present threshold limit value. References t. LiTCiiriiXB. j. T.. ]>., ami F. Wiuoxos: A Sinplffinl Mt'Uiod of F-valiarriifttf [taw-Elbct Espcrimcou. J. PAtrmacel. Exfill. Thtftf. .In'; Si) (, t AP00000I12