Document rBQzRDN2MokVm7Lwbp1NX8Y4V

* Reprinted from AMERICAN INDUSTRIAL HYGIENE ASSOCIATION JOURNAL Volume 27, Maxch-April, 1966 ^ ^ ^ The Application of Computer Science to Industrial Hygiene J. E. PETERSON. H. R. HOYLE, and E. J. SCHNEIDER Tht Biochemical Research Laboratory, Th* Door Chemical Company, Midland, Michigan 0 Automatic sampling and analysis of environmental atmospheres can result is voluminous amounts of data describing exposures to chemicals. Tbo uw of a digital computer to process such data and tii advantage* and dltadvaataga of the technique axe presented. Introduction A UTOMATIC AIR monitoring equipment is expensive. Nevertheless, if it is used to signal the need for action by operating per* sonnel to eliminate leaks and to effect needed repairs of equipment, its cost can easily be justified in terms of reduced exposures, re duced hazards, and reduced loss of process materiaL Furthermore, if sample locations an properly chosen, it provides data on the concentrations of air contaminants to which men are exposed. That information, handled properly, can bo used to show long-term trends of exposure which, correlated with plant operations, can result in still lower ex posures. in addition, such data can be used to determine the exposures of workmen in what is, in effect, a continuing industrial hygiene survey. These data, in turn, could be correlated with medical information on effects of the exposures to confirm or deny present standards or'to suggest new ones. At'The Dow Chemical Company, auto matic air sampling and analysis were first used in 1950 to monitor air concentration* of carbon tetrachloride in a production plant. At that time no method was available for handling the mass of data generated by the air monitoring equipment. Even though the instrument was successfully employed by plant Prantid at th Annual Ucatin* at Amirieia CWw anca ef Covanuaaatal laduMriat Hnlaaiia, Houiae*. Tun, May 196S. supervision to control exposures, full use of the data to estimate the average concentra tion to which men were exposed was not feasible. To illustrate: The instrument rec orded air concentrations at the rate of one every six seconds; this is 432,000 times per month. Because the fust continuous air monitor was a success at day-to-day control, others were installed in several plants during the 1950's. As each monitor was put into use, even more information on workmen's ex posures was "going to waste" simply because there was too much of it. The advent of digital computers suggested a solution. Tak ing advantage cf the fact that the Computa tions Research Laboratory bad a Burroughs ' B220 computer, the Environmental Research laboratory in 1961. acquired a machine for translating air concentration data from a recorder to punched paper tape. Continuous air concentration data could then be com bined with computer analysis to yield a much more complete description of inhalation ex posures than had heretofore been possible. This paper recounts some of the problems involved, from the installation of air moni toring equipment to the interpretation of the data as summarized by the computer. Actual data axe used, but only for illustrative pur poses, so plant, process, and even the air con taminant are immateriaL This is not a report of an environmental survey; it is a primer in the application of computer science to indus trial hygiene. 180 G'Sf EcgP Ii*S'.-f. i-f *v*t. ***.- -. r . .A - -'-.3g RF -4 L I j4T* #* ucc American Industrial Hygiene Association Journal 181 Obtaining Air Samples day number, hour, and minute. A switch selects a "type of data" digit (from 0 to 9) An automatic air monitoring system must which is incorporated into the digitizer out be sensitive enough to detect concentrations put. An extra "type of data" (for example of significance to health. Sensitivity must from an infrared spectrometer or from a com range from concentrations that may be im bustion-conductivity analyzer) digit can allow mediately hazardous to those that have little the computer to reject data that obviously do or no significance even for prolonged, re not belong with that being processed. To peated exposures. The instrument should be minimize costs, punched paper tape b used selective; it must respond only to the mate -to transfer data from the digitizer to the rial^) of interest. It must be stable; it computer. should be unaffected by minor (or even major) variations of temperature, humidity, For each datum the digitizer punches two vibration, 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 relatively long periods of time. Daily atten well as the "type of data" identification num tion by the operator, minor service weekly by an instrument man, and occasional perform ance testing should be adequate to keep it in ' operation. Finally, because a permanent rec ord of air concentrations can be important, concentration data must be automatically re ber. The second "word" contains die probe number and the actual datum which, in this case, is a number from 0 to 999, propose tional to the concentration of the air con taminant. This information is obtained at a rate that may vary from two times a minute corded. Any continuous analyzer that satisfies these specifications can be used. So far we have used only combustion-conductivity in struments and long-path gas cell infrared spectrometers; both types produce data suit able for computer analysis. to once in 2J> minutes. Because collecting each datum generated it not always neces sary, the digitizer can be programed to skip the collection of some data. In every case, once data from probe No. 1 are punched onto 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 plant is seldom economical Usually the sett of analyses. analyzer is positioned at a central location in the plant, and air samples are brought con Programing the Computer tinuously to it through probes of some sort. Tubing used for these probes must not react We decided that the minimum time in the with, or sorb, the air contaminant of interest data summary should be an eight-hour shift: e For this period the Dow Computations Re Preparing Date for Analysis search Laboratory programed the computer to calculate the mean concentration at each There is no method presently available for reading results automatically from multiple. point chart paper into a computer. This location, the standard deviation of these data, the percentage of time that the concentration was above several preselected levels,' and the means that a device must be used to "digi appropriate time-weighted averages. . tize" the data as they are obtained. At the iwiwinmml 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 outride 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 tnmimiw programing difficulties, the digitizer cognize several kinds of errors in the data includes a digital clock with an output of the aid use only "good" data for calculations: 182 March-April, 1966 Computer Output The basic computer output consists of the piftn concentration, standard deviation, nuns* her of analyses recorded, and percentage of time the concentration was above preselected levels, all at each location for each shift dur ing the survey. Shift or daily averages can aim be obtained over any selected time inter val such as a week or a month. These data, 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 time period of interest, concentrations at these data locations behaved in this manner: % of Time Concentration Exceeds Selected Values at Specified Locations Concentration Data Location Number greater than 1 7 4 12 25 ppm 50 100 250 500 26.63% 28.68% 93.44% 3135% 1639 22.13 20.08 23.77 635 10.65 17.62 11.47 1.63 2.04 1639 3.68 0.00 0.00 0.00 aoo>. The time-weighted percentage of tune spent by operators in concentrations above 25 ppm will then be: <nj) (ttJS) + (sij) (tn) + OM) <g >) + (HO) (SIJJ) + (114) (0) .............. -- an 100 the number of analyses used by the computer This means that on tht average, during the to obtain the shift averages. This has become time period of interest, men in the opera the normal signal for nonroutine maintenance tors classification encountered concentrations of the digitizing equipment. above 25 ppm 38.92% of the time, In addition to the basic output, the conn puter calculates time-weighted averages of two kinds. The first kind is the "usual" timeweighted average concentration to which men are exposed. It is obtained by combining the air analysis data with "job analysis" infor Similar calculations show that these men encountered concentrations above 50 ppm 28.24% of the time; above 100 ppm 1035% of the time; above 250 ppm 4.00% of the time; and above 500 ppm 0.00% of the time. 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 pre 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 Date Analysis Automatic air monitoring reveals that in an industrial situation the variation of con centration with time can be large despite the use of a rather large time base. Figure 1 is a plot of dally time-weighted average concen trations to which operators were exposed on each shift over one week. Each point on the graph is the mean of several hundred deter minations spaced equally over an eight-hour period. These are actual plant data. for an operator in a plant: On the second shift there was little varia Data Location Percentage of Time Spent tion during the first four days, but over the Number at That Location last three days of this period the shift average 1 123 ' varied by a factor of almost four. If the 7 313 threshold limit value (TLV) for this mate 4 183 rial were 50 ppm, an industrial hygienist tak 12 253 ing air'samples during the first three days on Unexposed ' 12.4 the second shift would probably have de clared that the hazard to health was low or 100.0 nonexistent. On the other hand, if he had American Industrial Hygiana Association Journal 183 ppm. sampled on the last day of this period (on the On the other hand, the spread or varia second shift) he could have experienced a bility of the data is indicated in a more mean strong inclination to "push the panic button." ingful manner by the percentage of time the Variation of concentration between shifts concentration exceeded certain levels. Figure can also be striking, arillustrated by a com* 3 is a plot of the time-weighted percentage of parison of the third shift with the first or time above .these levels on log-probability second shifts during the first three days. paper. The interval during which data were Figure 2 places Figure 1 in a broader con* gathered and the operational classification are text. Data foe Figure 2 are also time-weighted r identical to those in Figure 2. This graph average concentratidta to which operators shows that the median concentration to were exposed. The rather extreme variation which men on all shifts in this classification shown in Figure 1 for a time bans of one shift were exposed was 40 ppm and that they can be repeated for a dm. bans of one week. were exposed to 300 ppm or higher 2% of The information contained in Figure 2 is the rim.. based upon hundreds of thousands of indi- The conventional plot (Figure 2) shows a vidual air samples. iran'mnm concentration of about 135 ppm, Average concentrations and time*weighted but it is one of seven-day averages, whereas average concentrations are not the only data Figure 3 is a summation of instantaneous summaries provided by the computer; aver* . values: Both kinds of graph have advantages. age concentrations are accompanied by the A conventional plot illustrates better bow ex standard deviation of the data. Having the posures vary with time, and any trends be standard deviation and using it quantitatively come readily apparent. With this kind of are two different things, however. We do not graph, however, the only usable index of ex yet know how to use such information effi posure is the time-weighted average, a num ciently. ber of limited utility because it cannot reflect 184 Mareh-Aprit, 1966 concentration variations. A log-probability plot such as that shown - in Figure 3 is not very useful as a trend indi cator, but the fact that time*weighted txposurt 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 - um wm bmj i --l-- m --j-1 11 1 |I I 1 Fmvax 3. Date for tee period of shown in Figure 2 (all shifts). The air monitor was not being used bjr operating personnel to assist in the control of leaks. straight line is a complete summary of ex posure information, including measures both of the average concentration and of the con centration variability.1 . The data in Figure 3 were obtained by a survey using the equipment described. How are these data to be interpreted? If we still assume a TLV of 50 ppm, is the exposure represented by Figure 3 a hazardous one or not? These data are among the most com plete ever gathered for men industrially ex posed to a potentially hazardous vapor; what kind of aniwinl experiments should be con ducted to interpret these exposures? We can not yet answer these questions, but we know that they must be answered eventually. Despite this void in our knowledge, the data from this type of survey have several uses. Any information about the intensity of exposures helps to interpret medical findings. Even without complete interpretation, rank ing of exposures is possible. From graphs such as Figure 3 one can judge if tee ex posure ts obviously hazardous or nonhazardous. For instance, in Figure 4 the concentrations to which the men were ex posed are shown to exceed 15 ppm 50% of American Induitrial Hygiene Auociation Journal 185 the time. They exceed the assumed TLV of 50 ppm only about 14% of the time. With* out any further knowledge, the exposure rep* resented by this curve would probably be judged nonhazardous by most industrial hy gienists, provided that contacts with high 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 in the same plant, for the same group of men, and by the analyzer using the same sample points as for Figure 3. During the period of time when the information in Figure 2 was ob tained, the air analyzer, was being virtually ignored by operating personnel in the plant. On the other hand, the data in Figure 4 were obtained when the concentrations recorded by the air analyzer were used to prompt cor rective action. Even plants with a large num ber f 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. Furnas 4. Exposures for the nme people as shown In Figures 2 and 3, but for the period Jan uary 5 to June 18 when the air monitor was used to signal the need for maintenance The timeweighted avenge 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 gives a true picture of the manner in which air concentra tions of the contaminant vary with time. Coupled with a good analysis of the percentage'- 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 spots?* quite readily and, when used properly, can lead to dramatic reduction of concentrationsto which men 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 sir 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 a computer analysis of the data so obtained, can be an extremely potent weapon in the fight of industrial hy gienists against inhalation hazard* Mere installation of an air monitor, however, does not end the battle;. Data obtained by the monitor must be used by operating personnel in muds the 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 are 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 bans for judgment than does the present threshold limit value. References ' 1. lncsniu. J. T. ]*-, nd T. 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