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|Year : 2002 | Volume
| Issue : 14 | Page : 1--7
Monitoring exposure to solvent vapour in the workplace using a video-visualization technique
PT Walsh, RDR Clark, S Flaherty, IJ Plant
Health and Safety Laboratory, Sheffield, United Kingdom
P T Walsh
Health and Safety Laboratory, Broad Lane, Sheffield S3 7HQ
A video-visualization technique, using a combination of a fast-response, direct-reading, personal gas monitor (photo-ionization detector) and synchronized video monitoring (with a standard camcorder), has been applied to assess exposure to solvents in various industries. The objective is to understand how short-term peaks occur and how they can be controlled to minimise both short-term exposure and their contribution to long-term exposure. The technique was employed to identify peaks associated with work activities and their contribution to total exposure arising from use of tetrachloroethene in a dry cleaning establishment. The exposure and video data are simultaneously displayed on computer and the information is disseminated on CD (and video), forming the basis of a detailed occupational hygiene assessment or training material.
|How to cite this article:|
Walsh P T, Clark R, Flaherty S, Plant I J. Monitoring exposure to solvent vapour in the workplace using a video-visualization technique.Noise Health 2002;4:1-7
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Walsh P T, Clark R, Flaherty S, Plant I J. Monitoring exposure to solvent vapour in the workplace using a video-visualization technique. Noise Health [serial online] 2002 [cited 2022 Oct 2 ];4:1-7
Available from: https://www.noiseandhealth.org/text.asp?2002/4/14/1/31815
Exposure to solvent vapour in the workplace may be associated with ill-health. Short-term (peak) exposures may cause acute health effects and contribute towards long-term chronic exposure. It is therefore important to identify work activities which result in exposure peaks in order to understand how they occur and how they can then be controlled to minimise the risk.
Exposure video-visualization is a tool, based on the combination of video and fast-response, direct-reading, personal gas (or dust) sensors, to rapidly assess when and how exposure peaks occur (Walsh et al, 2000a;Walsh et al, 2000b). The information generated is then used to:
Improve work practices and methods of controlProduce educational and training information (on CD and video) on risk assessment and, specifically, good and bad work practice for workers, management and safety practitionersBuild up a database of short-term exposures to assist with exposure assessment.
There are several video-visualization systems (also known as PIMEX) currently in operation throughout the world (e.g. Rosen, 1993; McGlothlin et al, 1996; Heinonen et al, 2000; Martin et al, 1999). This paper describes our video-visualization system and illustrates its application to studies of workplace exposure to solvents of various kinds in various industries, particularly in a dry cleaning establishment where tetrachloroethene is used. Emphasis is given to the development of the system as an analytical tool for the occupational hygienist, and the use of the CD medium for data analysis and training material.
The exposure visualization system for monitoring exposure to gases and vapours consists of the following standard, commercially available equipment:
Hand-held camcorder (Sony DVCAM, 3CCD)Radio-telemetry transmitter and receiver (Satel radio modem 2ASXm plus Satelset battery)personal gas/vapour monitor (Rae Systems MiniRae 2000 photo-ionization detector)Hand-held or portable computer (Psion Workabout, notebook PC).
The use of digital technology in direct-reading instruments (i.e. the gas monitor) now enables better interfacing between the instrument and computer for processing of data. It also facilitates calibration of the system and improves reliability of transmission of data, compared to analogue systems, particularly when using telemetry.
The response time of the photo-ionization detector (PID) is approximately 1 s to 90% of its final reading. The PID and radio transmitter were mounted on a purpose-built harness which makes the personal monitoring as comfortable and unobtrusive as possible, and minimises its influence on the wearer's behaviour see [Figure 1]. Any additional samplers, e.g. diffusive, pumped samplers for calibration of the PID, can also be mounted on the harness in close proximity to the sampling inlet of the instrument.
The system is designed to be flexible and can be configured in various ways; for example, it can be used without telemetry when in potentially flammable atmospheres which requires all electrical equipment to be explosion protected. Here, the response from the instrument (the only intrinsically safe component of the system) can be datalogged and then mixed after the video has been recorded from outside the hazardous area, ensuring that the video and monitor signals are synchronized.
Video and instrument data are processed using application specific software developed by the Health and Safety Laboratory (HSL). The system can be used to show the exposure and the work activity concurrently in live mode on a computer screen, i.e. while the worker under study is actually working. However, the strength of this system is in the processing features performed after the videoing session has been completed. Two forms of output are used: CD and VHS video.
The visualization tool (VizCD) displays the synchronised video and exposure profile simultaneously on the computer screen and enables the exposure data to be annotated with activity descriptions and other events of relevance to the exposure scenario (e.g. ventilation on/off). The data analysis tool (ANA) enables these activities to be examined in detail; for example, calculation of their individual contributions to the total exposure, exploration of various 'what if' scenarios (e.g. what would the exposure be if a particular activity were removed or controlled to zero exposure?).
A snapshot of the VizCD output is shown in [Figure 2].
The visualization tool was developed for flexible, post-session processing, and included
the following features:
'Strip view' see [Figure 2] shows the data scrolling from right to left, the middle point of this view is synchronised with the video. To the left is 'Post Exposure Data': a review of exposure events which have just occurred (usually 2 minutes previous) and to the right is 'Pre Exposure Data': a preview of events about to happen (usually the next 2 minutes). The ability to see significant exposure coming, i.e. to 'see into the future', which primes the viewer to observe closely what will happen, is a powerful aspect of our system.Ability to view all of the exposure profile ('Full view' on [Figure 2]). In this view a cursor, synchronised with the video, scrolls across the data from left to right.Rapid review of video and monitor data on the computer screen (faster than review using a video cassette recorder).Labelling of tasks/activities, in a hierarchy, if required. (Current Actions/Status box in [Figure 2]).Running calculation of TWA using any time period and comparison with short-term limit (STEL) values, e.g. 15-min STELs.Auto-scaling or fixed scale (selectable) of charts, calibration and labelling of instrument response scale (scale in [Figure 2] is calibrated in ppm but other units are possible).
Video (VHS) output
The video-mixing tool displays the work activity as a standard video image with the simultaneous concentration profile of the gas derived from the monitor depicted in a window in the work activity video image, as in [Figure 1]. As the video is mixed after the session, the concentration profile to the left of the centre of the window represents the previous 30 seconds of exposure (i.e. equivalent to the Post Exposure Data section in [Figure 2], while the profile to the right represents exposure about to occur in the next 30 seconds (i.e. equivalent to the Pre Exposure Data section in [Figure 2]). The current time, as depicted in the video image, is represented by the centre of the concentration window; so, as the video runs, the concentration profile moves from right to left (similar to Strip View in VizCD).
Calibration of the gas monitor
For some applications, such as measuring a single substance vapour with no interferents present (e.g. some dry cleaning establishments where tetrachloroethene is effectively the only component present which the PID responds to), the PID was calibrated prior to the session using this vapour. For the dry-cleaning study, an indirect calibration was performed with a standard calibrant (isobutene) and then a response factor was applied to the reading to convert it into units of tetrachloroethene concentration. A response factor of 0.57 was used for tetrachloroethene relative to isobutene (Rae Systems, 2000), i.e. the PID is 1/0.57 times more sensitive to tetrachloroethene than isobutene. A comparison of the TWA from the PID datalogger can also be made with the TWA obtained from a pumped or diffusive sampler placed adjacent to the sampling inlet of the PID.
For the majority of other, more complex situations, the PID response was expressed as an isobutene equivalent. Also it was possible to calibrate the PID retrospectively using a diffusive or pumped sampler placed adjacent to the instrument and comparing their TWAs over the same period of exposure. For a multicomponent mixture, this assumes that the relative concentrations of the various components of the vapour remain constant over the averaging period, although this may not be true for evaporating solvents where the relative concentrations in the vapour phase may change over time.
Tetrachloroethene exposure in dry-cleaning establishments
Tetrachloroethene (also known as tetrachloroethylene or perchloroethylene) is used as the cleaning solvent in dry-cleaning establishments. The major sources of exposure arise from loading and unloading machines with washing, and the background level. The principal health effects of tetrachloroethene are central nervous system disturbance, eye irritation and possible reproductive toxicity. Tetrachloroethene is assigned an Occupational Exposure Standard in the UK: the long-term exposure limit (8-hr reference period) is 50 ppm (345 mg/m3) and the short-term exposure limit (15-min reference period) is 100 ppm (689 mg/m3). Short-term exposure to solvents, including tetrachloroethene, is also discussed by Stear (2001).
The exposure profile over a shift derived from the data analysis tool (ANA) is shown in [Figure 3] where the y-axis concentration units are in ppm.
There are seven cycles where items of clothing are loaded into and unloaded from the drycleaning machine (see video image in [Figure 3]). Peaks of several hundred to nearly 1000 ppm are observed. By analysing the video and exposure data, information on the contribution of activities to exposure can be derived. An example is shown in [Table 1].
Control measures which could be applied to reduce particularly the peak exposures include:
Loading the machine with a smaller amount of washingDistilling tetrachloroethene from the residue at the end or beginning of a shift and not between wash cycles would reduce exposure when loading/unloadingInstalling local exhaust ventilation adjacent to the dry-cleaner which would reduce exposure due to loading/unloading and the general background.
The data in [Table 2] can be used to estimate what the exposure would be if controls such as those above were applied. The results are shown in [Table 3], assuming 100% effectiveness of control.
This is a very simplistic treatment of the data as it assumes the background level would remain constant. However, it is a conservative estimate as, in practice, the background level would also be reduced as the amount of vapour released into the general area from loading/unloading would be less. These results are similar to those obtained in a more detailed study by Earnest (1996) solely using direct-reading instruments.
Video-visualization has been applied to the study of exposure to solvents and control in several industries. These are briefly summarised below:
Print works - exposure to white spirit type solvents when cleaning printing rollersRubber - exposure to solvents (e.g. toluene, methyl ethyl ketone) used in rubber solutionsFurniture - exposure to stripping solvents when cleaningBoat building - exposure to styrene when applying glass-fibre reinforced plastic (GRP) forming polystyrenePaint - exposure to paint solvents e.g. xylene.Distillery - exposure to ethanol when checking whisky barrels.
The exposure video-visualization technique allows a detailed analysis of how exposure occurs and suggests methods for improved work practices and methods of control. The use of the CD, in particular, and video outputs are effective means of displaying and disseminating the information which can then be used by occupational hygienists and other safety practitioners. It can also form the basis of training information on risk assessment and, specifically, good and bad work practice. Finally, it can be used to build up a database (graphical and numerical) of short-term exposures to assist with exposure assessment.
Many other applications of video-visualization are possible where suitable instrumentation exists. Examples are:
measurement of noise, particularly when coupled with solvent measurements as described aboveergonomics applications, e.g. using strain gauges, heat stress monitorsmeasurement of exposure to aerosols.
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