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|Year : 2002 | Volume
| Issue : 14 | Page : 9--13
The importance of controlling short term exposures to solvents
Occupational Hygiene, Health and Safety Executive, United Kingdom
Principal Specialist Inspector (Occupational Hygiene), Health and Safety Executive, HSE, Room 319, Magdalen House, Stanley Precinct, Bootle, Merseyside L20 3QZ
Knowledge of short term, task specific sources of exposure is becoming more important as, for many industries these represent the dominant source of exposure over the whole eight hour shift period. For solvents, workers can be exposed to very high peak or short term exposures, yet still be below the relevant occupational exposure limit. This article explains the importance of understanding short term exposures to solvents, and suggests that our appreciation of chronic ill health should go beyond total dose, and that any periods of high exposure should be controlled as far as reasonably practicable. It also shows that a better understaing of these exposures helps to target control measures and can reduce process running costs.
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Stear M. The importance of controlling short term exposures to solvents.Noise Health 2002;4:9-13
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Stear M. The importance of controlling short term exposures to solvents. Noise Health [serial online] 2002 [cited 2022 May 23 ];4:9-13
Available from: https://www.noiseandhealth.org/text.asp?2002/4/14/9/31814
Around two million workers use solvents in the UK for a range of applications, including coatings, adhesives, surface cleaning, printing, dry cleaning, and pesticides. They are also used in the chemical industry.
Solvents can be used safely, with exposures well below the relevant statutory occupational exposure limits. Conversely, where control is poor, occupational exposures can be high. Higher exposures tend to be found in the more diffusive applications, such as surface cleaning and paint stripping, although control is achievable where an adequate assessment has been carried out and the necessary control measures adopted. Most solvents are very volatile, and this is in part the reason why they are used; solvents 'flash off' from paints, dry from cleaned surfaces etc. It is this property and their ability to suspend or extract materials, without chemical change to the material or solvent, which has resulted in their extensive use. Control therefore requires consideration of this potential to generate high airborne concentrations.
There are many examples of industry sectors where control has been poor, with occupational exposure significantly above the relevant occupational exposure limits (OELs). There have been improvements in control, and generally there is a greater level of awareness, although high exposures to solvents do still occur.
Understanding Occupational Exposure
To achieve adequate control of exposure, you need to first understand how occupational exposure arises. For example, the end-of-shift cleaning may only take 20 minutes, but could provide 90% of the employee's full shift exposure. To explain this further, we can look at an example. Perchloroethylene is used in dry cleaning. It has occupational exposure standards (OES) of 50 ppm 8hr TWA (time weighted average) and 100 ppm 15 min TWA.
In our example, the dry cleaning operatives receive an 8hr TWA exposure of 25ppm. Time weighted average means that the average exposure was 25ppm throughout the shift (see the straight horizontal line at 25 ppm on [Figure 1]). Clearly this does not happen, and the level of exposure will fluctuate depending on what activities are being carried out. [Figure 1] also shows a typical workplace exposure profile with such fluctuations. Each peak and trough represents an activity, change in process condition, or even a movement by the operator as he / she carries out the task. Exposures could be many times higher than 25ppm on some occasions and nil at other times. To fully address control, we need to understand our dry cleaning workers' exposure profile, the peaks and troughs, and not just the 8 hour TWA.
This does not have to involve expensive measurement techniques. A lot can be understood from simply watching and talking to the operators to see which activities bring them into direct contact with the solvent. The shift can be broken down into its component activities. Determine which are the major contributors to exposure and tackle these first.
The activities that give rise to the highest routine exposure are loading and unloading of the dry cleaning machine. We can determine the duration of these tasks by watching the operator and determine the number in the shift by asking the operator. Therefore, the operator has long periods of no (or little) exposure with intermittent periods of high exposure. [Figure 2] illustrates this fact, showing the exposure profile during one loading and one unloading operation. These tasks will each be repeated throughout the shift depending on the number of cleaning cycles carried out.
This simple chart shows the approximate exposure values taken from a HSE study using a technique called video visualisation. Personal air sampling is carried out using a direct reading instrument for gases / vapours (using a photoionisation detector). The operator wears a harness with the direct reading instrument fixed to the chest and radio data modem on his / her back. This radio telemetry communicates with a remote modem connected to the datalogging and control computer. The activity is videoed, and back in the laboratory the data and video are overlayed together. This allows the activity and exposure profile to be viewed on screen at the same time; therefore the peaks in exposure and their causes are immediately apparent.
It takes about 2 minutes to load the dry cleaning machine and 2 minutes to unload it, with the operator carrying out several loads in a shift. As [Figure 2] shows, task specific exposures are far higher than was indicated by the 8hr TWA of 25 ppm. The 8 hr TWA is masking these peaks, the periods of little or no exposure reducing the full shift exposure. Do we need to worry about exposures up to 1000 ppm when the 8hr and 15 min TWAs are below their respective OESs?
Where a substance is assigned an OES, exposure is deemed to be adequately controlled if exposure is below the OES. The 8 hr TWA exposure in the above example is below the relevant OES and the 15 min TWA is at or below the relevant OES. If we assume that compliance is being achieved, then no further action is necessary. This is true from a legal perspective, but there are still good reasons why we need to understand what's happening with these short term task specific exposures and how to further reduce them.
1) Health risks?
We have known for decades that exposure to high concentrations of solvent vapour can result in acute effects, narcosis. Narcosis results from depression of the central nervous system. Many solvents have a short-term exposure limit (STEL) to address this health concern. However, there is concern that high short-term exposures could also result in chronic ill health.
A study of dockyard painters, by Dick et al 2000, suggested neurological deficits in those painters. Seaton's work suggests that chronic exposure to solvents may be important, even when the lifetime dose to solvents is lower than the OES for a full working lifetime. He concluded that the periods of significant exposure to solvents were important. Exposures during these periods may have been significantly higher than relevant 15min or 8hr TWAs.
In the study the painters' exposures were presented as 'OES years'. This represents the equivalent number of years' the painters were exposed to solvent concentrations at the OES. For example, one worker had a score of 14.6 OES years. This means that he was exposed to a calculated average solvent concentration at the level of the OES for a notional 14.6 years. The exposure of each subject was calculated by matching the solvent exposure estimate with his work history. A year at 2 times the OES would therefore be two OES years, and so on. It provides a simple representation of dose compared to the relevant OES. A worker exposed to the level of the OES for a working lifetime would have a value of 40 OES years. Seaton's study considered what the actual exposure profile was, and not just the total lifetime dose. The actual exposure profile over the individual's working lifetime would have been very different to this average represented by 'OES years' value. Typically there were periods of very high exposure interspersed between periods of low or no exposure. The painters reported occasions of intoxication and virtual unconsciousness when applying adhesives or paints
Therefore, the workers solvent exposures, time weighted for a lifetime's work, were probably below relevant occupational exposure limits. The authors still suggest there were neurological deficits. They conclude that the periods of substantial exposure were of more concern in the development of the neurological effects. For some brief periods solvent exposure would probably have been many times higher than the relevant OESs even though the time weighted lifetime exposure was below the OES. The interspersed periods of little or no exposure would reduce the overall lifetime dose.
It is worth noting in contrast to the above perchloroethylene example, that these painters' gross over exposure may have been for minutes, hours or even over several consecutive shifts depending on the nature of the work. Therefore, the painters would have been exposed to solvent vapour above the OES during some shifts, whereas in our example the dry cleaning operatives were not. However, the study suggests that our appreciation of chronic ill health should go beyond total dose, and that any periods of high exposure should be controlled as far as reasonably practicable.
2) Better control?
The second reason is to do with understanding sources of exposure. The dry cleaning operative in the above example spends only a fraction of his / her time (about 6%) loading / unloading the machine. But, this 6% of the shift contributes to 45% of the total exposure for the shift. If we target control resource at this 6% of activity then we can make a major impact on exposure. In fact the other 94% of the shift, representing 55% of exposure comes mainly from background levels of solvent vapour. Reducing exposure during loading / unloading of garments will also reduce this background exposure.
To control exposure we have to understand it. Why is exposure so high during loading / unloading when the clothes should be dry? Look at [Figure 2] again, why is exposure higher during loading? The most likely explanation is that the operator takes a large bundle of garments and forces them in, displacing a large plug of air from the drum. Why is this air laden with perchloroethylene? There will be residues of liquid / vapour from the previous cleaning cycle in the drum, which will be enough to create a high concentration inside the drum. Any activity which then displaces the air will result in high exposure. Worse still, the operator may lean inside to remove garments. A simple way of reducing exposure is to add smaller bundles of clothes. More importantly, preventative maintenance and correct working practices (e.g. cleaning cycle times) can reduce the level of perchloroethylene vapour in the drum. This will reduce exposure during loading and unloading.
But, we have done more than that. We have broken down the shift into the component tasks and addressed exposure during each of these. This has reduced not only the 8hr shift average, but also the task specific exposures and high peaks, and therefore the risk of ill health.
3) Good Health is Good Business
There's another added bonus - process costs have been reduced. Targeting the specific sources of exposure and controlling them reduces plant emissions. Increasing the containment of the solvent reduces solvent losses, and therefore the frequency of solvent topping-up. HSE publication, Health Risks Management: A guide to Working with Solvents
3, contains case studies of companies that have shown that Good Health is Good Business. For example, an industrial paint stripping company spent just £2,500 on new equipment, including a second-hand hoist. They identified unloading of components from the solvent bath as a major contributor to exposure. The hoist was installed to avoid the need for manual removal of parts. This targeting of the high short-term task specific exposures has significantly reduced exposure and significantly reduced the amount of solvent they use. This case study is reproduced at the end of this article. This guidance contains general advice on the control of exposure to solvents. HSE has also published new guidance on the control of hazardous substances to help employers, particularly SMEs (small to medium sized enterprises), comply with COSHH. This guidance, COSHH Essentials: Easy steps to Control Chemicals 1, is a step by step process which helps identify the correct control methods for the products and tasks in the workplace.
As increasing pressures from health, safety, and environmental legislation are placed on solvent users, employers need to ensure that emissions are reduced to meet these requirements. As has been shown in this article, fully understanding the workers' task specific exposure profile provides a fuller appreciation of the health risks and the actions needed to fully achieve control.
|1||COSHH Essentials (ISBN 0-7176-2421-8) is available from HSE books (tel:01787 881165) or www.hsebooks.co.uk|
|2||Dick, F, Semple, S., Seaton, A.(2000) Neurological deficits in solvent-exposed painters: a syndrome including impaired vision, cognitive effects, tremor and loss of vibration; Quarterly Journal of Medicine, 93; 655-661|
|3||Health Risks Management : A guide to Working with Solvents; HSG 188; ISBN 0-7176-1664-9 and Working Safely with Solvents; INDG 273 are available from HSE books (tel:01787 881165) or www.hsebooks.co.uk|