In our last post we saw how inhalation exposures to hazardous in the workplace are highly variable. The spread of results from an air sampling survey is usually quite wide and will usually conform to a skewed log-normal distribution.
This means that although there is a large spread of results the majority of exposures are at the lower end of the range with a relatively small number of high exposures. Nevertheless these higher results are valid and cannot be discarded.
This has a number of important implications for the design of air sampling surveys and the interpretation of the results. In particular for the number of samples that need to be taken to allow a clear picture of the range of exposures to be obtained.
If only one or two samples are taken for a particular process or task then it is most likely that they will be from the lower end of the range where the bulk of the results are clustered. It is less likely that a result from the higher end of the range, which could be 3 or more times greater, will be obtained. Care has to be taken, then when interpreting the results to decide whether exposure is adequately controlled. If the results from the small data set are close to the relevant occupational exposure limit, then it is likely that some workers on some occasions are likely to have exposures that will exceed the limit. Even if the results are substantially below the limit, this does not necessarily guarantee that exposure is adequately controlled.
One way to avoid this problem is to take enough samples so that a reasonable estimate of the range of exposures can be obtained. This is easier than it sounds. How many samples are enough? Unfortunately, guidance is sketchy.
The British Health and Safety Executive, in Appendix 2 of their guidance document HS(G) 173 “Monitoring strategies for toxic substances”, gives the following advice on the number of samples required is given:
- for worst case sampling, 1 in 5 workers should be monitored, unless a smaller number can be justified;
- for representative measurements, if there are fewer than 10 employees, only 5 need to be included, but although caution is advised for "more complex situations involving more then ten employees", no further advice is given on how to proceed.
However, many occupational hygienists would consider that even in simple cases, considerably more than 1 in 5 workers need to be included.
The American Industrial Hygiene Association, in their A Strategy for Assessing and Managing Occupational Exposures, suggest that 6 to 10 measurements are normally enough to obtain a reasonable estimate of worker exposures from each group. It will usually be practicable to collect this number of samples during a one day survey. However, there are many situations where only a small number of workers, fewer than this, carry out a given task. It may require repeated sampling on several days to obtain 6 samples
It is worth bearing in mind that, a better picture of exposure will be obtained if a larger proportion of workers are covered by the survey. A little common sense needs to be used, balancing the value of the results against the cost of the survey. Sampling is expensive and there is always pressure from clients to keep costs down.
The British Occupational Hygiene Society and the Nederlandse Vereniging van Arbeidsdeskundigen, have recently published a draft document on Testing Compliance with OELs for Airborne Substances (available here) which addresses these issues. It’s not an easy read but there will be a workshop on the proposals during the first day of the forthcoming BOHS Autumn Scientific Conference taking place in Leeds on 14th and 15th November. I’m sure there will be other opportunities to discuss it in the future. I’ll be returning to it in a future post.