Can any one offer challenging or supporting advice on the frequency rate for biological monitoring of exposure to a cacogenic substance (Benzene), at the moment we are doing spot monitoring measuring the PPM levels within the area and breathing zone. The task is vessel internal cleaning of residual sludge left over from draining, flushing and purging, unfortunately the levels can not always be guarantied and there is always the possibility of agitation disturbing the content and increasing the exposure level. For this reason the task is always undertaken in full PPE inclusive of full face BA. Spot monitoring of the vessel internal and exterior is at agreed frequency. As the control is PPE it is understandable that the best way to measure the effectiveness of the control is by biological monitoring (pre-test and post exposure) which to date has proven effective with no personnel exposure. There has been an argument put forward that the frequency of biological monitoring should be daily if the exposure is to run for an extended period of time. If the controls have been proven to work would this still be the case? Or what would be reasonable? Does anyone have case studies which demonstrate good control?

If you also do some personal monitoring for cases where the spot monitoring identifies there can be significant exposures, you will be able to get time-weighted average data. If these show potential exposures near or above the WEL then you are relying on the respiratory protective equipment to reduce exposure. Your face-fit testing data should allow you to calculate a worst-case actual exposure (i.e. the TWA figure divided by the protection factor). For the examples you mention it seems very unlikely that potential exposure will be anywhere near the WEL but, to confirm that, you could do occasional biological monitoring. No need to monitor everyone every shift, just some examples. If these show the actual exposures are very low, you've proved that your controls are fine, so don't need to carry on - provided you are confident these controls won't deteriorate over time. I presume you know that urine sampling is probably the most reliable indicator, and that you need before- and after-exposure samples, with the latter taken several hours after exposure ceases, or it won't be accurate.

I take your point Chris, but probably not for benzene? And certainly not for the task described, where those cleaning inside the vessel will have good skin protection everywhere (at least in my experience.