A little hard work reading through your post but I think I get it now.

You are wanting to re-use the masks for up to 8 hours in total, in 15 minute segments, am I right?

You say the manufacturer should be contacted and talked to re a deviation - why not do just that? Surely it will not cost anything and may provide the guidance you want and in your favour however if they say otherwise it will be "from the horses mouth".

If you are a manufacturer U should be using LEV as against RPE in the first place for day to day chemical experimentation etc areas!

And the law is first before instructions from manufacturers etc and proper risk assessment should answer your questions as there may be more than 1 factor at work

Chris - Yes you have it about right, The filters (Its a cartridge as ians points out) are designed for a single 8 hour shift but our exposure is of low level and short duration. I have already contacted the maunfacturer and they will not deviate from the recommendation to use for no more than one shift even if its only 15 minutes.

KD - Exposure has been measured by monitoring (personal diffusion badge)

Bob - We are using LEV, the RPE is to protect against residual risk, specifically because despite the low levels (most is removed by LEV) people are still experiencing symptoms therefore further control is necessary. It not RP to upgrade the LEV though its possible in the future as we expand the operation.

KD has a point about how the filter may be affected but there is also the manufacturers vested interest in selling us up to 20 cartridges a month insted of 1-2 and at over £5 each thats big difference.