Eddy,
I guess that given it is only ever likely to form an explosive atmosphere when an uncrotrolled release occurs, and as SaxonX says, becomes an aerosol, then you could classify the localised area a zone 2, under DSEAR, but this may require that other equipment in that loacised area needs to be at least of an IP 4 or 5 rating, obviously a very costly exercise!!
Personally I believe that if taken as read, the DSEAR regs can be a little misleading, for example, would you classify a small highstreet bakery as a Zone 21 or 22?? this would propably put most of them out of business, I think not!! However if you take the guidance word for word then youy clearly would, and I think the HSE are now starting to appreciate this type of scenario.
Hope this helps
Paul