If it is a control whose use has been decided upon following a risk assessment under Reg. 6 and 7 of COSHH then according to reg. it must be “it is maintained in an efficient state, in efficient working order…” The problem is that unlike LEV which has a set of standards to work from dilution ventilation dopes not have any standards (I could be wrong!) that you could apply. What you need to do is to establish, by a risk assessment ,  what your safe level for a particular contaminant is and make sure that the dilution keeps it below that level.