If there is doubt that the LEV is adequate then it would be prudent to have some additional monitoring carried out (most often personal dose monitoring , not "air monitoring"). You should be guided by the competent person who tests the system. Personal monitoring is often carried out at the commission stage of LEV to confirm all is well. Ideally you will have access to the original design and spec for the LEV and year-on-year records to show any potential decrease in efficiency of the LEV system. In the widest possible context, where there is doubt there is a legal (COSHH) requirement.