Therefore, the assessment of the risk, which is traditionally evaluated with reference to threshold limit
values (TLV) or permissible exposure levels (PEL) defined on the basis of daytime human exposures or
animal experiments, should be reconsidered also in the light of evening and night working shifts, as it has been
already suggested for prolonged 12-hours shifts. Also, as concerns biological monitoring, some chronobiological
studies have been published, aimed at defining the circadian pattern of excretion of toxic substances or
indicators of effect, both in exposed and non-exposed workers, for a better definition of the strategies of
control and protection in the field of the Occupational Medicine