Hazard Evaluation and Limiting Concentrations for Humans

The US EPA has developed so-called Reference Doses (RfD) (US EPA 1997, Vol. V). For a risk assessment of inorganic ionic mercury, it ivas concluded that the critical (= most sensitive adverse) effect is the formation of autoimmune glomerulonephritis. The production and deposition of IgG antibodies to the glomerular basement membrane was considered the first step in the formation of this (for details, see Section 17.6.6). On the basis of three animal experiments on this effect with Brown-Norway rats, lifetime LOAEL levels of 0.23 to 0.63 mg Hg kg per day were calculated, and from these an RfD for humans of 0.3 jg kg body weight per day was derived. This concerned an overall uncertainty factor of 1000 (10 from subchronic to chronic exposurexlO from LOAEL to NOAELxlO both from animal to humans and sensitive populations). 

Effects on the nervous system appeared to be the most sensitive toxicological endpoint 

Metallic liquid mercury was assessed to be less toxic. Therefore no reference value was established by the US EPA. 

In a similar way, ATSDR has set Minimal Risk Levels (MRLs) such as 7 pgkg per day for an acute and 2 pg kg per day for an intermediate oral intake of inorganic mercury. For methylmercury intake, an acute-intermediate MRL of 0.12 pgkg per day was established (ATSDR 1997). Again, the differences to the US EPA calculations result predominantly not from the data basis but from the assumed uncertainty factors. 

NIOSH determined a Recommended Exposure Limif (REL), time-weighted average, of 50 pg Hg m for mercury vapor, and 100 pg m for aryl and inorganic mercury compounds (NIOSFl 1983). ACGIFl established the same concentrations as 8-hour time-weighted averages as Threshold Limit Values (TLVs). 

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