Guidelines for Metals and Metalloids in Ambient ... - ARCHIVE: Defra

Expert Panel on Air Quality Standards
Figure 2.2: Arsenic – pulmonary absorption, metabolism, tissue distribution and
excretion. (a) 30-60% inhaled arsenic present in most respirable particulate matter is
deposited in the lung. (b) Particulate matter deposit onto the airway surface and the
soluble forms if arsenic move into the airway fluid. (c) Pentavalent arsenic is reduced
to trivalent arsenic in the blood. (d) Trivalent arsenic is taken up by the liver and
metabolised to monomethylarsonic acid and dimethylarsinic acid. (e)
monomethylarsonic acid and dimethylarsinic acid are eliminated from the liver and
excreted by the kidney (f) The majority of inhaled arsenic is excreted by the kidney.
a
As
As 5+
As 3+
c
b
As
As 3+ As 3+
Liver
Methylation
d
MMA + DMA
Tissue distribution
e
f
Excretion (75%)
Pentavalent arsenic tends to be eliminated rapidly. It is largely (50–70%) reduced in
the blood to the trivalent form, part of which is methylated in the liver (Marafante
et al., 1985; Vahter and Marafante, 1985). The efficiency of methylation decreases
with increasing dose level and with low protein intake (Vahter and Marafante, 1987).
22