The chemical speciation and transformations of arsenic in humans and in the environment

The human metabolism and biotransformation of arsenic were studied by the analytical speciation of urinary arsenic excreted by volunteers in a series of laboratory-controlled experiments, the general population and several groups subject to enhanced environmental or occupational exposure to inorganic arsenic. In additon, the post-depositional remobilisation of arsenic in the aquatic environment was investigated by the establishment of the chemical forms of arsenic in the sediment porewaters of Loch Lomond and the Dubh Lochan and in the sediments of nearby Scottish sealochs. Total arsenic concentrations were measured by hydride generation-atomic absorption spectrometry (HGAAS) following acid digestion of human urine and biological material and dry ashing of sediment samples. Urine samples were rapidly directly screened for the approximate sum of the hydride-forming species arsenate (As(V)), arsenite (As(III)), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), which were subsequently separated and accurately determined by ion-exchange chromatography/HGAAS, a procedure uninfluenced by the presence of the common dietary organoarsenicals from seafood. In two experiments, the ingestion of several hundred micrograms of arsenic as the stable organoarsenical in seafood and as inorganic As(V) in mineral water resulted in the rapid gastrointestinal absorption and major urinary excretion of arsenic in under a week. A two-component model showed that urinary arsenic excretion was faster in the former, as reflected in first and second component half lives of 6.9 - 11hr and 3 days compared with 17 - 24hr and 7.1 - 8.6 days for the latter. However, while the seafood organoarsenical was excreted unchanged in urine, inorganic As(V) was rapidly reduced to As(III) and further biotransformed into the less toxic methylated metabolites MMAA and DMAA. After only 12hrs, DMAA became the predominant species to such an extent that over 7 days As(V) constituted only 9 - 10% of the total eliminated arsenic, As(III) 12 - 15%, MMAA 9-18% and DMAA 57 - 69%. In a subsequent experiment involving regular oral intake of As(V), and equilibrium was established in which 40 - 60% of the daily dose of 60 - 70ug As(V) was excreted in urine. As(V), As(III), MMAA and DMAA were eliminated in a steady speciation pattern with mean proportions of 5.8/15.2/14.3.64.7% respectively. Some possible longer term tissue retention of As(III) was indicated. For the general population, the concentration of the sum of the hydride-forming species As(V), As(III), MMAA and DMAA rarely exceeds 10ug/g creatinine, of which DMAA is the dominant species (> 80%). With calculated intakes derived from urinary arsenic data comparable to M.A.F.F. dietary estimates and well below recommended limits, there is a negligible risk to the health of the general population from typical exposure to inorganic arsenic. In the mineralised south west of England, where it has been suggested that the highly enriched soil arsenic concentrations may at least be a cofactor in the increased incidence of skin cancer, urinary arsenic levels were only slightly elevated. The potential for increased uptake by the local population was, however, reflected in the more frequent occurrence of As(III) and MMAA in urine and, of especial significance, in two comparatively highly elevated sum of species concentrations of 48.7 and 20.8ugAs/g creatinine recorded for 2 pre-school children. Elsewhere, for workers occupationally exposed to inorganic arsenic, mean urinary arsenic excretion varied from < 10ug/g creatinine for those in the electronics industry to 67.4ug/g creatinine for timber treatment workers applying arsenical wood preservatives, to 79.4ug/g creatinine for a group of glassworkers using arsenic trioxide and 244.8ug/g creatinine for chemical workers involved in the manufacture of arsenical compounds. The maximum urinary arsenic (As(V)+As(III)+MMAA+DMAA) concentration recorded was 956.4ug/g creatinine. For the most exposed groups, the average species proportions ranged from 1 - 6% As(V), 11