 methylation is considered the detoxification pathway for inorganic arsenic ( inas ) , an established human carcinogen . 
 urinary speciation analysis is used to assess the distribution of metabolites [ monomethylarsonate ( mma ) , dimethylarsinate ( dma ) , and unmethylated arsenic ( inas ) ] , as indicators of methylation capacity . 
 we conducted a large biomarker study in northern chile of a population chronically exposed to high levels of arsenic in drinking water . 
 we report the results of the methylation study , which focused on the effects of exposure and other variables on the percent inas , mma , dma , and the ratio of mma to dma in urine . 
 the study consisted of 122 people in a town with arsenic water levels around 600 micrograms / l and 98 participants in a neighboring town with arsenic levels in water of about 15 micrograms / l . 
 the corresponding mean urinary arsenic levels were 580 micrograms / l and 60 micrograms / l , of which 18.4% and 14.9% were inas , respectively . 
 the main differences were found for mma : dma ; exposure , smoking , and being male were associated with higher mma : dma , while longer residence , atacameo ethnicity , and being female were associated with lower mma : dma . 
 together , these variables explained about 30% of the variability in mma : dma . 
 overall , there was no evidence of a threshold for methylation capacity , even at very high exposures , and the interindividual differences were within a much wider range than those attributed to the variables investigated . 
 the differences in percent inas were small and within the ranges of other studies of background exposure levels . 
 the biological significance of mma : dma , which was more than 1.5 times greater in the exposed group , and its relationship to sex , length of exposure , and ethnicity need further investigation because its relevance to health risk is not clear.imagesp620-afigure 1.figure 2.figure 2.figure 2.figure 2 . 
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