Abstract
The inputs of As in forest ecosystems have declined since the eighties when the higher concentrations of that metalloid were observed due to industrial activities. The As inputs to the forest floor include throughfall and litterfall where dry deposition is an appreciable percentage. This is manifested by the higher As concentration in older needles of conifers and the enrichment of throughfall relative to the bulk deposition. The throughfall and the forest floor convert the inorganic As into methylated organic As and in this way reduce its toxicity. In unpolluted forests the vast percentage of As is retained in soils because the oxides of Fe and Al are very efficient holders. In polluted forested soils the As can become mobile and enrich the surface runoff waters approaching even the threshold value set by the World Health Organization. For this reason forest soils with high concentration of As due to former high loads should be monitored.