Speciation of metals and metalloids in tobacco and tobacco smoke : implications for health and regulation

Abstract

Some metals and metalloids make significant contributions to the harmful effects of tobacco consumption although understanding the mechanisms involved in toxicity is hampered by the lack of information on their chemical and valence species, both in tobacco and in smoke. This research addresses the speciation of the metals and metalloids most frequently implicated, particularly those elements that exist in nature in multiple valence states, namely arsenic (As) and chromium (Cr), there being considerable differences in toxicity with oxidation state. A strategy was devised to overcome some of the problems that have thwarted earlier attempts at speciation. Firstly tobacco plants were cultivated under controlled conditions in compost burdened with high levels of metals and metalloids resulting in leaf with up to 250 µg g⁻¹ As, although Cr uptake was less successful. Secondly valence speciation even at the exceptionally low concentrations of As and Cr in smoke from unburdened tobacco was achieved with XANES analysis using the exceptionally bright Diamond synchrotron source. This revealed that combustion of tobacco has a marked effect on valence speciation with As(III), the reduced form of As, dominating (and persisting) in condensate of tobacco smoke while ash is dominated by the oxidised form, As(V). Chromium also appears to be present in smoke mainly as reduced Cr(III) species. HPLC-ICPMS analysis of arsenic indicates the dominance of inorganic over organic species (~4:1). Other metals were investigated in less detail. These findings establish that arsenic is present in smoke in its most toxic form and represents a significant risk to health. Conversely smokers appear to be exposed to the less harmful species of chromium. These results support a recent WHO report that includes As but not Cr in a list of four metals and metalloids recommended for regulation in crops and commercial products in the interests of public health.