Environmental Research 93 (2003) 202–215 Surface reactivity of volcanic ash from the eruption of Soufrie`re Hills volcano, Montserrat, West Indies with implications for health hazards Claire J. Horwell, a, Ivana Fenoglio, b K. Vala Ragnarsdottir, a R. Steve J. Sparks, a and Bice Fubini b a Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK b Dipartimento di Chimica IFM, Universita` di Torino, Torino 10126, Italy Received 7 August 2002; received in revised form 28 February 2003; accepted 10 March 2003 Abstract The fine-grained character of volcanic ash generated in the long-lived eruption of the Soufrie`re Hills volcano, Montserrat, West Indies, raises the issue of its possible health hazards. Surface- and free-radical production has been closely linked to bioreactivity of dusts within the lung. In this study, electron paramagnetic resonance (EPR) techniques have been used, for the first time, on volcanic ash to measure the production of radicals from the surface of particles. Results show that concentrations of hydroxyl radicals (HO d ) in respirable ash are two to three times higher than a toxic quartz standard. The dome-collapse ash contains cristobalite, a crystalline silica polymorph that may cause adverse health effects. EPR experiments indicate, however, that cristobalite in the ash does not contribute to HO d generation. Our results show that the main cause of reactivity is removable divalent iron (Fe 2+ ), which is present in abundance on the surfaces of the particles and is very reactive in the lung. Our analyses show that fresh ash generates more HO d than weathered ash (which has undergone progressive oxidation and leaching of iron from exposed surfaces), an effect replicated experimentally by incubating fresh ash in dilute acid. HO d production experiments also indicate that iron-rich silicate minerals are responsible for surface reactivity in the Soufrie`re Hills ash. r 2003 Elsevier Science (USA). All rights reserved. Keywords: Volcanic ash; Montserrat; Respiratory health; Surface reactivity; Particles 1. Introduction Millions of people worldwide are frequently exposed to volcanic dusts from resuspension of particles from volcanic soils by quarry or construction work, farming, domestic activities, and wind. The respirable health hazards of volcanic ash were first studied in detail after the eruption of Mount St. Helens, USA in 1980. The results of toxicological and epidemiological studies carried out in the wake of the Mount St. Helens event indicate that volcanic ash is less toxic than pure crystalline silica in normal lungs and may be similar to inert agents, such as titanium dioxide, but could cause pulmonary fibrosis in large doses or over long time periods. These results are summarized in a series of overview papers in the American Journal of Public Health Supplement (Health Effects of Volcanoes: An Approach to Evaluating the Health Effects of an Environmental hazard, Vol. 76, 1986) and in Horwell et al. (manuscript in preparation). Interest in the health implications of volcanic ash has been heightened by the long-lived and ongoing eruption of the Soufrie`re Hills volcano, Montserrat, West Indies. The volcano has been producing ash since the onset of the current eruption in July 1995 (Robertson et al., 2000; Bonadonna et al., 2002). As the eruption progressed into a long-lived state, the potential hazard of the ash to human health became a concern (Baxter et al., 1999). Several toxicological studies carried out on the Soufrie`re Hills volcanic ash (e.g., Wilson et al., 2000; Cullen et al., 2002) have shown that it is toxic, albeit less than quartz (DQ12) but markedly more than TiO 2 . These studies also indicate that prolonged exposure could lead to fibrosis. Other toxicological studies (Housley et al., 2002) have found that ash samples caused little reaction in rat lungs and cells. ARTICLE IN PRESS  Corresponding author. Fax: +44-0-117-925-3385. E-mail address: claire.horwell@bristol.ac.uk (C.J. Horwell). 0013-9351/03/$ - see front matter r 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0013-9351(03)00044-6