Mt. Vesuvius, Italy, is regarded as one of the world'smost dangerous volcanoes because of the potential for vast numbers of people to be affected by the renewal of volcanic activity; more than 600000 people live within 10 km of the summit alone. Vesuvius has been quiescent since 1944 andwith continued dormancy, the more likely it is that the next eruption will be explosive. At that point, wide-spread concern is likely over the potential health hazard of the ash, away from the zone of primary volcanic hazards. Analyses of the mineralogical and geochemical characteristics of ash provide us with critical information on the potential toxicity of the particles, for example, whether particles are sufficiently small to enter the lungs and whether the particles have reactive properties which could trigger disease. Rapid assessment of these characteristics allows real-time decision making on hazard mitigation issues (e.g. distribution of dust masks) and allows considered judgement on whether to embark on major medical/toxicological studies. The study presented here is the first time that the potential respiratory health hazard of ash from Vesuvius volcano has been considered and allows planning for future eruption scenarios. Twenty-one ash samples, representing the range of eruption styles at Vesuvius, were collected and analysed. The results demonstrate that the physical processes of fragmentation play an important role in determining the grain size and, therefore, hazard, of the ash. Here, the finest samples derive from the interaction of magma and water during the final, phreatomagmatic phases of plinian and subplinian eruptions (∼16 vol.% b4 µm material), while the low-intensity explosivity activity, associated with lava effusion, produces coarse ash posing a lesser hazard. The quantity of material found in the different health-pertinent fractions is strongly correlated, allowing prediction of these fractions where only coarser sieve data are available. Since Vesuvius produces silica undersaturated products, ‘free’ crystalline silica in the ash does not pose a significant health hazard (b2 wt.% cristobalite and b3 wt.% quartz). Surface tests showed that the capability of the ash to generate the highly-reactive hydroxyl free radical varies considerably amongst samples, with available surface iron correlating well with reactivity potential.
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