The continuous ash and gas emissions from the Tavurvur cone in Rabaul caldera, Papua New Guinea, during 2007-08, raised concerns regarding how exposure would affect the respiratory health of nearby populations and impact on the environment. As part of a formal evaluation of the effects of volcanic emissions on public health, we investigated the potential health hazard of the ash using a suite of selected mineralogical analyses and in vitro toxicity screening tests. The trachy-andesitic ash comprised 2.1-6.7 vol.% respirable (sub-4 μm diameter) particles. The crystalline silica content was 1.9-5.0 wt.% cristobalite (in the bulk sample) with trace amounts of quartz and/or tridymite. Scanning electron microscopy showed that the ash particles were angular with sparse, fibre-like particles (∼3-60 μm max. diameter) observed in some samples, which we confirmed to be CaSO 4 (gypsum, at <6 wt.% in the bulk samples) and not asbestiform fibres. The ash specific surface area was low (0.1-2.7 m 2 g −1 ). The leached solution from one of the ash samples was slightly acidic (pH 5.6), but did not contain high levels of toxic metals (such as F, Cu, Zn, Mn, As, Ni and Cd) when compared to previously tested volcanic ash leachates. Ash Editorial responsibility: H. Delgado Electronic supplementary material The online version of this article (