The Soufrière Hills stratovolcano on the Caribbean island of Montserrat has been erupting since 18th July 1995. An enormous amount of respirable volcanic ash has been suspended into the atmosphere by the eruptions and wind re-suspension of deposited ash. The large amount of fine, airborne particulate matter, in particular the component 10 μm equivalent aerodynamic diameter (PM10), is a cause of medical concern. Airborne levels have frequently exceeded the UK environmental standard for PM10, (50 μg/m3), although it is noted that this standard was primarily set for urban PM10. The crystalline silica in the ash is mostly cristobalite, at reported levels up to 20%. The UK government’s Chief Medical Officer, referring to Montserrat, has suggested that long-term exposure to high levels of volcanic ash could lead to silicosis. These concerns have prompted government-funded investigations into the potential toxicity of well characterized volcanic ash samples from Montserrat. Given the well established toxicity of cristobalite, particular attention was paid to the amount of this mineral in the ash samples. Three ash samples were tested: (1) a vulcanian eruption ash, (2) ash released in a domecollapse pyroclastic flow, and (3) ash from a major vulcanian explosion that was wind-transported to, and deposited on, the neighbouring island of Antigua. Comparative toxicological studies were carried out on respirable preparations of these three samples together with appropriate control mineral dusts that matched the major components of the Montserrat samples: anorthite, labradorite, cristobalite/ obsidian and cristobalite. Alpha quartz (DQ12) was the positive control. All samples, including the controls, were characterized to establish particle-size distributions, particle morphologies, and to confirm the mineralogy. Rats were challenged with 1 mg via intratracheal instillation, and groups sacrificed at three time points (1, 3 and 9 weeks). Health assessment was made by examining endpoints of increasing lung damage such as inflammation, permeability (oedema), changes in epithelium, and increase in the size of broncho-thoracic lymph nodes. The data indicate that Montserrat respirable ash, derived from dome collapse pyroclastic flows or vulcanian explosions, has minimal acute bioreactivity in the lung. The feldspar standards showed low bioreactivity, in stark contrast to the cristobalite standard that showed progressive increases in lung damage. These results suggest that either the mass of cristobalite present in the Montserrat ash was insufficient to cause an effect in the lung, or the cristobalite in the ash was, for some as yet unknown reason, markedly less bioreactive than our pure cristobalite standard.