WHAT THE LUNGS SEE: MINERALOGICAL FACTORS AFFECTING THE POTENTIAL TOXICITY OF VOLCANIC ASH

Multidisciplinary research into volcanic ash as a respiratory health hazard arose from the eruption of Mt. St. Helens (MSH), USA in 1980 and returned to prominence following the 2010 Eyjafjallajökull, Iceland eruption. Exposure to ash is known to trigger acute respiratory diseases and can potentially instigate chronic disease if the ash is sufficiently fine to deposit in the alveolar region of the lungs. One suspected disease-causing mechanism is due to the presence of crystalline silica, specifically as cristobalite, a group I human carcinogen. Recently, we have established the potential toxicity of volcanic ash is likely to vary depending on the type and style of eruption; dome-forming eruptions in particular tend to generate substantial quantities of crystalline silica and dome collapse results in the generation of ultra-fine, respirable cristobalite-rich ash.

The effects of mineralogy on the potential toxicity of volcanic cristobalite are ill-defined as the influence of structure and composition on the physiological burden have been sufficiently studied at only one location, Soufrière Hills volcano (SHV), Montserrat. Here we present results from a systematic characterisation of the mineralogical properties of cyrstalline silica at a suite of locations: MSH, SHV, Merapi, Unzen, and Santiaguito. To further define the disease-causing potential of ash and the properties responsible for adverse biological responses, the abundance, purity, crystallographic form, and crystal shape of volcanic silica were determined using XRD, electron microprobe, SEM, and Raman spectroscopy. As with previous observations at SHV, cristobalite is found within the groundmass as well as growing into vugs in dome rock in both platy and euhedral forms. The composition is impure, containing traces of other cations such as aluminium (up to 3 wt%). The quantity of cristobalite differs between volcanic settings due to variability in activity and duration of dome growth, with MSH dome rock elevated relative to SHV (4-15 wt. % compared with 1-10 wt. %). As higher levels of cristobalite in respirable ash raise concerns about the onset of chronic, injurious disease, it will be possible to recognize respiratory hazards based on local geology and significantly aid the risk mitigation work of disaster managers globally.