Grain-size analysis of volcanic ash for the rapid assessment of respiratory health hazard

Abstract

Volcanic ash has the potential to cause acute and chronic respiratory diseases if the particles are sufficiently fine to enter the respiratory system. Characterization of the grain-size distribution (GSD) of volcanic ash is, therefore, a critical first step in assessing its health hazard. Quantification of health-relevant size fractions is challenging without state-of-the-art technology, such as the laser diffractometer. Here, several methods for GSD characterization for health assessment are considered, the potential for low-cost measurements is investigated and the first database of health-pertinent GSD data is presented for a suite of ash samples from around the world. Methodologies for accurate measurement of the GSD of volcanic ash by laser diffraction are presented by experimental analysis of optimal refractive indices for different magmatic compositions. Techniques for representative sampling of small quantities of ash are also experimentally investigated. GSD results for health-pertinent fractions for a suite of 63 ash samples show that the fraction of respirable (<4 microm) material ranges from 0-17 vol%, with the variation reflecting factors such as the style of the eruption and the distance from the source. A strong correlation between the amount of <4 and <10 microm material is observed for all ash types. This relationship is stable at all distances from the volcano and with all eruption styles and can be applied to volcanic plume and ash fallout models. A weaker relationship between the <4 and <63 microm fractions provides a novel means of estimating the quantity of respirable material from data obtained by sieving.

Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile

“…The refractive indices for size-distribution calculation were 1.48 and 1.50; based on the high SiO 2 content of the ash, the absorption index was set to 0.10, as discussed by Horwell (2007).…”

Statistical analysis of dispersal and deposition patterns of volcanic emissions from Mt. Sakurajima, Japan

“…Volcanic ash is released in a range of sizes (2 mm down to μm scale), varying with each volcano and even for different eruption styles for a specific volcano. The health impact of volcanic ash critically depends on the size of the ash, and thus the distribution of ash size (typically referred to as the grain size distribution; GSD) for an eruption is crucial for health impact studies (Horwell, 2007). Aside from the physiological effects, long-term exposure to volcanic ash (and increased air pollution in general) can also cause psychological stress and lead to increased absenteeism from school or work (Pope et al, 1995;Jenkins et al, 2015).…”

Aircraft observations and model simulations of concentration and particle size distribution in the Eyjafjallajökull volcanic ash cloud

“…For example, Scasso et al (1994) found that the mean and median particle sizes deposited at the surface from the Hudson volcano decreased rapidly up to 270 km from the volcano vent, beyond that point they were more or less constant up to 550 km. Similarly, in a study combining ash fallout data observed up to 100 km from the volcano, for a range of volcanic eruptions, Horwell (2007) found that there was a strong linear relationship between 4 µm and 10 µm ash particles but a weaker non-linear relationship between 4 µm and 63 µm fractions. Both these results show that the coarse mode in the PSD shifts to finer sizes with distance from the volcano in the near-source region.…”