Long term exposure to respirable volcanic ash on Montserrat: a time series simulation |
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| Authors: | T K Hincks W P Aspinall P J Baxter A Searl R S J Sparks G Woo |
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| Institution: | (1) Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK;(2) Aspinall and Associates, Woodside Close Beaconsfield, Bucks, HP9 1JQ, UK;(3) Department of Public Health and Primary Care, University of Cambridge, Cambridge, CB1 2ES, UK;(4) Institute of Occupational Medicine, 8 Roxburgh Place, Edinburgh, EH8 9SU, UK;(5) Risk Management Solutions, Peninsular House, 30 Monument Street, London, EC3R 8NB, UK;(6) Institute for Advanced Studies and Department of Earth Sciences, University of Bristol, Bristol, BS8 1UJ, UK |
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| Abstract: | Frequent ash fallout from long-lived eruptions (with active phases greater than 5 years) may lead to local populations experiencing
unacceptably high cumulative exposures to respirable particulate matter. Ash from Montserrat has been shown to contain significant
levels of cristobalite and other reactive agents that are associated with an increased risk of developing pneumoconiosis (including
silicosis) and other long-term health problems. There are a number of difficulties associated with estimating risks in populations
due to uncertain and wide ranging individual exposures, change in behaviour with time and the natural variation in individual
response. Present estimates of risk in workers and other population groups are simplifications based on a limited number of
exposure measurements taken on Montserrat (1996–1999), and exposure−response curves from epidemiological studies of coal workers
exposed to siliceous dust. In this paper we present a method for calculating the long-term cumulative exposure to cristobalite
from volcanic ash by Monte Carlo simulation. Code has been written to generate synthetic time series for volcanic activity,
rainfall, ash deposition and erosion to give daily ash deposit values and cristobalite fraction at a range of locations. The
daily mean personal exposure for PM10 and cristobalite is obtained by sampling from a probability distribution, with distribution parameters dependent on occupation,
ground deposit depth and daily weather conditions. Output from multiple runs is processed to calculate the exceedance probability
for cumulative exposure over a range of occupation types, locations and exposure periods. Results are interpreted in terms
of current occupational standards, and epidemiological exposure−response functions for silicosis are applied to quantify the
long-term health risk. Assuming continuing volcanic activity, median risk of silicosis (profusion 1/0 or higher) for an average
adult after 20 years continuous exposure is estimated to be approximately 0.5% in northern Montserrat to 1.6% in Cork Hill.
The occupational group with the highest exposure to ash are gardeners, with a corresponding 2% to 4% risk of silicosis. In
situations where opportunities for in-depth exposure studies are limited, computer simulations provide a good indication of
risk based on current expert knowledge. By running the code for a range of input scenarios, the cost-benefit of mitigation
measures (such as a programme of active ash clearance) can be estimated. Results also may be used to identify situations where
full exposure studies or fieldwork would be beneficial.
Editorial responsibility: J Stix |
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| Keywords: | Monte Carlo Montserrat Respirable health Risk Silicosis Volcanic ash Cristobalite |
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