Long term exposure to respirable volcanic ash on Montserrat: a time series simulation

1. Long term exposure to respirable volcanic ash on Montserrat: a time series simulation T. Hincks, R.S.J. SparksUniversity of Bristol W.P. Aspinall Aspinall and Associates and University of Bristol P.J. Baxter Dept of Public Health and Primary Care, Univ. Cambridge A. Searl Institute of Occupational Medicine, Edinburgh G. Woo Risk Management Solutions

2. Aim: • Estimate risk of silicosis from cumulative exposure to cristobalite • Volcanic activity • Ash composition • Deposition and erosion • Weather conditions • Human activity / occupation

3. Risk assessment for Montserrat • Active since 1995 - high cumulative exposures? • 1996 - DfID and DoH research on health risks associated with volcanic ash • 3 - 24 weight % crystalline silica in <10mm (inhalable) fraction of ground deposits • Baxter et al. (1999) • preferentially fractionated in PM4 (<4mm) • Horwell et al. (2003) •  silicosis, lung cancer, pulmonary tuberculosis, autoimmune diseases • Health risks? • Exposure to date • Continuing volcanic activity

4. Uncertainty and expert judgment • Difficult to assess risk in populations • Information highly uncertain and poorly constrained • probabilistic risk assessment Observations • eruptive history • ash isopach data • air quality monitoring • weather data • ash dispersal • weather simulation Numerical models Expert elicitation formal and unbiased process for obtaining information from experts limited data full extent of system behavior? • Aleatory uncertainty Observables represented by probability density functions

5. Time series simulation • Code generates daily exposures for PM10 (<10mm) and cristobalite crystalline silica • 6 occupation groups • 4 sites • Multiple runs (10,000 runs/simulation) • Sample from PDF for each parameter • Correlated sampling

6. Model structure

7. Model structure

8. Dome growth • Periodic dome growth function controls replacement of material •  frequency of collapses •  increased probability of vulcanian explosions at high growth rates Growth rate m3s-1

9. Rainfall • Rainfall time series simulated as two part process: • Incidence of rain (true/false) • Quantity of rain (24h) Mean Rain depth: Lognormal distribution with time dependent parameters Standard deviation

10. Volcanic activity • 6 event categories are considered:  significant ash deposits • 3 -10 x 106 m3 dome collapse • 10 -30 x 106 m3 dome collapse • 30 - 50 x 106 m3 dome collapse • 50 - 75 x 106 m3 dome collapse • >75 x 106 m3 dome collapse • Series of 0.4 x 106 m3 vulcanian explosions • Assume event magnitudes and frequencies ~similar to past 10 years activity  daily P(event) • Probability of Vulcanian explosions increases after major dome collapses and during periods of high extrusion rate

11. Ash deposition • Ash deposits generated with HAZMAP • 2-D advection diffusion model for ash transport • (Bonadonna et al. 2002) • 3 years of daily wind data • Dome collapse pyroclastic flows down 5 valleys • Single source Vulcanian explosions • correlated lognormal deposits distributions for 4 locations

12. Ash removal (wind and rain) • Approximate with 4 deposit levels • Use beta distribution to represent variation in deposit lifetime • Expert elicitation for mean, upper and lower bounds

13. Individual exposure High exposure Gardeners Public works department Low exposure indoor occupations elderly • PDF: VARIATION IN EXPOSURE Beta distribution function of • deposit depth • cristobalite content of ash • occupation • Modified to account for rainfall dust trak data

14. Individual exposure Sum daily exposure values over 5, 10 and 20 years  estimate cumulative exposure  risk of silicosis

15. Simulated time series

16. Simulated time series

17. Simulated time series • UK HSE recommended maximum occupational exposure to crystalline silica: 0.3 mg m-3 • Suggested limit: 0.1 mg m-3 8h time weighted average • US NIOSH recommended limit: 0.05 mg m-3 • time weighted average for up to 10 hour work day during 40 h working week UK HSE (2003) suggested limit US NIOSH recommended limit

18. Results: 20 year cumulative cristobalite exposure f w % trials exceeding exposure s c cumulative cristobalite exposure mg.m-3.year cumulative cristobalite exposure mg.m-3.year

19. Estimating risk: exposure-response functions for silicosis • Upper limit of risk: • Buchanan et al. 2003 • Study of silicosis in Scottish coalminers • High intensity exposure > 0.1 mg m-3 • Heavy ash fall areas only

20. Estimating risk: exposure-response functions for silicosis Upper limit of risk: Buchanan et al. 2003 Study of silicosis in Scottish coalminers Most analogous: Hughes et al. 1998 occupational exposure for diatomaceous earth workers > 0.5 mg m-3 ≤ 0.5 mg m-3 exposure intensity affects risk

21. 20 year risk of silicosis • 20 years continuous exposure • Hughes et al. (1998) risk function TYPICAL ADULT CMO risk scale Estimated exposures lie within bottom 20% of Hughes cohort (<100 cases) RISK?

22. 20 year risk of silicosis • 20 years continuous exposure • Hughes et al. (1998) risk function TYPICAL ADULT CMO risk scale OUTDOOR WORKER

23. Validation & future work • Medical studies • 2000: x-ray survey of 421 high risk workers showed no evidence of chest abnormalities (< 5 years exposure) • X-ray survey after 10 years exposure • Risk to children highly uncertain • limited data - better estimates of cumulative exposure? • applicability of exposure response functions? • Field data • Continuous PM10 measurement • + weather data • Personal exposure sampling • Ash erosion rates - very poorly constrained • Duration of hazard • Implications for lahar and flood hazard assessment

24. Validation & future work • Exposure control measures • dust masks for outdoor workers in ash affected areas • minimize exposure during cleanup operations • minimize children’s exposure (clear sports & play areas after ash fall)

25. Further applications • Ash-leachates • water contamination • risk to livestock • crop damage • Popocatépetl • PM10, ash leachates • Guadeloupe • concerns about contamination of aquifer

26. Acknowledgements • Thanks to my PhD supervisors: Steve Sparks, Willy Aspinall and Gordon Woo • Constanza Bonadonna for reconfiguring and running HAZMAP ash dispersal code • DATA • Ash data from Clare Horwell, Univ. Cambridge • Personal exposure DustTrak data from TheInstitute of Occupational Medicine • Montserrat & Antigua rainfall data from the Montserrat Volcano Observatory and IOM • Guadeloupe rainfall data from the Hong Kong Observatorywww.hko.gov.hk • CODE • SCYTHE C++ Statistical Library GNU GPL 2001 A.D. Martin and K.M. Quinn • MT19937 Mersenne Twister random number generator 2002 T. Nishimura and M. Matsumoto

27. References • Buchanan, D., B. G. Miller, et al. (2003). "Quantitative relations between exposure to respirable quartz and risk of silicosis." Occupational and Environmental Medicine 60(3): 159-164 • Burmaster, D. E. and P. D. Anderson (1994). "Principles of Good Practice for the Use of Monte Carlo Techniques in Human Health and Ecological Risk Assessments." Risk Analysis 14(4): 477-481 • Cooke, R. M. (1991) Experts in Uncertainty: Opinion and Subjective Probability in Science. Environmental Ethics and Science Policy Series. Oxford University Press, New York. • Hughes et al. (1998) Radiographic Evidence of Silicosis Risk in the Diatomaceous Earth Industry. Am. J. Respir. Crit. Care Med., Volume 158, Number 3, 807-814 • Horwell, C.J., Sparks, R.S.J., Brewer, T.S., Llewellin, E.W., and Williamson, B.J. (2003). The characterisation of respirable volcanic ash from the Soufrière Hills Volcano, Montserrat, with implications for health hazard. Bull. Volcanol., DOI: 10.1007/S00445-002-0266-6. • National Institute for Occupational Safety and Health (2002). NIOSH Hazard Review: Health effects of occupational exposure to respirable crystalline silica.