Acute health effects of the Mount Ruapehu (New Zealand) volcanic eruption of June 1996

On 17 June 1996, Mt Ruapehu in the central North Island of New Zealand erupted, creating an ash plume extending for several hundred kilometers over populated areas to the north-east of the mountain. Out study of this natural experiment aimed to investigate whether the eruption (intervention) had any adverse effect on the health of those people living within the ashfall area, when compared with temporal and spatial controls. The study population comprised those people living in areas receiving > 0.25 mm ashfall, and the control population was unexposed to volcanic ash. Discharge rates from public hospitals were measured for the 3-month period following the eruption and compared with the aggregated corresponding 3-month periods in the previous 7 years. The conditions studied were respiratory disease, stroke and ischaemic heart disease (IHD). Analysis used multivariate Poisson regression models, with tests for independence and estimation of interaction effects for change in time before and after the eruption within the study and control areas. The only detrimental effects detected were a borderline increase in acute bronchitis (rate ratio, RR = 1.44. p = 0.031) and conflicting evidence for some cardiovascular diseases. If there were any health effects of exposure to ash following the eruption, these were too small to be clearly evident with this study design. It is therefore unlikely that any such health effects will pose a significant public health problem following similar future eruptions. No targeted public health interventions are therefore indicated beyond those that remove people from the immediate area under direct physical threat from the volcano.