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 erup ted, creating an ash plume extending for several hundred kilometers over po pulated 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 are a, when compared with temporal and spatial controls. The study population c omprised those people living in areas receiving > 0.25 mm ashfall, and the control population was unexposed to volcanic ash. Discharge rates from publ ic hospitals were measured for the 3-month period following the eruption an d compared with the aggregated corresponding 3-month periods in the previou s 7 years. The conditions studied were respiratory disease, stroke and isch aemic heart disease (IHD). Analysis used multivariate Poisson regression mo dels, 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 evidenc e for some cardiovascular diseases. If there were any health effects of exp osure to ash following the eruption, these were too small to be clearly evi dent with this study design. It is therefore unlikely that any such health effects will pose a significant public health problem following similar fut ure eruptions. No targeted public health interventions are therefore indica ted beyond those that remove people from the immediate area under direct ph ysical threat from the volcano.