Since the last eruption of the Fossa crater in 1888-1890, intense volcanic degassing has been remaining on Vulcano Island of Sicily (Italy). Toxic sulfur dioxide (SO2) of the solfataric action in this area represents, when inhaled, a permanent natural hazard harming humans. Approximately 500 permanent residents live and 15,000 tourists visit during the summer time the Porto village in the North of Vulcano Island. A cross-disciplinary fuzzy logic risk assessment has been conducted to evaluate health risks of human individuals exposed to higher SO2-concentrations C over certain exposure times t. The simple approach, based on fuzzy set theory, explains health risks semantically by words rather than by numbers. Advantages of this approach are, first, experts, non-experts, decision makers, or the public are able to understand and communicate risk degrees by words without using numbers. Second, in comparison to other risk definitions, the risk is not equal to the vulnerability; it is based on the hazard (SO2-gas clouds) and vulnerability (health effects) in combination. Third, risk levels can be still estimated even when limited or no statistical information is available, e.g., high SO2-concentrations or long exposure times. Moreover, human health risks were determined for C-t-scenarios based on threshold values of the European Union and the World Health Organization. Independently, two additional methods were used to determine the proportions of the population who are exposed to levels of SO2 at which health effects may be expected and also safety zones for civil protection around the degassing fields. In conclusion, SO2-gas concentrations in many parts of Vulcano Island go beyond the proclaimed alert threshold of the European Union and the World Health Organization. For example, the results show that sensitive individuals, such as asthmatics, young children, or elderly people, should not be exposed at any time to the degassing areas in Porto di Levante and at the NE-rim of the Fossa crater. In contrast, healthy non-sensitive individuals should be exposed less than 10 min to the SO2-clouds at these degassing areas, while hiking on the crater rim.
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