A five-year study into Rotorua's hydrogen sulphide levels has found the gas has no negative impacts on people suffering from asthma.
The finding is being described as fantastic news for the city and its visitor industry, putting to bed myths the gas, which gives off the city's iconic sulphur smell, is bad for asthma sufferers.
The study, by the University of Otago, Wellington, the University of California, Berkeley, and Stanford University, is the largest of its kind between respiratory health and the smell of hydrogen sulphide or H2S.
There has long been medical and scientific debate about the health impacts of H2S gas.
The study looked at a variety of health effects, including asthma, among more than 1600 adult Rotorua residents.
Rotorua was chosen because it is the largest centre anywhere with long-term exposure to hydrogen sulphide.
Exposure was estimated from measurements taken around Rotorua in summer and winter, and then each person's exposure was estimated from where they lived and worked.
Overall the findings show there is no evidence people in the highest exposure group are more likely to have been diagnosed with asthma or have any more asthma symptoms that those in the lowest group.
In fact, asthma symptoms, particularly wheezing, were less often reported in the highest exposure group, raising the slight but intriguing possibility that low levels of H2S might be beneficial for people with asthma.
"The results, showing no ill effects, are reassuring not only for people in Rotorua, but also for those with asthma who are exposed to lower levels of H2S in industrial or work situations," said respiratory specialist and co-author, Professor Julian Crane, from the University of Otago, Wellington.
However Professor Crane points out that H2S can be toxic at higher concentrations and no conclusions can be drawn from this study about health impacts of very high levels of this gas in industrial situations or occurring naturally.
One of the objectives of this research project is to provide further evidence of the toxicity of H2S so this can be used to provide guidelines for industry internationally, where many deaths have been reported from H2S poisoning, particularly in confined areas.
The researchers will be examining the effects of H2S on the nervous system and the eye, particularly cataract, in future analyses.
Rotorua Mayor Kevin Winters said the results were good news for Rotorua residents and the visitor industry.
"The study should put to bed some popular myths about long term exposure to hydrogen sulphide causing or exacerbating asthma."
The study has been published in the international journal Environmental Research and was funded by National Institute of Environmental Health Sciences in the United States.
No one from the Rotorua Asthma Society was available for comment.
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