Generations of visiting children have compared the smell to "rotten eggs"; it's the powerful aroma of sulfur that hangs over the popular New Zealand tourist destination and geothermal resort of Rotorua.

But a study by New Zealand and US scientists has revealed that the otherwise toxic hydrogen sulfide (H2S) gas that causes the smell might be hiding some unexpected health benefits, the University of Otago announced Wednesday.

One of the study's objectives was to find evidence of the toxicity of H2S in order to set guidelines for industry internationally, where many deaths had been reported from H2S poisoning, particularly in confined areas, said a statement from the university.

Concentrations of the gas could be produced as a by-product in factories, as well as naturally in volcanic and geothermal regions like Rotorua, which has a population of almost 60,000 and is the largest center anywhere with long-term exposure to H2S.

The five-year study by the University of Otago, the University of California, Berkeley, and Stanford University examined a variety of health effects, including asthma, amongst over 1,600 adult residents.

Overall the findings showed no evidence that those with the highest exposure to H2S were more likely to be diagnosed with asthma or have more asthma symptoms than those with the lowest exposure.

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," Otago University respiratory specialist and study co-author Professor Julian Crane said in the statement.

Recently H2S had been shown to occur naturally in humans and animal studies showed it could reduce inflammation after injury and might have other potentially beneficial effects.

However, it was too soon to conclude that the anti-inflammatory effect was working in Rotorua, said Crane.

The researchers would examine the effects of H2S on the nervous system and the eye in future analyses.