By SIMON COLLINS science reporter
Scientists are concerned about mounting evidence of long-term health effects from the geothermal gases that give Rotorua its "pong".
A study has found that hospital admissions for asthma and other breathing diseases in six Rotorua districts between 1991 and 2001 were five to 10 times the local average.
Another study has found hospital admission rates between 1993 and 1996 were between 1.5 and 2.7 times the national average for a range of breathing and nerve-related illnesses in the parts of Rotorua that are most exposed to geothermal gases.
Bay of Plenty Medical Officer of Health Phil Shoemack, an author of the latter study, said the health effects of the gases were "likely to be relatively small".
But he and the other two authors of the study want further research.
"The results of this study strengthen the suggestion that there are chronic health effects from hydrogen sulphide exposure," they wrote.
Rotorua's 65,000 people are believed to be the largest population in the world exposed to natural emissions of hydrogen sulphide, which gives the city its "rotten eggs" smell.
The gas blows out of the ground in geysers and hot pools, and also seeps imperceptibly out of the soil.
Long soaking in hot pools, or concentrations of the gas seeping into buildings through floors, walls and sewers, have been blamed for 11 deaths since 1946 - most recently that of Austrian actress Ellen Umlauf-Rueprecht in February 2000.
"We know that hydrogen sulphide is emitted in geothermal areas and we know that it can be very toxic in acute settings once it reaches quite high levels," Dr Shoemack said.
"What we don't know is the extent of any impacts at low levels over many years."
The latest study, by Canterbury University geographers Jeff Wilson and Mike Durand, has found clusters of asthma, bronchitis and other chronic breathing diseases at five to 10 times the expected rates in six of the city's 39 census areas: Kuirau, Victoria, Glenholme West and East, Fenton and Ngapuna.
The expected rates were the local Rotorua average rates of hospital admissions for those diseases, adjusted for the age, sex, ethnicity, socio-economic status and smoking rates in each census area.
The six affected areas are all close to the main geothermal fields in the centre and south of Rotorua.
The area surrounding the most famous geothermal field, Whakarewarewa, was also part of the most general cluster of breathing illnesses, but did not show up in the clusters for some of the individual diseases such as asthma.
Mr Wilson, a doctoral student from Texas, told the New Zealand Geographical Society conference in Auckland yesterday that the statistical clusters matched what local people said in interviews.
"People talk about moving into one building in the central business district, after living in Rotorua for years, and starting to have migraine headaches and watery eyes," he said.
Staff at Doyles in Eruera St spoke of seeing spiders in the shop for the first time in years after installing a vent to extract gas emerging from underneath the footpath outside.
The other study, published overseas last September by Dr Shoemack and two scientists then with the Institute of Environmental Science and Research (ESR), Dr Michael Bates and Nick Garrett, divided Rotorua into three zones of high, medium and low exposure to hydrogen sulphide.
It found rates of asthma and other chronic breathing diseases in the high-exposure zones in the centre and south of the city were 1.6 times the national average.
Hospital admissions in the high exposure zone were twice the national average for ear problems, 2.3 times for eye problems and 2.7 times for migraine, epilepsy and other nervous system disorders.
Rotorua deaths attributed to hydrogen sulphide:
1946: Man collapses and dies in public spa pool area.
1948: Two men overcome by gases in sewer pipe during maintenance. One later dies.
February 1954: Five men overcome entering septic tank. One later dies.
February 1954: Man overcome by gas in hot pool and later drowned.
June 1954: Man digging sump hole is overcome and dies.
Feb 1962: Couple found dead at home. Blamed on leaking pipe in a groundwater-fed heating system.
May 1962: Man died on toilet on CBD site. Hydrogen sulphide ingress found throughout building.
September 1987: Couple killed during the night in motel unit. A faulty water trap in the unit's shower tray let gas into room.
February 2000: Austrian actress Ellen Umlauf-Rueprecht died in hotel room. Investigation found no potential gas sources on site, but victim was known to have spent many hours at local spa baths.
Source: Dr Michael Durand, Canterbury University.