The discovery of hydrogen sulphide in spent mushroom compost was new to the Health and Safety Authority, a spokesman for the organisation said yesterday.
The production of the lethal gas from compost stored in poorly drained areas had prompted a major inquiry which was continuing.
While the danger from slurry gases was well known and had caused 2 per cent of farm deaths in 10 years, it had not encountered the gas being generated from used mushroom compost before.
"It is a clear gas with a characteristic smell of rotten eggs. Its smell cannot be used as an indicator of gas being present. Remember no smell could actually indicate a high concentration of hydrogen sulphide rather than its absence," he warned.
Its exposure limit is 15 parts per million and gas levels over 2000 ppm at slurry storage level have been found - these are lethal for humans.
The HSE has urged farmers to use gas masks fitted with independent air systems while dealing with the gas.