Hydrogen sulphide, or sour gas, as it is known, smells like rotten eggs. It escapes from deep well drilling and extraction of natural gas and oil. It is one of the world's deadliest substances. Just a whiff of hydrogen sulphide can kill, as more than 30 oil field workers in Alberta have been during the last three decades.

               In 1999 the USA Environmental Protection Agency was requested to list it as a hazardous air pollutant (see the correspondence) stating that in "chronic, low level exposures, one may observe abnormal neurobehavioral functioning and altered mood states (e.g., depression, fatigue, tension, vigor). In addition, numerous CNS-brain effects occur including: changes in brain density, headache, memory loss, reduced sense of smell, loss of balance, dizziness, sleep difficulties, and fatigue."
               It cites: "Kaye Kilburn, PhD, is a public health scientist who has been conducting research on the health effects of exposure to H2S for many years. Describing a new study, he unequivocally stated at the conference that 'H2S poisons the brain, and the poisoning is irreversible'. Demonstrable symptoms of chronic exposure include pronounced deficits in balance and reaction time, as well as such ailments as dizziness, insomnia, and overpowering fatigue."