High levels of hydrogen sulphide were detected in the air in Guadarranque, Algeciras (Cádiz), on Tuesday evening prompting an investigation.
Between 21:00 and 21:30, levels in excess of 134 micrograms per cubic metre were recorded, 100 micrograms being the maximum permitted on health grounds.
Hydrogen sulphide is a natural constituent of the air we breathe, but in large doses can become toxic.
Most of the gas in the atmosphere has a natural origin, produced by the decomposition of organic matter near lakes or in areas with geothermal activity and natural gas wells. However, it is also a byproduct of some industrial processes such as paper pulping, oil refining and textile manufacturing.
The gas is colourless, soluble in various liquids and has a characteristic odour (it stinks to high heaven) even at low levels. Short-term exposures to high concentrations can cause headache, dizziness and vomiting.