A chemical expert believes the mysterious gas smell in Perth yesterday morning was from organic matter.
Dr Mark Imisides said the smell could be explained by the same weather pattern that caused a blanket of smoke from fires in Denmark last weekend.
"Chemicals called mercaptans are added to gas to make it smell," he said. "These sulphur-based compounds also occur naturally where you have rotting organic matter, such as in a rubbish tip.
"The smell spread to such a wide area because we have had a temperature inversion, which was why we got that smoke haze a few days ago."
He said temperature inversions, where layers of cold air are trapped under layers of hot air, could trap odours.