Methane is a colourless gas. At low concentrations it is odourless, but it has a sweet smell at high concentrations. At mixtures more than about 5-15% in air, it is explosive. It has a lifetime of around ten years because it is only very gradually destroyed by other chemicals in air. Methane is one of a group of chemicals known as the volatile organic compounds (VOCs). It is produced both naturally and from man's activities.