A: The problem with fossil fuels is they are non renewable (can not be reused) they can also cause more carbon dioxide to be released into the atmosphere. A: The combustion (burning) of fossil fuels releases poisonous gases into the atmosphere. Oil/Petrol, for instance, is a Hydrocarbon (a mix of Carbon and Hydrogen atoms) and when burnt with Oxygen creates Carbon Monoxide and Carbon Dioxide. Small amounts of Sulphur in the fuel may also create Sulphur Dioxide.