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…showed last 75 words of 327 total…the burning of fuel, and respiration, a similar process in animals, produces carbon dioxide and water when burned completely and cleanly. To much carbon dioxide may contribute to the greenhouse effect. Unfortunately, that is not the only consequence of combustion. Fuel is usually not burned cleanly or completely because it may contain impurities, other chemicals which produce dangerous pollutants, and many not be burned efficiently. Fossil fuels, coal, oil, and natural gas, contain such impurities.