Hemp may also be used to produce ethanol (grain alcohol). The United States government has developed a way to make this automobile fuel additive from cellulosic biomass. Hemp is an excellent source of high quality cellulosic biomass. One other way to use hemp as fuel is to use the oil from the hemp seed-some diesel engines can run on pure pressed hemp seed oil. However, the oil is more useful for other purposes, even if we could produce and press enough hemp seed to power many millions of cars. Biomass fuels are clean and virtually free from metals and sulfur, so they do not cause nearly as much air pollution as fossil fuels. Even more importantly, burning biomass fuels does not increase the total amount of carbon dioxide in the Earth’s atmosphere. When petroleum products are burned, carbon that has been stored underground for millions of years is added to the air; this may contribute to global warming through the `Greenhouse Effect’, (a popular theory which says that certain gases will act like a wool blanket over the entire Earth, preventing heat from escaping into space.) In order to make biomass fuels, this carbon dioxide has to be taken out of the air to begin with-when they are burned it is just being put back where it started. Another advantage over fossil fuels