Biodiesel is a form of biofuel used as a substitute for diesel. It is safe, biodegradable, and produces less air pollutants than petroleum-based diesel. Biodiesel is meant to be used in standard diesel engines and is thus distinct from the vegetable and waste oils used to fuel converted diesel engines. Biodiesel can be used in pure form, or blended with petrodiesel in any proportions. Biodiesel blends can also be used as heating oil.  It also can be obtained from Pongamia, field pennycress and jatropha and other crops such as mustard, jojoba, flax, sunflower, palm oil, coconut and hemp. Several economic studies have been conducted regarding the economic impact of biodiesel production.