components of diesel exhaust were confirmed as an animal carcinogen in 1988 by the national institute for occupational safety and health, and by 2002, the u.s. environmental protection agency (epa) considered it "likely to be carcinogenic to humans". the particulate matter of diesel exhaust has been linked to (among other health effects) lung cancer, chronic bronchitis, and aggravated asthma; it has also been identified as a greenhouse gas. " for these and other reasons, alternatives and improvements to standard diesel fuel have been developed.