Most tourism-related air pollution comes from automobiles (Andereck, 1993, p. 27). Automobiles emit by far the most carbon monoxide of all transportation modes. In 1997, they emitted 26 million short tons of carbon monoxide, compared with 1.7 million short tons from recreational marine vehicles, and 1 million from aircraft (U.S. Environmental Protection Agency [EPA],1998, December, Table A-1). Specific information on tour bus emissions was not available, but all heavy-duty diesel vehicles (most tour buses fall into this category) emitted 1.4 million short tons in 1997.