Environmental concerns in the city involve managing its extraordinary population density. Mass transit use is the highest in the nation and gasoline consumption in the city is at the rate the national average was in the 1920s. New York City's dense population and low automobile dependence help make New York among the most energy efficient in the United States. The city's greenhouse gas emission levels are relatively low when measured per capita, at 7.1 metric tons per person, below San Francisco, at 11.2 metric tons, and the national average, at 24.5. New Yorkers are collectively responsible for one percent of the nation's total greenhouse gas emissions, though comprise 2.7% of the nation's population. The average New Yorker consumes less than half the electricity used by a resident of San Francisco and nearly one-quarter the electricity consumed by a resident of Dallas.