New York's population density has environmental pros and cons. It facilitates the highest mass transit use in the United States, but also concentrates pollution. Gasoline consumption in the city is at the rate the national average was in the 1920s, and greenhouse gas emissions are a fraction of the national average, at 7.1 metric tons per person per year, below San Francisco, at 11.2 metric tons, and the national average, at 24.5 metric tons. New York City accounts for only 1% of United States greenhouse gas emissions while housing 2.7% of its population.