Environmental concerns in the city involve managing the city's 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. Large amounts of concentrated pollution in New York City lead to high incidence of asthma and other respiratory conditions among the city's residents. In recent years the city has focused on reducing its environmental impact. The city government is required to purchase only the most energy-efficient equipment for use in city offices and public housing. New York has the largest clean air diesel-hybrid and compressed natural gas bus fleet in the country, and some of the first hybrid taxis. The city is also a leader in the construction of energy-efficient green office buildings, including the Hearst Tower among others. New York City is supplied with drinking water by the protected Catskill Mountains watershed. As a result of the watershed's integrity and undisturbed natural water filtration process, New York is one of only five major cities in the United States with drinking water pure enough not to require purification by water treatment plants.