Concentrated pollution in New York City leads 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.