the city. The system loop contains nearly of chilled or heated water at any time. Cornwall, Ontario operates a district heating system which serves a number of city buildings and schools. Markham, Ontario: Markham District Energy operates several district heating sites: Warden Energy Centre (c. 2000), Clegg Energy Centre and Birchmount Energy Centre serving customers in the Markham Centre area Bur Oak Energy Centre (c. 2012) serving customers in the Cornell Centre area Many Canadian universities operate central campus heating plants. United States The Holly Steam Combination Company was the first steam heating company to commercially distribute district heating from a central steam heating system. As of 2013, approximately 2,500 district heating and cooling systems existed in the United States, in one form or another, with the majority providing heat. Consolidated Edison of New York (Con Ed) operates the New York City steam system, the largest commercial district heating system in the United States. The system has operated continuously since March 3, 1882 and serves Manhattan Island from the Battery through 96th Street. In addition to providing space- and water-heating, steam from the system is used in numerous restaurants for food preparation, for process heat in laundries and dry cleaners, for steam sterilization, and to power absorption chillers for air conditioning. On July 18, 2007, one person was killed and numerous others injured when a steam pipe exploded on 41st Street at Lexington. On August 19, 1989, three people were killed in an explosion in Gramercy Park. Milwaukee, Wisconsin has been using district heating for its central business district since the Valley Power Plant commenced operations in 1968. The air quality in the immediate vicinity of the plant, has been measured with significantly reduced ozone levels. The 2012 conversion of the plant, which changed the fuel input from coal to natural gas, is expected to further improve air quality at both the local César Chavez sensor as well as Antarctic sensors The Wisconsin power plants double as breeding grounds for peregrine falcons. Denver's district steam system is the oldest continuously operated commercial district heating system in the world. It began service November 5, 1880 and continues to serve 135 customers. The system is partially powered by the Xcel Energy Zuni Cogeneration Station, which was originally built in 1900. NRG Energy operates district systems in the cities of San Francisco, Harrisburg, Minneapolis, Omaha, Pittsburgh, and San Diego. Seattle Steam Company, a