By 1900, 7,531 people lived in the city. The population increased gradually to 13,913 in 1910. At about this time, the U.S. Veterans Administration had begun construction on the present Veterans Hospital. Many veterans who had been gassed in World War I and were in need of respiratory therapy began coming to Tucson after the war, due to the clean dry air. Over the following years the city continued to grow, with the population increasing to 20,292 in 1920 and 36,818 in 1940. In 2006 the population of Pima County, in which Tucson is located, passed one million while the City of Tucson's population was 535,000.