The ancient Romans tried to keep their homes cool in the summer by pumping water from aqueducts through the walls. In Southeast Asia, people hung bundles of wet grass over their windows to lower the temperature inside. Modern air conditioning, which arrived in 1902, is just a refinement of this rudimentary principle. The basic scientific theories of air conditioning were developed by the American engineer Willis Carrier (1876-1950) of Buffalo, New York. His first system was designed for a printing factory. Changes in the temperature and humidity of the air inside the printing plant altered the properties of the paper, impairing the quality of the print. Carrier's task was to solve this problem. His first system, which used sprayers to cool and dehumidify the air, was, however, large, expensive and dangerous because it used ammonia as a cooling agent. For years, his machines were used only to cool other machines. But when their potential was realized, they began to be installed in office buildings, hotels, and hospitals—as well as in the United States Senate and the White House itself. Air conditioning made life more comfortable, boosted economic activity in the summer months, and reduced the death rate from heat-related illnesses by up to 40 percent. Carrier continued to refine the design and, in 1922, installed an air-conditioning system in a Los Angeles theater that supplied air through the ceiling and exhausted it at floor level. Carrier Corporation continues to manufacture and install air-conditioning systems throughout the world.