In a 2005 NASA-sponsored study, it was estimated that the area covered by lawns in the United States to be about , making it the nation's largest irrigated crop by area. Lawn care is thus a popular business in the United States; proper maintenance, construction and management of lawns of various kinds being the focus of much of the modern horticulture industry. Estimates of the amount spent on professional lawn care services vary, but a Harris Survey put the total at $28.9 billion in 2002 (approximately $1,200 per household using such services). In her book The Lawn: A History of an American Obsession (1994) Virginia Scott Jenkins traces the historic desire to kill weeds. She notes that the current rage for a chemically dependent lawn emerged after World War II and argues that "American front lawns are a symbol of man's control of, or superiority over, his environment." According to the EPA, "of the 26 billion gallons of water consumed daily in the United States, approximately 7.8 billion gallons, or 30 percent, is devoted to outdoor uses. The majority of this is used for irrigation." Along with trees, lawns are a vital element in the fight against urban heat islanding. Lawns provide: Oxygen conversion, Filtering of air particulates, Erosion control, Air and surface cooling to offset asphalt, cement, and rooftops, A place for recreation and enjoyment. In comparison to bare dirt, a lawn may be 20 °F cooler on a hot day, and up to 40 °F cooler than cement surfaces. Lawnmowers Some companies and brands of lawn mowers in the US include: The Grasshopper Company Sensation Lawn Mowers, a commercial lawnmower brand US lawn mower patents US patent Lawn mowers , Filed April 12, 2000, Issued October 10, 2006. Self-propelled lawn mower [ed., self-propelled lawn mower comprising a microprocessor and at least one cutter] , Filed September 11, 1989, Issued October 23, 1990. [ed., remote control lawn mower] , Filied March 28, 1910, Issued January 21, 1913 [ed., Stephen Foster Briggs gas engine igniter] , Lawn mower. Filed September 8, 1898, Issued May 9, 1899. (ed., John Albert Burr, African-American inventor) , Filed September 30, 1878, Issued December 10, 1878. [ed., leaf catcher] , Issued May 22, 1877. [ed., leaf catcher] , Filed November 28, 1874, Issued January 19, 1875. [ed., roller mower] [ed. Titled "lawn mower".] US patent Mowing machines , Issued March 12, 1861. Designs , Filed