Next developed country is America, this country's air pollution is also serious nowadays. Because traffic jam often occurs in New York, Washington, Boston etc. big cities in US. So, U.S. cities' parks and its trees have significant influence to produce fresh air to provide U.S. residents who are living in cities to breach for their body health. David J. & Gordon, M. ( 2016) indicated " In U.S. these urban parks are estimated to contain about 370 million trees with a structural value of approximately $300 billion." The number of park trees varies by region of the country, but which can produce significant air quality effects in and near parks, related to air temperatures, air pollution, ultraviolet indication and carbon dioxide ( a dominant greenhouse gas related to global climate change). Additional open space and other vacant lands in cities, which may contain trees and other vegetation. Contribute significant additional benefits, effects of parks and open space at the city scale can vary significantly depending on the amount of parkland and amount of tree cover within the parkland. The reasons why parks can reduce air pollution. Parks generally have lower air temperature than surrounding areas. Temperatures are usually cooler toward the center of a park than around its edges. At night, the center of a large park may be 13 degree cooler than surrounding city areas. The cooler air from parks often moves out into adjacent developed neighborhoods. This cooling of surrounding areas tends to increase with park size and percentage of the park covered by trees. So, cooler air temperature is provided by urban parks can have significant impacts on human health. During heat wave events, which can kill hundreds of people, park areas may provide city dwellers with some respite from high air temperature, particularly in the evening, during hot, sunny days tree shade can greatly increase human comfort. Because park influences on air temperature extend to developed areas outside of parks, local energy use for heating and cooling buildings is also effected. Although, the net around effect of parks on energy costs has been by reducing temperature is difficult to estimate at least in the southern United States the effect will usually be a net annual benefit. Furthermore, large park trees will reduce winds and may provide a benefit of winter heating of buildings near the park. Although, the overall economic effect of urban trees and parks on air temperature reduction is not fully billions of dollars annually at the national scale in terms of improved environmental quality and human health. In fact, trees and vegetation in parks can help reduce air pollution both by directly removing pollutants and by reducing air temperatures and building energy use in and near parks. There tree effects can reduce pollutant emissions and formation. However, park vegetation can increase some pollutants by either directly emitting volatile organic compounds that can contribute to ocean and carbon monoxide formation or indirectly by the emission of air pollutants through vegetation maintenance practices, such as operation of chain and use of transportation fuels. David J. & Gordon, M. ( 2016) showed "Annual pollution removal and economic benefits by U.S. un bank park trees is estimated at about 75,000 tones ( $500 million) or 80 pounds per acre of tree cover ( $300 per acre of tree cover).