The economic costs of noise to society are several. Airports are currently op- erating at less than capacity because of noise regulations, which restrict their 62. ______. hours of operation. For instance, at Washington’s national airport no jet traffic is allowed from 11 p. m. to 7 a. m. Other airports restrict the use of certain run- ways. One estimated is that noise restrictions reduce possible airport use by 20 63. ______. percent. The profitable cargo trade is especially affected by night restrictions. In the case of airports, jet engines may be modifying to reduce their noise lev- 64. ______. el, or insulation (隔音) from air traffic noise may be provided by the purchasing 65. ______. of land around airports. It is estimated that $ 5.7 billion would be required to equip all existing jet engines for noise control devices. However, 66. ______. consider the current state of the art, even taking this step will not reduce noise 67. ______. levels at all points to acceptable values. Some combination of methods is proba- bly necessary. If all aircraft are made quieter by existing methods, there would be a number 68. ______. of economic benefits. An increase in airport capacity would occur. Property val- ues near airports might rise. Transportation costs to and from airports could be increased since the airports now could be located closer to population centers. 69. ______. Much research still needs to be done on the economic aspects of noise re- duction and noise effects. Although some of the effects of noise pollution are known, more must be discovered about their effects on health, productivity, prop- 70. ______. erty values and the quality of life. Furthermore. the cost of noise pollution to the economy as a whole needs to be investigated. The public must be alert about the dangers and economic costs of noise poi- 71. ______. lution so that people may make intelligent choices and exert appropriate pres- sures.