The cost of substantially reducing industrial pollution is high; how to finance it without undue economic burden remains a question. Some experts hold that since population growth automatically increases waste production, pollution can best be combated by population control. Another view is that worldwide proliferation of industry and technology is the chief culprit, posing the threat of global warming and requiring curtailment if pollution is to be conquered. The early 1990s brought discussion of more effective means to calculate the true costs of pollution in terms of its effects on health, productivity, and quality of life. There is considerable agreement, nonetheless, on the need for revised technology to diminish industrial and automotive emissions, to produce degradable wastes, and to dispose of all wastes in ways less damaging to the environment—for example, by returning sewage to the farm as fertilizer and by recycling glass and metal materials. Finally, improvement is required in techniques for preventing pollution by especially hazardous wastes. The difficulty of finding adequate permanent storage locations has been increased by opposition from residents of potential sites, who are concerned about health hazards. In 1997 more than 1.3 million people in the United States were employed in environmental industries related to pollution control.