||8 THE THIRTY-SIXTH ANNUAL REYNOLDS LECTURE mental factors. The same observation can be made about cigarette smoking; before we eliminated these contagious diseases as killers (at least in the upper and middle classes) smoking probably had little effect on overall life expectancy. Now that these other causes of death are practically gone, smoking has a real effect on life expectancy. So also with air pollution. It is informative to contrast the situation on air pollution, for which we have come to awareness and action so recently, with water pollution, for which we have had active programs for many years. Water pollution problems were of an entirely different type. The worst water problems were caused by contamination of drinking water with human sewage. This quickly spreads cholera, typhoid, and amoebic dysentery. These diseases are sudden and dramatic in onset and often swiftly fatal. Their connection with polluted water is easily demonstrated. Thus, we responded to the water pollution problem much sooner and more vigorously than we have to the air pollution problem. The evidence of air pollution damage to health is much more difficult to adduce than that for water pollution. One can seldom point to a pile of corpses and say "they died of air pollution" as one can after a cholera outbreak due to polluted water. The situation is more like smoking, where we can seldom say "he died of smoking" but we can say "smoking can be shown, by perfectly unarguable statistics, to decrease the life expectancy of the smoker." The same can be said for air pollution; living in an air-polluted environment can be shown, by perfectly unarguable statistics, to lower life expectancy. It is clear from the fact that so many people â€" including educated people â€" smoke that this type of argument is not as persuasive as the sight of the corpses after a water-pollution outbreak. Similarly many people cannot take very seriously the loss of life and health due to air pollution, since its demonstration is, as they say, "only statistical." Another obvious analogy between air pollution and smoking is the fact that many people can be found who have lived in very badly air polluted environments all their lives, and have excellent lungs and hearts. The same is true of smokers; everyone knows someone who lived to be a vigorous 95 and smoked his cigarettes or cigars every day. Those examples exist; the counter-examples died younger, of diseases to which air pollution and/or smoking contributed. Another puzzling fact is that the public awareness of air pollution as an issue has occurred at a period when in many respects the problem is less severe than it was previously. To cite several local examples: apparently before the introduction of natural gas for home heating in Salt Lake City the winter air here was much dirtier with coal soot than it is now.