It was, as one scientist put it, "the largest scratch-and-sniff test in the history of mankind."
Last year, National Geographic magazine mailed nearly 11 million copies of its September issue, each stuffed with a set of six scratch-and-sniff panels, and asked readers to report what they smelled. The odors were those of banana, musk, cloves, rose, a chemical in sweat called androstenone and mercaptans, the chemical compounds added to natural gas to make it smell bad and help identify leaks.
About 1.5 million readers took the test and sent back the forms. Preliminary analyses of 26,200 forms chosen at random by two smell specialists will be published in the October issue of the magazine. They confirm some long-held beliefs, suggest intriguing new questions about individual differences and raise warnings that some people may be in danger as a result of misunderstood smelling ability.
Among the findings reported yesterday:
Women are slightly more likely to have a good sense of smell than are men, but the ability declines with age for both sexes. Both findings confirmed previous beliefs.
Contrary to popular belief, however, pregnant women were poorer at smelling than their nonpregnant counterparts.
Factory workers, who tend to think their smelling ability is extremely poor, are actually far above average at identifying odors. By contrast, people who work outdoors usually think they have a superior sense of smell, but are really below average at identifying odors. They are, however, more sensitive to faint odors. Office workers, who rate their abilities below average, are the best at identifying odors.
Smoking does not necessarily dull the sense of smell. Smokers were more sensitive to some odors (banana and especially musk) and less sensitive to others (sweat, cloves and the chemical in natural gas).
The older people get, the less likely they are to regard the smell of escaping gas as highly unpleasant, a finding that raises questions about the effectiveness of the chemical used by gas companies.
The study was led by Avery N. Gilbert and Charles J. Wysocki, both of the Monell Chemical Senses Center in Philadelphia, a privately funded research facility devoted to the senses of smell and taste.
Both scientists said that although the number of people surveyed is large, it is not representative of the country as a whole. National Geographic readers are more likely to be white and to be better educated and more affluent than Americans in general.
"Still, it does give us an unprecedented sample that's already telling us things we didn't know before," Gilbert said.
For example, 1.2 percent of those responding said they could not smell any of the six odors, a proportion of "odor blindness" that Wysocki said was much higher than expected and warranted a study to see how people's lives were being affected by this potential handicap.
"Odor blind spots," the inability to detect specific odors, were far more prevalent, the survey results show. While more than 99 percent were able to smell banana, cloves and rose (though not necessarily name the odor), the figure dropped for the mercaptans (gas). Among women, 97.8 percent could smell it, slightly better than the 97 percent for men.
Odor detection was far lower for sweat and musk. Only 70.5 percent of women and 62.8 percent of men could smell androstenone. The figures for musk -- actually a synthetic version called galaxolide that is added to some cosmetics -- were only slightly better. About three-quarters of women and two-thirds of men could smell it.
Even if people could smell an odor, they could not invariably identify it. For example, although almost everyone could smell the odors of cloves and roses, only 80 to 90 percent could identify them. More than 99 percent could smell banana but only about half could identify it.
The poorest identifications were for androstenone (sweat) and musk. Only about one-quarter of either sex identified androstenone. About one-third of women identified musk but less than a quarter of men did.
In addition to the responses from the United States, the scientists tallied reports from about 100,000 National Geographic readers overseas, concentrating on the ability to smell androstenone. Readers living in the United States were poorest at detecting the odor and those in Africa the best.
The researchers said they had no explanation for the variations. They noted, however, that further study of the data, the largest mass of information about the human sense of smell ever gathered, is likely to continue for years.