The oldest age at death for humans has been rising for more than a century and shows no signs of leveling off, according to a demographer at the University of California, Berkeley.

This new finding, based on Swedish national death records for each year since 1861, calls into question the belief by many scientists that the human life span has a set end-point of 120 years.

In research published today in Science, the nation's premier science journal, UC Berkeley associate professor of demography John Wilmoth and his colleagues in Sweden and the United States show that, in the1860s in Sweden, the oldest ages at death for men and women centered around 101. That average maximum age moved up slowly throughout the century to about 105 in the 1960s and then accelerated to 108 in the 1990s.

"We have shown that the maximum life span is changing. It is not a biological constant. Whether or not this can go on indefinitely is difficult to say. There is no hint yet that the upward trend is slowing down," said Wilmoth.

Wilmoth said that Swedish demographic statistics - considered the world's best records on birth and death - are a good indication of patterns in other industrialized nations, where it has become commonplace to survive to a very old age.

But until now, there has been no evidence that the maximum age at death was being pushed back, leading to a lengthening of the human life span, said Wilmoth.

"Human progress is real, somehow," he said. "We are changing the limits of the human life span over time."

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For individuals alive now, the current life span makes it rare to live past 110 years, said Wilmoth. "But future generations could have a higher range."

Such statements run counter to recent predictions by other experts in the field who theorize that the human life span is biologically limited to 115 or 120 years.

"Those numbers are out of thin air," said Wilmoth. "There is no scientific basis on which to estimate a fixed upper limit. Whether 115 or 120 years, it is a legend created by scientists who are quoting each other."

Historical records, on the other hand, show that the entire configuration of ages at death in Sweden has been shifting upward for 138 years, he said. The upward trend accelerated suddenly around 1970, more than doubling the rate at which the life span was growing, from less than one year of age for every two decades to more than one year per decade.

This has happened because of medical and public health advances throughout the century, said Wilmoth, whose analysis ruled out simple population growth as a factor. Some scientists had thought that the increased number of very old people could be due to a larger population base, but Wilmoth's data show that the main cause is increased survival after age 70.

Earlier in the century, the lengthened life span was probably due to public health measures such as better sanitation, a safer water supply and control of infectious diseases.

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While it is well known that these measures increased life expectancy - the average age at death - especially by lowering child mortality, Wilmoth believes that better public health also increased the maximum age at death by creating a healthier population in old age.

"The elderly today are benefiting from the fact they were not as sick when they were children as in past generations, and these changes took place 80 to 100 years ago," he said.

As for the spurt upward in 1970, Wilmoth's data shows that survival after 70 was due overwhelmingly to improved medical practice concerning heart disease, stroke, smoking cessation, and the development of new drugs.

"After 1970, the trend began to slope upward rapidly," said Wilmoth. "That corresponds closely with breakthroughs in certain medical practices such as understanding and treating heart disease and stroke."

Wilmoth also analyzed the age at death for the individual who lived the longest from each yearly birth cohort between 1756 and 1884 and found a similar upward trend in the life span.

The longest-lived person born in 1756, for instance died in 1857 at the age of 101, while the longest-lived person born in 1884 died in 1993 at the age of 109.

As in most other demographic data, men had a slightly lower maximum life span than did women, but the gender difference was much smaller than with life expectancy. Life expectancy is the average age to which people are expected to live when they are born.

Men in the United States can expect to live, on average, to age 72 while women live to 79, a difference of about seven years. But in great old age, the gender difference shrank to less than two years in Sweden's records on maximum age.

In the world today, said Wilmoth, "there are currently about two dozen documented cases of people over 110 years of age - a handful of men and a few handfuls of women."