Human lifespan has been increasing for decades because of improvements in diet and public health as well as advances in the field of medicine that made possible the prevention and treatment of many diseases.

Despite these gains, the analysis of an international mortality database showed that there may be a limit on how long humans can live and that humans may have hit their longevity limit.

For the new study published in the journal Nature on Oct. 5, Brandon Milholland of Albert Einstein College of Medicine and colleagues looked at the Human Mortality Database, which contains data on how long people have lived in recent decades.

They found that in at least 40 countries and territories, the number of people who survived at least 70 years rose since 1900 suggesting that the life expectancy of humans, the estimate of how long people can expect to live, has increased.

Babies born in the U.S. today, for instance, can expect to live until 79 years old. Those who were born in 1900, however, have a life expectancy of only 47 years.

The researchers, however, said that while the percentage of people who lived to old age kept increasing since the 1900, the likelihood of those who made it to the 100-year mark to survive after their 100th birthday did not increase much regardless of what year they were born.

Age at the time of death were found to have increased a bit between the 1970s and the early 1990s, but this appears to have leveled off since this period.

The longest documented lifespan of a person in history belongs to Jeanne Calment, who died in 1997 at the age of 122. The age at death of the oldest person in the world has not increased since Calment's death and since then, the trend has been for the oldest person to reach about 115 years old. The researchers said that this would remain stable in the foreseeable future.

Based on longevity data, the researchers believe that the average maximum life span is 115 years old and the absolute limit of human life span is 125 years. Researchers likewise said that the chances that any person would reach 125 years old in a given year is less than 1 in 10,000.

Why can't humans live longer? Researchers propose possible reasons.

"We suspect that the accumulation of damage with age, especially mutations in the individual cells of the body, somatic mutations, ultimately places a limit on lifespan," Milholland said.