Smoking is bad for us. We know that. We also know that it's one of the leading causes of death for people around the world. Not smoking, all told, is a pretty clever thing for you to do.

But for a small minority of very lucky people, smoking doesn't seem to cause all the life-shortening illnesses that threaten most other smokers. Indeed, some of the world's oldest people reach extreme ages while being smokers. The world's documented longest-living person, Jeanne Calment, was a smoker for most of her life, and another claimant to the title is said to smoke a pack a day. What's their secret?

According to new research, the lifespans of such long-living smokers aren't a coincidence. A study published in The Journals of Gerontology, Series A: Biological Sciences & Medical Sciences this month has found that SNP (single nucleotide polymorphism) sequences of DNA in some people appears to help them better withstand and mitigate the environmental damage caused by long-term smoking.

"We identified a set of genetic markers that together seem to promote longevity," said Morgan E. Levine, corresponding author of the study, in a press release. "What's more, many of these markers are in pathways that were discovered to be important for ageing and lifespan in animal models."

The researchers sequenced the genomes of 90 long-lived smokers who lived past 80 years of age – "a group whose survival may signify innate resilience" – and contrasted them with the genomes of 730 smokers who died before they reached 70.

They identified a network of SNPs in genes that conferred significant anti-ageing benefits, offering those with the longevity genes effectively a 22 percent increase in the likelihood of reaching 90–99 years of age, and a threefold increase in the likelihood of becoming a centenarian. And those same genes are associated with an almost 11 percent lower cancer prevalence.

"There is evidence that these genes may facilitate lifespan extension by increasing cellular maintenance and repair," said Levine. "Therefore, even though some individuals are exposed to high levels of biological stressors, like those found in cigarette smoke, their bodies may be better set up to cope with and repair the damage."

The researchers conclude that long-lived smokers may represent a "biologically distinct group, endowed with genetic variants allowing them to respond differentially to environmental stressors".

As Levine told Ariana Eunjung Cha of The Washington Post, in the future it's quite possible that individualised testing will be available to consumers to help them determine whether they carry the genetic markers that could help them resist the effects of ageing and stave off illnesses.

But there's a limit to how much the knowledge will do for them.

"[The proportion] of people who have a 'genetic signature' that would help them cope with the biological stresses of smoking is extremely small, and therefore, nobody should use this paper as an excuse to continue smoking," he said.