Despite being a very bad and unpleasant habit with many bad effects, smoking may permanently affect some genes.

A Candian team led by Wan L Lam and Stephen Lam from the BC Cancer Agency conducted in a study, in order to explain why the former smokers are still still more susceptible to lung cancer than those who have never smoked.

The researchers took samples from 24 people, eight current smokers, 12 former smokers and four people who had never smoke. and they analyzed the patterns of gene activity by using  technique called serial analysis of gene expression (SAGE).

Researchers were able to identify 600 genes that were differently expressed in current and non-smokers.

Former smokers fell somewhere in the middle, with changes in one third of the affected genes proving to be irreversible.

According to their findings, the changes induced by smoking in the gene activity are followint into two categories: reversible and irreversible

The reversible genes were particularly involved in xenobiotic functions (managing chemicals not produced in the body), nucleotide metabolism and mucus secretion. Some DNA repair genes are irreversibly damaged by smoking, and smoking also switched off genes that help combat lung cancer development.

The researchers identified a number of genes not previously associated with smoking that are switched on in active smokers. One example is CABYR, a gene involved in helping sperm to swim and associated with brain tumours, which may have a ciliary function. The team also further investigated changes in genes involved in airway repair and regeneration, and within this group identified genes that fell into three categories following cessation of smoking: reversible (TFF3, encoding a structural component of mucus; CABYR, in it's newly discovered bronchial role), partially reversible (MUC5AC, a mucin gene) and irreversible (GSK3B, involved in COX2 regulation). These findings were tested against a second cohort of current, former and non-smokers.

"Those genes and functions which do not revert to normal levels upon smoking cessation may provide insight into why former smokers still maintain a risk of developing lung cancer," according to Raj Chari, first author of the study.

Though, as Dr. Calum MacAulay, one of the authors, pointed out the study should not discourage current smokers to quit. "This should not be construed that it's no use to quit smoking," he said.

He explained that about one-third of the genes affected by smoking return to normal after a smoker quits.

According to the World Health Organization, tobacco-related disease will kill 8.3 million people by 2030. The international health experts warned that tighter laws on the tobacco industry could save 200 million lives by 2050.

Also WHO said that if more aggressive measures will be introduced against smoking (such as higher taxes, banning cigarette advertising and making offices and public places totally tobacco-free), the smoking rates could halve by 2050.