Smoking increases a person’s risk of dying early. Now a new UK study of 1 million women finds those who quit smoking by age 30 can almost completely eliminate the increased risk of dying early compared to never smokers, while those who quit by the age of 40, can cut it by 90%.

A paper on the largest-ever study of smoking among women in the UK, written by researchers at the University of Oxford on behalf of the Million Women Study Collaborators, is published on 27 October in The Lancet.

The most important result of this new study is that the risks posed by smoking are bigger than previous research suggests, and, quitting smoking has a bigger effect on reducing those risks than previously thought.

The researchers found that female smokers in the UK die about 10 years earlier on average than never smokers. But by giving up the habit before the age of 40, and preferably well before then, they can cut more than 90% off the risk of losing those 10 years.

Their analysis shows that most of the difference between smokers and non-smokers, as far as cause of death is concerned, is smoking-related.

It shows that two-thirds of smokers in the study who died in their 50s, 60s, and 70s, did so as a result of a smoking-related diseases such as lung cancer, chronic lung disease, heart disease and stroke.

But women who quit by the age of 30 avoided 97% of the increased risk of smoking-related premature death.

Release of the study coincides with the centenary of the birth of Sir Richard Doll, renowned epidemiologist and an early pioneer of research linking smoking to lung cancer. He died in 2005 aged 92, and was Regius Professor of Medicine at Oxford University from 1969 to 1979.

Many of the researchers involved in this latest study worked with Doll and have continued building on the methods he developed in using large epidemiological studies and randomized trials in medical research.

Professor Sir Richard Peto of the University of Oxford, is one of the lead authors of the study. He worked with Doll for 30 years and says in a press statement that Doll’s work has helped millions of people worldwide escape an early death.

Peto also explains why we have had to wait until now to discover these latest revelations:

“Both in the UK and in the USA, women born around 1940 were the first generation in which many smoked substantial numbers of cigarettes throughout adult life.”

“Hence, only in the 21st century could we observe directly the full effects of prolonged smoking, and of prolonged cessation, on premature mortality among women,” he adds, but also points out that for both men and women, “smokers who stop before reaching middle age will on average gain about an extra ten years of life“.

The Million Women Study recruited 1.3 million women in the UK aged 50 to 65 over the period 1996 to 2001. On entry to the prospective study, they completed a detailed survey of their smoking status, lifestyle, medical conditions and social factors, and then again three years later.

On enrollment to the study, 20% of participants were current smokers, 28% were former smokers, and 52% had never smoked.

Then for an average of 12 years after enrollment, the researchers used NHS records to find out which of the participants died and the cause of death.

When they analyzed the results, they found the women who were still smoking when surveyed 3 years after enrollment, were nearly three times more likely to die over the ensuing 9 years than non-smokers.

The results also showed that risk of dying rose steeply with the number of cigarettes smoked, and even “light” smokers (who had between 1 and 9 cigarettes per day) had double the risk of dying in the following 9 years as non-smokers.

Funds for The Million Women Study come from Cancer Research UK, the Medical Research Council, and the Health and Safety Executive.

Written by Catharine Paddock PhD