“Smoking Kills” is more than just a catchy PSA or smoking cessation campaign slogan—it’s verifiable fact. Since the mid-1900s, study after study has generated compelling evidence linking smoking to increased mortality rates. Arguably, the most influential of these is the 1956 publication of smoking data on the “British Doctors Study,” which presented compelling evidence that over half of smokers would eventually die due to smoking-related complications. A new study published in BMC Medicine asserts that this mortality rate may even be as high as 66 percent, meaning that two out of three smokers will eventually die from conditions associated with their smoking.

This study, put together by investigators from the National Centre for Epidemiology and Population Health at the Australian National University, followed 204,953 men and women over 45 years old from New South Wales, Australia. These participants were categorized into groups of smokers, past smokers, and never smokers.

The investigators found the percentage of smokers was comparable among men and women. They also found that current smokers were likely to be younger than never smokers, and they were less likely to live in urban areas. Current smokers had lower income and education when compared to never smokers, and they were less likely to hold private health insurance. Finally, current smokers were more likely to report consuming over 15 alcoholic drinks a week and were more likely to have a higher body mass index.

(These factors were accounted for when the mortality statistics were examined.)

Person-years are a measure of time used in epidemiological studies, in which the years studied for all participants in a study are added together. For example, if three people were studied for 10 years each, 30 total person years would be reported in the study. In the study published in BMC Medicine, a total of 874,120 person-years were examined, and during those person-years, 5,593 deaths occurred among the study population.

Epidemiological outcomes are typically reported in terms of “Relative Risk”, which describes the proportion of the risk of an outcome that can be attributed to a specific factor. In this study, the relative risk of death (known as mortality) for male and female smokers showed that they were approximately 2.76 and 2.95 times more likely to die than never smokers. Quitting helps; male and female past smokers were 1.27 and 1.39 times more likely to die than never smokers. These numbers, while not surprising given the large body of data on the risks of smoking, are nonetheless a staggering reminder of the quantifiable risks of smoking.

The investigators also examined the mortality rate in relation to the number of cigarettes smoked per day. They found that there was a two-fold increase in mortality for participants who smoked 1-14 cigarettes per day and a four-fold increase in mortality for those who smoked more than 25 cigarettes per day (compared to never smokers).

This study does report the promising finding that quitting smoking, even later in life, can considerably reduce the risk of mortality associated with smoking, a finding that’s been demonstrated several times in the literature on smoking. The researchers found that when they compared participants who had quit smoking at ages 45-54, the relative risk of dying for men and women was only 1.36 or 1.52 times the risk of dying in never smokers. They found that the risk of death declined progressively, and this was proportional to the amount of time that passed since the participant quit smoking.

Though the findings of this study are extremely similar to smoking studies conducted in the US and the UK, this is the first large-scale population study conducted in Australia, and demonstrating that the risks of smoking transcend cultural lifestyle factors. Additionally, despite the fact that this finding is similar to the existing literature on the risks of smoking, this striking data on the nearly three-fold increase in mortality for smokers remains relevant, as many people seem to either ignore or never fully register that message.

The good news is that it’s not too late to quit smoking to improve your future health.

BMC Medicine, 2015. DOI: 10.1186/s12916-015-0281-z (About DOIs).