People who are routinely exposed to a lot of second-hand smoke, such as workers in bars and restaurants, can see their risk of lung cancer triple, a new study says.
The Canadian study provides some of the most compelling scientific evidence yet for a total ban on workplace smoking, including bars and restaurants.
The research, published in the International Journal of Cancer, found that the more people smoke in a workplace, the greater the risks to non-smokers.
"These data absolutely back a smoking ban in bars," said Dr. Kenneth Johnson, senior epidemiologist at the surveillance and risk-assessment division of Health Canada and the lead researcher.
Dr. Roberta Ferrence, director of the Ontario Tobacco Research Unit, said "What's important about this research is it demonstrates a dose-response: The more exposure you have, the higher your risk.
"While this may seem obvious, it has long been contested by industry."
Dr. Ferrence said she hopes that this "strong new evidence will prompt strong new action" to expand workplace smoking bans.
The study also looked at the risk of second-hand smoke in the home.
Dr. Johnson and a team at Health Canada analyzed a lifetime of information from 761 women who had never smoked, and 71 others with lung cancer.
They found that a non-smoking woman who lives with a smoker has a 21-per-cent higher risk of developing lung cancer over her adult lifetime. But if the woman lived with a smoking parent as a child, her risk jumps 63 per cent, above that of someone who has always lived in a smoke-free home.
A woman who has always lived in a smoke-free home but works where smoking is permitted sees her risk of developing lung cancer jump by 27 per cent. That risk climbs steadily over time, and increases based on the number of smokers in the workplace.
"There's an underlying [belief] that second-hand smoke increases your risk of developing lung cancer by 20-25 per cent, and maybe that can be explained away by publication bias," Dr. Johnson said.
"But when you see the risk rising by 75 per cent right up to a tripling of the risk, it's hard to argue that nothing is going on."
The new research found that when the number of "occupational smoker years" (the number of smokers in the workplace multiplied by the worker's years of service) reaches 26, the risk of lung cancer has doubled.
(That could mean two smoking co-workers over 13 years or five smoking co-workers over five years. It could also mean 26 customers daily for a year in a bar.)
When researchers looked at the upper third of workers -- those exposed to the most second-hand smoke -- they found the lung cancer risk was more than tripled.
Since the early 1980s, more than three dozen studies have examined the impact of secondhand smoke on non-smokers, but the Health Canada research is the first original Canadian data.
In the International Journal of Cancer, Dr. Johnson wrote that it is not surprising to see higher risks associated with workplace exposure because studies have consistently demonstrated that the intensity of exposure is higher on the job than at home. The level of nicotine in the air of bars is up to 15 times higher than in the home of a smoker.
He said the new research suggests that earlier studies on the impact of secondhand smoke in the home were flawed because they failed to take into account workplace exposure. Similarly, previous research on childhood exposure has tended to look at the impact during childhood only, while the new data demonstrate that the risks are cumulative if exposure continues through adulthood.
The Health Canada researchers collected data on the women from the cancer registries of eight provinces.
Women are usually the focus of research on secondhand smoke because, historically, they have been far less likely than men to smoke and far more likely to live and work with smokers.
Twenty-four per cent of Canadians over the age of 15 now smoke, the lowest rate since Health Canada began collecting data in the 1960s. Then, fully half the population smoked.
Lung cancer is the biggest cancer killer in both women and men. It killed an estimated 17,700 Canadians last year. Smoking is also a leading contributor to heart disease, Canada's biggest killer.