Living with a smoker can be like breathing the air in the world’s most polluted cities, according to a new study from Scotland.

“The message is pretty simple really - smoking in your home leads to really poor air quality and results in concentrations of fine particles, that you can’t see, that would cause real concern to us if they were found outside,” said lead author Sean Semple, of the Scottish Center for Indoor Air at the University of Aberdeen.

Tiny particles 2.5 microns in diameter or smaller, known as PM2.5, can penetrate deep into the lungs and even enter the blood. They’ve been linked to heart disease, strokes and cancer.

“Making your home smoke-free is key to reducing your exposure to PM2.5; for non-smokers who live with a smoker the impact of implementing smoke-free house rules would reduce their daily intake of PM2.5 by 70 percent or more,” Semple told Reuters Health in an email.

Such tiny particles typically result from combustion. Outdoors, the primary sources are vehicle exhaust, power plants and wildfires. Indoors, wood-burning or coal-burning stoves, gas cooking and heating fires and tobacco smoke are the most common sources of PM2.5 in the air.

For outdoor air, the World Health Organization says the safe exposure limit for PM2.5 particles is an average of 25 micrograms, or 25 millionths of a gram, per cubic meter of air over a 24-hour period, or average annual levels of 10 micrograms per cubic meter.

The U.S. Environmental Protection Agency sets the 24-hour limit at an average of 12 micrograms.

Semple and his colleagues wanted to bring together two scientific communities: those involved in tobacco control work and those interested in outdoor air pollution and health. “We think there is a lot that each can learn from the other,” he said.

Many studies have examined outdoor air pollution or indoor air quality in workplaces. But home is where most people spend the majority of their time, particularly small children and homebound elderly people, the researchers write. By comparing indoor air pollution in the homes of smokers and non-smokers, then comparing that to the most polluted cities, they hoped to illustrate the perils of indoor tobacco smoke over a lifetime.

The study team looked at data from four separate studies that measured PM2.5 levels in 93 Scottish homes where people smoked and 17 homes that were smoke free.

On average, PM2.5 levels in smokers' homes were around 31 micrograms per cubic meter – 10 times greater than the average of 3 micrograms in non-smoking homes.

There was a wide range of smoke concentrations in the smokers’ homes, however, and in one quarter of them, the 24-hour averages were 111 micrograms.

Semple pointed out, “A considerable proportion of smokers’ homes had air pollution levels that were the same or higher than the annual average PM2.5 concentration measured in Beijing,” a heavily polluted city.

The study team estimates that over a lifetime, a non-smoker living with a smoker will inhale about 6 grams more particulate matter than a non-smoker living in a smoke-free home.

Semple said that isn’t much, but this amount is likely to "have a substantial effect on the risk of developing diseases of the cardiovascular and respiratory systems.”

Semple said smokers often express the view that outdoor traffic pollution is a bigger problem than second-hand smoke pollution in the home.

“What this work shows is that, for most people living outside of major heavily polluted mega-cities like Beijing or Delhi, outdoor air pollution is much, much lower than what is measured inside homes where someone smokes,” he said.

“We have a lot of data and it’s an established fact how bad secondhand smoke is,” said Lucy Popova, from the Center for Tobacco Control Research and Education at the University of California, San Francisco. “There’s no safe level of exposure to it.”

“Smoke-free rules help not only by reducing the particulate matters for non-smokers but it actually helps smokers to quit too,” said Popova, who was not involved in the Scottish study. “Research shows when you have smoke-free rules in your home, that motivates smokers to make more cessation attempts and decrease the number of cigarettes that they smoke.”