Non-smokers who live in homes where someone else smokes are exposed to damaging air particles that are three times higher than recommended safe levels, a new study has found.

According to the findings, living with a smoker is the same as living in a non-smoking home in a heavily polluted city such as Beijing.

The researchers from Scotland looked at air particles known as fine particulate matter (PM2.5), such as soot or fine dust that is suspended in the air. This is often used as a marker for secondhand smoke exposure. For example, pubs where smoking is allowed display levels of smoke-derived PM2.5 that are often well above the safe limits suggested by the World Health Organization (WHO).

The researchers set out to compare PM2.5 levels in non-smoking homes compared to smoking homes. They analysed data from four studies carried out in Scotland between 2009 and 2013. Together, the four studies provided information on 93 smoking homes and 17 non-smoking homes.

The study found that PM2.5 levels in the 93 smoking homes were around 10 times higher than the levels found in the non-smoking homes. Furthermore, non-smokers who lived with smokers tended to have PM2.5 exposure levels that were at least three times higher than the WHO's recommended safe levels.

The researchers noted that many non-smokers who lived with smokers inhaled similar amounts of PM2.5 as people who lived in smoke-free homes but were exposed to high levels of air pollution, such as residents of Beijing or London.

The study also noted that in homes where heavy smoking took place that was not restricted in any way, secondhand smoke concentrations were around 10 times higher than in homes where there was an attempt to restrict smoking.

The study pointed out that over an 80-year period, a person living in a non-smoking home could expect to inhale 0.76g of PM2.5. However a non-smoker living in a home where smoking was permitted could expect to inhale 5.82g over the same period.

The researchers believe that the amount of PM2.5 inhaled on a daily basis by a non-smoker in a home where smoking is permitted could be reduced by 70% if the home was to become smoke-free.

They insisted that their findings show the importance of reducing secondhand smoke exposure in the home.

"Smokers often express the view that outdoor air pollution is just as much a concern as the secondhand smoke in their home. These measurements show that secondhand tobacco smoke can produce very high levels of toxic particles in your home - much higher than anything experienced outside in most towns and cities. Making your home smoke-free is the most effective way of dramatically reducing the amount of damaging fine particles you inhale," said the study's lead author, Dr Sean Semple, of the University of Aberdeen.

Details of these findings are published in the journal, Tobacco Control.