Non-smokers who live with a smoker are exposed to three times the officially recommended safe levels of damaging air particles, according to a study.
The researchers found that living with smokers is the same as living in smoke-free homes in heavily polluted cities such as Beijing or London.
They said moving to smoke-free homes could have major health benefits for non-smokers.
The study has been published online in the BMJ's Tobacco Control journal.
It was carried out by researchers at the universities of Edinburgh and Aberdeen.
They said there was already strong evidence to suggest that exposure to second-hand smoke is linked to a wide range of adverse health events such as respiratory and heart illness.
Accordingly, many governments have introduced measures to restrict their population's exposure to second hand smoke within workplace and leisure settings.
Fine particulate matter (PM2.5), such as fine dust or soot suspended in the air, has been widely used as a marker for second-hand smoke exposure.
Main outdoor sources of particulate matter include exhaust fumes from motor vehicles and industrial emissions, and more is known about what impact this has on health than the impact within indoor environments.
Therefore, the Scottish researchers set out to estimate the amount of PM2.5 inhaled by people living in smoking and non-smoking homes.
They studied data from four linked studies carried out in Scotland between 2009 and 2013 that had real time measurements of PM2.5 in homes, and combined them with data on typical breathing rates and time-activity patterns.
Collectively, the studies produced air quality data from 93 smoking homes with a further 17 non-smoking households. Most sampling was for a 24-hour period with the exception of one study data, which was generally carried out over a period of 6-7 days.
The results showed that the average PM2.5 concentrations from the 93 smoking homes were about 10 times those found in the 17 non-smoking homes.
Non-smokers living with smokers typically had average PM2.5 exposure levels more than three times higher than the World Health Organisation's guidance for annual exposure to PM2.5.
Many non-smokers living in smoking homes inhaled similar quantities of PM2.5 to non-smokers who lived and worked in smoke-free environments in cities such as Beijing or London with high levels of air pollution.
The researchers also calculated that overall, homes where unrestricted, heavy-smoking activity took place produced second-hand smoke concentrations that were, on average, about 10 times higher than homes where efforts to reduce or restrict second-hand smoke exposure were more common.
Some homes studied had particularly high rates of smoking. Around a quarter of homes had 24 hour average concentrations more than 11 times that recommended as an annual average concentration by WHO.
The researchers also estimated that the overall mass of PM2.5 inhaled over an 80-year period for a person living in a typical smoke free home was about 0.76g compared with a similar person living in a smoking home, who would inhale about 5.82g.
Non-smokers living in smoking households would experience reductions of more than 70% in their daily inhaled PM2.5 intake if their home became smoke-free, the researchers calculated, and the reduction was likely to be greatest for the very young and for older members of the population.
They concluded: "These findings ultimately support the need for efforts to reduce SHS [second hand smoke] exposure in the home, most notably through the implementation of smoke free home rules and smoke free multi-unit housing policies."
Lead author Dr Sean Semple, of Aberdeen University, said: "Smokers often express the view that outdoor air pollution is just as much a concern as the second-hand smoke in their home.
"These measurements show that second-hand tobacco smoke can produce very high levels of toxic particles in your home: much higher than anything experienced outside in most towns and cities in the UK.
"Making your home smoke-free is the most effective way of dramatically reducing the amount of damaging fine particles you inhale."