Researchers suggest that non-smokers who live with people who smoke indoors are exposed to three times the healthy limit of dangerous air particles, as recommended by the World Health Organization.

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Secondhand smoke from burning tobacco products is associated with a variety of diseases.

According to a study published in Tobacco Control, living with people who smoke indoors is equivalent to living in a smoke-free house in a heavily polluted city, such as London or Beijing.

The dangers of secondhand smoke are well documented. The Centers for Disease Control and Prevention (CDC) report that since 1964, 2.5 million non-smokers have died as a result of exposure to secondhand smoke.

Secondhand smoke contains over 7,000 different chemicals – including many that are toxic and around 70 that are known to cause cancer. Even in adults who have never smoked in their lives, secondhand smoke has been found to cause both cardiovascular disease and lung cancer.

Exposure to secondhand smoke is often assessed by measuring the presence of fine particulate matter (PM2.5), such as fine dust or soot, suspended in the air. According to the researchers, the indoor levels of PM2.5 in a non-smoking home are around 60-70% of those outdoors.

The health impact of outdoor sources of PM2.5 – such as vehicle exhaust fumes and industrial emissions – has been widely documented, but less is known about the impact these particles have on individuals within indoor environments.

Based in Scotland, UK, the researchers assessed data taken from four linked studies carried out between 2009 and 2013 that included real-time measurements of PM2.5 in homes. These data were combined with other data regarding time-activity patterns and typical breathing rates.

Air quality data were taken from 93 smoking and 17 non-smoking households. Three of the four studies measured PM2.5 concentrations with an aerosol monitor positioned in the houses’ main living areas for 24 hours. The other study utilized a new particle-counting device, which took readings over the course of a week.

The researchers found that the average PM2.5 concentrations in the smoking households were around 10 times higher than those in the non-smoking homes.

On average, non-smokers living in smoking households had PM2.5 exposure levels over three times higher than the World Health Organization’s (WHO) recommendation for annual exposure to PM2.5 (10 μg/m3). These non-smokers were found to inhale similar levels of PM2.5 to non-smokers living and working in smoke-free environments within heavily polluted cities.

These levels increased in homes with higher levels of smoking; about a quarter of smoking households had average concentration levels of 111 μg/m3, over 11 times the WHO’s recommended annual average.

The overall amount of PM2.5 that would be inhaled over the course of 80 years by a person living in a smoke-free home was calculated to be 0.76 g, compared with 5.82 g that would be inhaled by someone living for the same amount of time in a smoking household.

The authors of the study acknowledge that the samples used for their research were not particularly representative and that the short timeframe of some of the studies left room for potential behavioral bias from the study participants. However, in the longer-running study, the researchers observed no evidence of lower PM2.5 on the first day of measurement.

The CDC report that levels of secondhand smoke have been falling over the last few decades. This fall is largely attributable to laws prohibiting smoking in certain places and changing personal habits, such as voluntary smoke-free home rules.

For non-smokers living in smoking households, a move to a smoke-free home would reduce their daily inhaled PM2.5 intake by more than 70%, according to the authors of the study.

Lead author Dr. Sean Semple, of the University of Aberdeen, Scotland, says that their findings conflict with the opinion held by some smokers that outdoor air pollution is as much of a health concern as secondhand smoke at 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 in the UK. Making your home smoke-free is the most effective way of dramatically reducing the amount of damaging fine particles you inhale.”

Recently, Medical News Today reported on a study finding that approximately 14 million major medical conditions experienced by American adults are attributable to smoking.