A long-term Swiss study has for the first time shown some of the risks of passive smoking on healthy adult non-smokers.
Environmental tobacco smoke (ETS) is more likely to affect so-called never-smokers with a genetic predisposition to respiratory disease.
The first stage of the Swiss Study on Air Pollution and Lung Diseases in Adults was launched in 1991, with a follow-up in 2002. Over 1,600 people without a previous history of respiratory distress were assessed.
While passive smoking is now considered a risk factor for children and probably adults, the study constitutes a first in that it focuses on healthy adults over a long period.
Previous research had provided only a snapshot of people's health, but had shown that exposure to smoke increased the likelihood of respiratory disease.
Around three quarters of the participants were not exposed to passive smoking during the period of the study. Approximately one fifth were exposed at the beginning, but not at the end, while less than ten per cent had tobacco smoke inflicted on them in 1991 and 2002.
"The results of our assessment of ETS effects in never-smokers showed that exposure [...] was associated with the development of respiratory symptoms," said Margaret Gerbase, a pulmonary specialist at Geneva University Hospital.
The study group was also tested for bronchial hyperreactivity (BHR), which affects the respiratory tract. BHR, which has a strong genetic element, is associated with asthma and other breathing problems.
Earlier research on smokers showed that this hyper-reactivity increases the risk of respiratory distress and diseases. The latest Swiss study notes this is also the case with passive smoking.
"A particularly strong effect of continued exposure to ETS was observed among previously asymptomatic individuals with bronchial hyper-reactivity," added Gerbase.
The researchers found that exposure to passive smoke was strongly associated with the development of cough. In subjects with hyper-reactivity, this exposure also led to shortness of breath, while other symptoms like wheeze and chronic bronchitis were noted.
The study showed that individuals with BHR who are persistently exposed to passive smoking are at particular risk of developing early-onset chronic respiratory disease. Gerbase said that as symptoms developed, the subjects peripheral airways narrowed.
Earlier research was used to factor out elements such as air pollution and smoke in bars and restaurants. "The study covered eight regions, so we had take into account for example stronger pollution in cities like Geneva and Lugano," Gerbase told swissinfo.
The findings of the study, which come as debate continues unabated about potential measures against passive smoking, have public health implications according to the authors.
"We don't know who is affected by bronchial hyperreactivity," said Gerbase. "So any measures taken to reduce the impact of passive smoking need to target the entire population of non-smokers."
swissinfo, Scott Capper
1,661 adults were tested for the study in 1991 and 2002.
People with respiratory symptoms such as cough, wheeze, short breath or chronic bronchitis were excluded beforehand, as well as those only exposed to passive smoke in 2002.
The group was also checked for bronchial hyper-reactivity to find if passive smoke increased the likelihood of respiratory problems later.
The results are published in the American Journal of Respiratory and Critical Care Medicine.
Since 1992, non-smokers have been protected from passive smoking in the workplace under Swiss law.
But voters haven't always been in favour of restrictions on tobacco. In 1993, more than 70 per cent turned down proposals to ban tobacco and alcohol advertising.
However, a number of cantons are drawing up legislation offering non-smokers more protection, while in others votes on smoking restrictions look likely to take place.
The southern canton of Ticino showed the way in March, with voters massively approving a ban on smoking in public spaces.
The Swiss Federal Railways declared its trains smoke-free last December.