Second-hand smoke increases the risk of the bone disease osteoporosis, researchers have found.
Osteoporosis can lead to bone fractures
The US/Chinese study showed exposure to passive smoking boosted pre-menopausal women's osteoporosis risk threefold.
An International Osteoporosis Foundation meeting in Toronto, also heard men, as well as women, increase osteoporosis risk by smoking.
Experts said the studies added to understanding of the link between smoking and the bone disease.
Osteoporosis, which affects one in three women and one in 12 men, is responsible for 200,000 broken bones per year in the UK - and 40 deaths a day.
It is often known as a silent illness because many people do not know they have it until it is too late.
Although it is thought of as a disease of old age, its roots are thought to lie in adolescence.
Harvard School of Public Health researchers looked at over 14,000 men and pre and post-menopausal women in rural China.
The scientists measured hip bone mineral density, and recorded non-spine fractures and smoking history.
Second-hand smoking was defined as living with one or more people who smoked each day.
Pre-menopausal non-smoking women who lived with one smoker had more than double the risk of osteoporosis compared to those who lived with none.
And those who lived with two or more smokers exposed to second-hand smoke were found to have a threefold greater risk of the condition.
They also had a 2.6 times greater risk for a non-spine fracture, compared to non-smokers.
A separate study by Gothenburg University researchers looked at 1,000 men aged between 18 and 20-years-old .
It was found that smokers' bone density in the spine, hip, and body as a whole, was lower than in their non-smoking peers.
A CAT (Computer Tomography) scanner was used to obtain 3D images of bone.
The researchers say smoking appeared to primarily affect a specific type of bone called cortical bone, by reducing its thickness.
This very dense bone forms a layer, similar to the enamel on teeth, around softer, spongy bone.
The effects was most striking in the hip, where the mineral density was over 5% lower than in non-smokers.
Fractures are more common in people with low bone mineral density.
A spokeswoman for the UK's National Osteoporosis Society said: "We know that smoking is becoming a much more significant risk factor for predicting whether you will break a bone because of osteoporosis due to the increasing amount of research being done into this area.
"However, our understanding of the effects of passive smoking on bone health is not quite so well researched so this study is an interesting addition to our cumulative knowledge of what smoking does to our skeletons.
"It does all add up to the fact that smoking is just plain bad for bones, whether you smoke yourself or if you live with someone who smokes.
"So it's another good reason to consider either giving up yourself or encouraging and supporting your loved ones to quit."
Amanda Sandford, of Action on Smoking and Health (ASH) said: "If there is a link between actual smoking and a condition, it's not surprising if a further link is found between passive smoking - albeit at a lower rate.
"The carcinogens in tobacco smoke can affect any organ of the body, once it's absorbed.
"This is further evidence for a ban in public places."