People exposed to passive smoking, also known as second hand smoking, are at a greater risk of stroke, a new study says. This is also true of a non-smoker married to a smoking partner, respiratory health experts say.

The study, carried out by a Harvard University team, looked at records of more than 16,000 people aged over 50 and their spouses over a period of, on average, slightly more than nine years. The results showed that non-smokers living in the same house as a smoker were at far greater risk of stroke.

People who had never smoked but are living with a smoker have 42 percent more risk of stroke. However, the risk factor rises to as high as 72 percent if he or she had smoked at some point in their lives.

According to the Journal of the American Medical Association, an estimated 53,000 non-smokers die each year in the U.S. as a result of second hand smoke. Almost 3 million children in the United States under the age of 6 years old breathe secondhand smoke at home at least four days per week.

Children, pregnant women, older people, and people with heart or breathing problems need to be especially careful.

The study appears in the latest issue of the American Journal of Preventative Medicine.