For years, American health agencies have issued warnings about the dangers of secondhand smoke.

Now, for the first time, researchers say they have hard evidence that exposure to secondhand smoke causes changes in the lungs.

Kathy Smith, 47 years old, has been smoking for most of her adult life.

She's also been trying to quit for almost that long -- for her sake and for the health of her three kids.

"They are worried about what it's doing to me,” she admits. “They're worried about what it's doing to them."

We've all heard that secondhand smoke can hurt our lungs, but now there's proof.

For the first time, researchers are able to see the damage.

In a clinical study, smokers and nonsmokers inhaled a special helium mixture before having an MRI scan.

The machine measured the movement of the helium atoms and spotted lung damage.

Researchers found that almost one-third of nonsmokers had damage to their lungs similar to that of smokers.

"We've shown that secondhand smoke does have the potential to damage the lungs, and so, for smokers, you're really not just hurting yourself,” explains Dr. Talissa Altes. “You are hurting the people around you and the people you love."

Exposure to secondhand smoke has been linked to asthma, ear infections, pneumonia, and SIDS in children.

In adults, it increases the risk of lung cancer, heart disease, blood clots, and stroke.

This new study is the first to prove secondhand smoke damages the lungs.

"I don't even need that information to feel bad about my habit," Smith admits.

For people like Kathy, it's another reason to try to kick the habit again.

To minimize your risk of secondhand smoke, never let anyone smoke inside your house or car.

If someone in your home does smoke, consider using an air purifier. And remember to clean and re-paint walls and surfaces frequently.

Sugar cravings may be related to genes

If you drool at the sight of chocolate, don't blame a sweet tooth...

You may be able to blame your genes!

Toronto scientists say they've pinpointed a genetic variation they think is the reason why some people consume more sugar than others.

It's called GLUT2. During a recent study, those with the variant said they ate more sugary foods.

This doesn't prove GLUT2 is totally responsible for sugar cravings, but researchers say it warrants more study.