Jan. 10, 2012 -- Woodstock generation, breathe easy. One of the largest and longest studies ever to look at the effect of marijuana smoking on lung health finds that pot smoking doesn’t appear to cause chronic breathing trouble.

The study has followed more than 5,000 young adults in four cities for more than two decades. More than half of the people in the study reported smoking tobacco, marijuana, or both.

Over time, researchers repeatedly checked two measures of lung function: One was a test that measured the amount of air forcefully exhaled in a single second. The second test measured the total amount of air exhaled after taking the deepest possible breath.

Those tests help doctors diagnose chronic, irreversible breathing problems like chronic obstructive pulmonary disease (COPD).

Cigarette smoking is a leading cause of COPD. And marijuana smoke contains many of the same chemicals as tobacco smoke.

As more states legalize marijuana -- 16 states and the District of Columbia now allow its medical use -- experts have worried that the kinds of lung damage caused by cigarettes could also be brought on by pot smoking.

Indeed, cigarette smokers in the study saw their lung function drop significantly over 20 years.

But that didn’t happen to people who only smoked marijuana.

In fact, the study found that the lung function of most marijuana smokers actually improved slightly over time.