Smoking just one joint has the same impact on the lungs as smoking up to five cigarettes, says recent research out of New Zealand.

In the study, adults aged 25 to 49 were divided into four groups: those who only smoked marijuana, those who only smoked tobacco, those who smoked marijuana and tobacco, and those who didn't smoke at all.

Researchers tracked their respiratory function for a 10-year period, using high-resolution CT scans, lung function tests and a respiratory and smoking questionnaire.

Those participants who smoked only marijuana reported wheezing, coughing, chest tightness and phlegm. The researchers found that their smoking habits damaged their lungs and stopped them from functioning properly.

They discovered that the small airways in the lungs of pot smokers had narrowed, meaning less oxygen and waste products could be transported to and from the blood vessels.

They also noted damage to the large airway, which blocked airflow and forced the lungs to work much harder.

The effect on the lungs of each joint was found to be equivalent to smoking between 21/2 and five cigarettes at one time.

Researchers theorize that the more pronounced effect on the lungs by marijuana is caused by pot smoking habits. Marijuana smokers tend to inhale more deeply, smoke without a filter, and smoke to a shorter butt length for a prolonged period of time - and at a higher temperature.

The end result is that more tar and carbon dioxide is taken into the lungs.

The good news for pot smokers is that the study found that chronic users of marijuana were less likely to be diagnosed with emphysema, a chronic and progressive disease of the lungs which leads to shortness of breath.

Only 1.3 per cent of pot smokers were found to have the disease, while 16.3 per cent of the marijuana-tobacco smokers and 18.9 per cent of the tobacco smokers developed it.

The study - Effects of cannabis on pulmonary structure, function and symptoms - will be published in Thorax, an international journal of respiratory medicine.

The authors warn that the results need to be interpreted with caution as the study relied on patients to self-report their marijuana use.