Researchers at Emory University, in Atlanta, Georgia, analysing public US health data have concluded that while long term marijuana smoking can inflame the airways, there is no significant change in lung function.

From the data collected by the National Health and Nutrition Examination Survey (NHANES) from 2007 to 2010, over 5,000 adult survey participants had used cannabis, with 59.1% having used the drug at least once in their lifetime, while 12.2% had used it in the prior month.

Almost 3,000 of the adult participants had patterns of lifetime marijuana use. The researchers measured their use of the drug in terms of "joint-years", whereby an individual had smoked an average of one joint per day for a year.

In the participants that had been exposed to cannabis for less than 20 joint-years, the researchers found that there was no significant change in lung function.

As for the people who had been exposed to marijuana for more than 20 joint-years, there was a clinically detectable change in their lung function, but the symptoms did not match those observed in obstructive lung disease due to tobacco use.

Instead, people who had smoked cannabis for over 20 joint-years were more likely to have inflamed airways.

Cannabis increases respiratory irritation

"While over 20 joint-years is significantly associated with a change in lung function, it is inconclusive whether or not this represents early lung function impairment similar to long-term tobacco use," said lead author Jordan A Kempker, MD, MSc, clinical research fellow in the Division of Pulmonary and Critical Care Medicine in Emory University School of Medicine.

"Furthermore, smoking marijuana seems to increase symptoms of respiratory irritation, such as bronchitis, and our study was inconclusive about whether those effects are permanent. We also did not study the association of marijuana smoking with the development of cancer."

The study, entitled "Effects of Marijuana Exposure on Expiratory Airflow: A Study of Adults who Participated in the U.S. National Health and Nutrition Examination Study" is published in the journal Annals of the American Thoracic Society.

The NHANES survey is an ongoing nationally representative survey conducted annually by the National Center for Health Statistics.

Moderate use could actually improve lung function

Data collection for the survey includes both self-reporting survey questions, as well as high quality, standardised spirometry tests to measure lung function such as forced vital capacity (FVC) and forced expiratory volume (FEV1).

FVC measures the volume of air a person can force out of their lungs, while FEV1 measures the volume of air forced out in the first second of the manoeuvre.

The researchers' results contradict a study presented at the British Thoracic Society conference in December 2014, which found that young adults in their 20s ended up having the "lungs of 80-year-olds" after less than 10 years of marijuana use.

However, a landmark US study published in 2012 that monitored 5,115 participants over 20 years from the age of 18-30 also found that cannabis did not affect the lungs as badly as tobacco smoke.

Researchers from several institutions across the US found that moderate use of cannabis (two or three times a month) actually led to improved lung function, while heavy smokers of marijuana (over 20 times a month) had the same lung function of people who did not use the drug at all.