By Mary Kekatos

People who cut down on cigarettes thinking it will improve their lung health are actually putting themselves at greater risk of disease, new research shows.

Heavy smokers who quit had lesser odds of suffering from lung disease than light smokers, a study from Northwestern University in Illinois found.

Those who continue to smoke 10 cigarettes per day were more likely to develop emphysema than those who used to smoke 20 cigarettes per day or more and have since stopped entirely.

Light smokers also suffer a larger decrease in lung function than those who have quit, regardless of how long they smoked for previously.

Experts long-believed that smokers used intermittent smoking as a bridge to quitting.

However, one-quarter of all smokers are considered light smokers who have no intention of ceasing their habit.

Researchers analyzed 30 years of data from 3,140 participants in the Coronary Artery Risk Development in Young Adults (CARDIA) study.

The participants were 25 years old on average, were living in four US cities and nearly half had smoked at some point in their lives.

They underwent periodic spirometry - which measures how much air you inhale, how much air you exhale and how quickly you exhale - to assess lung function and a chest CT scans 15, 20 and 25 years after enrolling.

Compared to those who never smoked, stable heavy smokers experienced the greatest decline in lung function as measured by FEV1, or the maximum amount of air you can forcefully blow out of your lungs in one second.

Additionally, stable heavy smokers were 26 times more likely to develop emphysema and nearly eight times more likely to develop obstructive lung disease than nonsmokers.

The researchers then looked at the health risk of light smokers, which is defined as having less than a 10 pack-years smoking history, versus quitters.

Pack-years was estimated by multiplying the number of cigarette packs smoked per day by the number of years that person smoked.

Smokers who quit had a 9.8 pack-year smoking history and those who were light smokers had 6.4 pack-years smoking history.

The team found that, despite having smoked more cigarettes, those who had quit preserved more lung function with less FEV1 decline than stable low rate smokers of -33.8 mL/year against -35.7 mL/year.

Quitters also had lower emphysema odds of 2.8 compared to 7.7.

'We were surprised to find that those who quit had lower disease risk than the group we identified as stable, low-rate smokers, even though those who quit had a greater lifetime exposure to cigarettes,' said Dr Amanda Matthew, a research assistant professor at Northwestern University Feinberg School of Medicine.

'There is no safe threshold of smoking on lung health. Cutting down can be a great first step, but quitting for good is the most effective way to reduce lung disease risk.'

The new research comes on the heels of past studies showing that low-intensity smoking still translates to a higher risk of early death compared to nonsmokers.

A study published in January 2017 found that smokers of one cigarette per day were nine times likelier to die from lung cancer and those who smoked 10 per day was 12 times likelier to die of the disease.

Intermittent smoking is not only popular among heavy smokers trying to quit, but also among younger people because it is seen as less harmful.

Although more than 15 out of every 100 US adults age 18 or older currently smoke cigarettes, with six million people being killed each year, the rates have been on the decline.

According to the CDC, smoking has dwindled from 20.9 percent in 2005 to 15.5 in 2016.