May 22: Light cigarettes, long marketed by the tobacco industry as a "healthier" option than regular cigarettes, offer no health benefits and may instead explain a puzzling rise in a type of cancer deep in the lungs, researchers said today.

A study by US researchers has suggested that the so-called mild or light cigarettes with relatively low tar levels may have caused more harm, contributing to the rise in adenocarcinoma, now the commonest type of lung cancer.

Cigarette manufacturers have over the past six decades introduced low-tar cigarettes through features such as cellulose acetate filters that trap tar, porous cigarette paper that allows toxic chemicals to escape, and ventilation holes in filter tips that dilute smoke with air.

The ventilation holes are critical to the claims of low tar. When analysed by a machine, smoke from a light cigarette has lower tar than smoke from a regular cigarette. But, a US National Cancer Institute document says, a machine cannot predict how much tar a smoker inhales.

The institute has pointed out that the tar exposure from a light cigarette could be just as high as that from a regular cigarette if a smoker took long, deep or more frequent puffs.

As for the holes, they were introduced "to fool smokers and the public health community into thinking that they were actually safer", Peter Shields, deputy director of the Ohio State University Comprehensive Cancer Centre and lung medical oncologist, said in a media release.

The study by Shields and his colleagues was published today in the Journal of the National Cancer Institute. It has revealed what Shields says is a "clear relationship between the addition of ventilation holes and (the) increasing rates of lung adenocarcinoma seen over the past 20 years".

Shields and his collaborators analysed multiple studies on toxicology, smoking behaviour and cancer risk and hypothesised that the higher incidence of adenocarcinoma may be linked to the ventilation holes that cause smokers to inhale smoke that has higher levels of carcinogens and other toxins.

"The filter ventilation holes change how tobacco is burned, producing more carcinogens, which (the heavier load of carcinogens) then also allows the smoke to reach the deeper parts of the lung where adenocarcinoma frequently occurs," Shields said.

The findings, the researchers say, may explain why the incidence of lung adenocarcinoma has increased over the past 50 years while other types of lung cancer have been decreasing, in tandem with fewer people smoking. Adenocarcinoma is now the most common type of lung cancer.

Last year, a team of doctors at the Tata Memorial Hospital, Mumbai, had in a review of lung cancer in India pointed out that the "global trend of a rise in adenocarcinoma appears to be paralleled in India".

Doctors who specialise in addiction treatment services in India say the domestic market for low-tar and light or mild cigarettes is driven by assumptions that they are safer than regular cigarettes.

Current smokers "falsely assume that these products reduce the risk of tobacco use", Sonali Jhanjee, an associate professor at the All India Institute of Medical Sciences, New Delhi, who was not associated with the US study, had said last year in the Indian Journal of Medical Research.

The US National Cancer Institute had earlier cited studies suggesting that people who switched from regular cigarettes to light cigarettes were likely to have inhaled the same amount of toxic chemicals and remained at high risk of developing smoking-related cancers.