Smoking is linked to at least 15 types of cancer, while e-cigarettes are no safe alternative, a Chiang Mai cancer specialist warned on World No Tobacco Day on Sunday.
Dr Thanika Ketpueak of Chiang Mai University's Faculty of Medicine said many people still mistakenly associate smoking primarily with lung cancer.
In addition to lung cancer, smoking increases the risk of cancers of the mouth, throat, larynx, oesophagus, stomach, liver, pancreas, kidney, bladder and cervix, as well as acute leukaemia, she said.
Smokers are 15 to 30 times more likely to develop lung cancer than non-smokers.
"There is no safe level of smoking. Even one cigarette a day increases cancer risk," Dr Thanika said.
Tobacco smoke contains more than 7,000 chemicals, including over 70 known carcinogens, and is classified by the World Health Organisation as a Group 1 carcinogen, the highest category indicating it causes cancer in humans.
Dr Thanika also warned of second-hand smoke exposure, saying toxic substances such as benzene, formaldehyde and nitrosamines can harm non-smokers, particularly children and pregnant women.
Smoking-related diseases often develop silently over many years, meaning patients may appear healthy until cancer reaches an advanced stage, she said.
Common warning signs include a persistent cough, coughing up blood, hoarseness, unexplained weight loss, fatigue and chest pain, although these symptoms often emerge late.
Attention is also growing around e-cigarettes, which are often perceived as a safer option. Dr Thanika said this perception is misleading.
"Even if some e-cigarettes contain fewer carcinogens than conventional cigarettes, they still expose users to toxic chemicals, heavy metals and nicotine. Heating and vapour processes can trigger long-term lung inflammation and cellular damage," she said.
She said carcinogens damage DNA and disrupt normal cell repair mechanisms, leading to chronic inflammation and abnormal cell growth that can eventually develop into cancer. These substances can also circulate through the bloodstream and affect multiple organs, including the bladder.
Doctors assess smoking-related risk using the "pack-year" system, calculated by multiplying the number of cigarette packs smoked per day by the number of years smoked. A score above 20 places an individual in a high-risk category for lung cancer.
Dr Thanika urged smokers to quit as early as possible, noting that health improvements begin soon after smoking cessation, with inflammation decreasing and cancer risk gradually declining over time.
