Tobacco smoking is the practice where tobacco is burned and the vapors either tasted or inhaled. The practice began as early as 5000–3000 BCE. Many civilizations burnt incense during religious rituals, which was later adopted for pleasure or as a social tool. Tobacco was introduced to the Old World in the late 16th century where it followed common trade routes. The substance was met with frequent criticism, but became popular nonetheless.
German scientists formally identified the link between smoking and lung cancer in the late 1920s leading the first anti-smoking campaign in modern history. The movement failed to reach across enemy lines during the Second World War, and quickly became unpopular thereafter.
In 1950, health authorities again began to suggest a relationship between smoking and cancer. Scientific evidence mounted in the 1980s, which prompted political action against the practice. Rates of consumption from 1965 onward in the developed world have either peaked or declined. However, they continue to climb in the developing world.
Smoking is the most common method of consuming tobacco, and tobacco is the most common substance smoked. The agricultural product is often mixed with other additives and then pyrolyzed. The resulting vapors are then inhaled and the active substances absorbed through the alveoli in the lungs. The active substances trigger chemical reactions in nerve endings which heightens heart rate, memory, alertness, and reaction time. Dopamine and later endorphins are released, which are often associated with pleasure. As of 2000, smoking is practiced by some 1.22 billion people. Men are more likely to smoke than women, though the gender gap declines with younger age. The poor are more likely to smoke than the wealthy, and people of developing countries than those of developed countries.
Many smokers begin during adolescence or early adulthood. Usually during the early stages, smoking provides pleasurable sensations, serving as a source of positive reinforcement. After an individual has smoked for many years, the avoidance of withdrawal symptoms and negative reinforcement become the key motivations to continue.