Smoking and cancer, more than just the lungs

Author: Xiao Dan, Director of the Department of Tobacco Control and Respiratory Disease Prevention and Control, China-Japan Friendship Hospital, Researcher

Reviewer: Tang Qin, Deputy Secretary-General of the Science Popularization Expert Committee of the Chinese Medical Association, Researcher

When you know that the intersection between smoking and cancer is not limited to lung cancer, you will probably first think of oral cancer, which is most directly related to smoking behavior.

What may surprise you is that smoking also increases the risk of many seemingly unrelated cancers, such as breast cancer and bladder cancer.

What are the types? How harmful are they? Let’s learn more about them in this article.

First of all, we still need to affirm the close relationship between smoking and lung cancer in common sense. The "China Smoking Health Harm Report 2020" clearly points out that smoking is the primary risk factor for lung cancer by summarizing research literature on the health hazards of smoking at home and abroad.

According to the latest global cancer burden data for 2020 released by the World Health Organization's International Agency for Research on Cancer (IARC), while breast cancer incidence has surpassed lung cancer to rank first in the world, lung cancer is still the cancer with the highest incidence in my country. At the same time, it is also the cancer with the highest mortality rate in China and the world.

Figure 1 Number of cancer cases in China in 2020

(Picture from IARC official website)

Figure 2 Number of cancer deaths in China in 2020

(Picture from IARC official website)

Oral and oropharyngeal malignancies

Human health is harmed by tobacco smoke, and the first areas to be affected are the oral cavity and oropharynx, which are the first areas to be touched by smoke when it enters the human body. Due to the burning of cigarettes and the stimulation of harmful substances, smokers will have varying degrees of inflammatory hyperplasia on their oral mucosa. In severe cases, the mucosa will proliferate and turn white, which is called leukoplakia. Leukoplakia is often regarded as a precancerous condition, which can develop into squamous cell carcinoma over time.

Smoking is causally related to oral and oropharyngeal malignancies, and any type of tobacco can cause oral and oropharyngeal cancers. The names of cigarettes, cigars, smokeless tobacco, etc. are just fancy tricks and do not represent qualitative differences.

Gastric cancer

In 2020, the incidence and mortality rates of gastric cancer in my country ranked third among cancer incidence and mortality rates.

There are many risk factors for gastric cancer. In addition to eating foods containing aflatoxin, pickled foods, smoked foods, etc., smoking is also one of them.

Smoking reduces the body's immunity. Smoking also stimulates the nervous system, increases gastric motility, promotes gastric acid secretion, damages the gastric mucosal barrier function, and leads to gastritis and even gastric cancer. For patients with gastritis, smoking slows down the healing of ulcers and may turn gastritis into a chronic disease. In addition, nitrogen oxides in tobacco can also synthesize nitrites in the human body that have a significant promoting effect on many cancers.

The results of a study on the Chinese population showed that smokers have a 1.66 times higher risk of developing stomach cancer than non-smokers, and male smokers have a nearly 2 times higher risk of developing stomach cancer than non-smokers. By the way, the much-criticized image of the male protagonist in online novels, "Nine out of ten CEOs have stomach problems, and eight smell like tobacco," may have some internal connection - after all, people are "smelled" of tobacco, so it is inevitable that their stomachs will be damaged.

Figure 3 Comparison of gastric cancer risk between smokers and non-smokers

(Drawing by the author)

Liver cancer

In 2020, the incidence and mortality rates of liver cancer in my country ranked 5th and 2nd among cancer incidence and mortality rates, respectively.

The liver is an important organ for the metabolism and transformation of chemical substances in the human body. In the human body, the battle between the liver and toxic substances is a positional war and a protracted war.

The smoke produced by burning tobacco contains more than 7,000 chemicals, including at least 69 carcinogens. Once these substances are inhaled into the human body, they need to be transformed by the liver. As time goes by, the liver will naturally be damaged by toxic substances.

After collecting a large amount of research evidence, the "China Report on the Health Harms of Smoking" and the "U.S. Surgeon General's Report" concluded that smoking can cause liver cancer.

Breast cancer

Data released by the World Health Organization's International Agency for Research on Cancer show that breast cancer deaths account for 15.5% of all cancer deaths among women. In my country, the incidence of breast cancer has ranked first among female cancers, becoming the disease that poses the greatest threat to women's health.

A study on breast cancer risk factors in Asian women showed that smoking can increase the risk of breast cancer in Asian women by 50%. The results of multiple studies have shown that smoking is one of the risk factors for breast cancer in premenopausal women. Other studies have shown that smoking before first pregnancy may be one of the risk factors for breast cancer in women.

Bladder cancer

The harmful substances inhaled by people circulate in the blood and harm the whole body, participating in the metabolism of the whole body, and finally being filtered by the kidneys and accumulated in the bladder with urine. Tobacco contains aromatic amines and acrolein, and the thermal decomposition products of these compounds are strong carcinogens of bladder cancer. Before the urge to urinate occurs, the accumulated toxic urine contacts the inner wall mucosa in the bladder for a long time, thus increasing the probability of bladder cancer.

Some scholars have calculated that if a smoker smokes five more cigarettes a day, the risk of bladder cancer will increase by 18%; for every five years of smoking, the risk of bladder cancer will increase by 14%. For those who smoke more than two packs a day and inhale deeply, the risk of bladder cancer is seven times higher than that of non-smokers.

Figure 4 Smoking and risk of bladder cancer

(Drawing by the author)

The World Health Organization's International Agency for Research on Cancer believes that if the whole society quits smoking, the risk of bladder cancer in men can be reduced by 50% and the risk of bladder cancer in women can be reduced by 33%.

Conclusion

For smokers, in addition to the above-mentioned cancers, the risk of developing other cancers, such as laryngeal cancer, cervical cancer, pancreatic cancer, esophageal cancer, and nasopharyngeal cancer, is higher than that of non-smokers. Generally speaking, the more smokers smoke and the longer they smoke, the higher their risk of developing cancer.

Some of the harms of smoking to health can be reduced by quitting smoking. For those who successfully quit smoking, the risk of developing some cancers will gradually decrease as the time of quitting smoking increases.

In general, it still comes down to one sentence: "It is better to smoke less than more, and it is better not to smoke than to smoke."