A carcinogen is a substance with the potential to cause cancer in living tissues. Tobacco products and their smoke contain thousands of chemicals, with at least 70 identified as known carcinogens. These substances originate from the tobacco plant itself, the curing process, and the act of combustion.
Key Carcinogenic Compounds in Tobacco
A group of carcinogens are the tobacco-specific nitrosamines (TSNAs), which form during the curing and processing of tobacco leaves. Present in both unburned tobacco and smoke, key examples are NNN and NNK. The concentration of these compounds varies depending on the specific curing processes used.
Polycyclic aromatic hydrocarbons (PAHs) are another class of carcinogens formed during the incomplete combustion of tobacco. Benzo[a]pyrene is a well-known PAH in tobacco smoke with DNA-damaging capabilities. The amount of PAHs produced is directly related to burning, making them a primary hazard of smoked tobacco.
Combustion also produces aromatic amines and aldehydes. Aromatic amines like 2-naphthylamine and 4-aminobiphenyl are recognized human carcinogens. Aldehydes, including formaldehyde and acetaldehyde, are present in high concentrations in tobacco smoke and damage cellular genetic material.
Tobacco plants absorb toxic metals and radioactive elements from the soil and fertilizers used during cultivation. Metals such as arsenic, cadmium, and lead are present in the leaves and transferred into the smoke. The plants also absorb radioactive elements like polonium-210, which are inhaled and deliver low levels of radiation to lung tissues.
The Biological Impact of Tobacco Carcinogens
Many carcinogens in tobacco smoke are initially inert and must be activated to become harmful. This process, known as metabolic activation, occurs when enzymes in the body attempt to break down the chemicals for excretion. Instead, this process can convert them into highly reactive molecules.
Once activated, these reactive molecules can bind directly to a cell’s DNA, forming what are known as DNA adducts. These adducts are chemical modifications to the DNA structure that alter its normal state. Their formation is a central event in the initiation of cancer because they create physical obstructions on the DNA strand.
When a cell attempts to replicate its DNA, the cellular machinery can misread the sequence at the site of an adduct. This misreading results in a permanent change, or mutation, in the genetic code. These errors can accumulate in new cells with continued exposure to tobacco.
Mutations are particularly damaging when they affect genes that control cell growth. They can damage tumor suppressor genes, like the p53 gene, which normally prevent cells from becoming cancerous. Mutations can also activate oncogenes, which promote uncontrolled cell growth and lead to the development of a tumor.
Specific Cancers Caused by Tobacco Exposure
Carcinogens in tobacco smoke cause cancer in tissues they come into direct contact with. Lung cancer is the most widely recognized cancer caused by smoking, as lung tissues are constantly exposed to inhaled smoke. Other cancers strongly linked to this direct exposure include:
- Oral cavity (mouth, lips, tongue)
- Larynx (voice box)
- Pharynx (throat)
- Esophagus
The danger is not confined to areas of direct contact. Carcinogens are absorbed from the lungs into the bloodstream and circulated throughout the entire body, exposing nearly every organ. Organs that filter waste from the blood, such as the bladder and kidneys, are particularly vulnerable as they process and concentrate these toxins before removal.
This circulation explains why tobacco use is linked to a broad range of cancers in distant organs. Cancers causally linked to smoking include:
- Pancreatic cancer
- Stomach cancer
- Liver cancer
- Acute myeloid leukemia (a cancer of the blood and bone marrow)
- Colorectal cancer
- Cervical cancer
Carcinogen Profile in Various Tobacco Forms
Different tobacco products expose users to varying profiles of carcinogens. In conventional cigarettes, combustion generates many chemicals. Additives like sugars, intended to make the smoke less harsh, can also form cancer-causing chemicals when burned.
Smokeless tobacco products, like chewing tobacco and snuff, do not involve burning but deliver high concentrations of TSNAs. This direct, prolonged contact with tissues of the mouth and throat makes smokeless tobacco a primary cause of oral and esophageal cancers.
Cigars and pipe tobacco also rely on combustion. Due to the larger amount of tobacco in a single cigar and the less porous wrapper, cigar smoke can have higher concentrations of TSNAs and other toxic compounds compared to cigarette smoke.
Using a hookah, or water pipe, does not eliminate the dangers of tobacco smoke. It is a misconception that the water filters out harmful substances. Hookah smoke contains high levels of carcinogens from both the tobacco and the charcoal used as a heat source.