Carcinogenicity and Toxicity: Substances that are capable of forming cancerous cells in living tissues are known as carcinogens. This ability of a cancer-causing agent to cause cancer disease is known as carcinogenicity. These substances may include chemicals, radiations, etc. Cancer is a deadly disease that is caused due to uncontrolled cell division inside the body that leads to the formation of tumour cells or changes in DNA sequence or genetic code, also called ‘mutation.’

We often go through the slogans written on tobacco containing substances like cigarettes like “Smoking Kills”! This means that tobacco is a carcinogenic substance that may cause a deadly disease, i.e., cancer. Like tobacco which is chemically known as TSNAs (Tobacco Specific Nitrosamines), there are numerous other carcinogenic chemicals such as benzene, carbon monoxide, Poly Aromatic Hydrocarbons (PAHs), and many other aromatic organic compounds. Though we come across various carcinogenic substances in our daily life, they cause cancer only when they are ingested, inhaled, applied to the skin to a certain uncontrolled level. In short, we can say that frequent exposure to any carcinogenic substance to a certain level may be poisonous or toxic. This property of a substance being toxic is known as toxicity.

What are Carcinogens?

Any substance, a radionuclide, or radiation that is an agent directly involved in causing cancer is known as a carcinogen. These carcinogens have the ability to damage the genome, disturbs cellular metabolic activities, and cause uncontrolled cell growth and cell division that leads to the formation of a tumour. Most radioactive substances are carcinogenic due to radiations that they emit, such as gamma rays and alpha particles. The process of tumour formation is shown below:

Examples of Carcinogens

People can be exposed to carcinogens in their workplace, in the environment, through certain food products, by using certain cosmetic products and through various lifestyle factors too. Some examples of carcinogens with the type of cancer caused by them are listed below:

Carcinogen / Industries producing carcinogens	Type of Cancer caused
Herbicides, benzene, x-ray, ethylene oxide	Hematopoietic and lymphatic (Multiple organs involved in the development of immune system)
Leather industry, Rubber industry, benzidine, aluminium production, \(4\)-aminobiphenyl	Bladder
UV-rays from Sunlight, Coal tar, mineral oil, Arsenic	Skin
Formaldehyde, Mustard gas	Pharynx (Throat)
Isopropyl alcohol, Asbestos	Larynx (Voicebox)
Radon, Silica, tar, soot, nickel refining, chromium compounds, foundry substances, Asbestos, coke oven fumes, cadmium	Lungs
Chlorophenols, Chlorophenoxy	Soft-tissue sarcoma (Bones or soft and fibrous tissues like cartilage, fibrous tissues, etc.)

What is Toxicity?

The degree or extent to which a chemical can cause damage to the living body is defined as toxicity. A substance may be toxic to the whole organism like humans, plants, and animals or may be restricted to a specific body area. Though we know all toxic substances are not carcinogens, many of them are. The term ‘toxic’ is primarily doses dependent. Even vitamins, minerals, water, etc., can also be toxic if taken in too much quantity. The basic difference between a drug and medicine is related to ‘dose’ only. If a medicine is taken beyond the prescribed dose, it turns out to be a toxic drug.

Different Types of Toxicity

Toxicity is caused due to various types of toxic agents such as chemical, biological, physical, and radiation. Based on the extent of toxicity caused by any toxic substance, it can be classified as:

1. Acute Toxicity – It is the minor toxicity of a substance that shows its effects within \(24\) to \(72\) hours of exposure. Mostly the effect occurs within \(14\) days and remains for a short span of time, such as in the case of alcohol drunkenness.
2. Chronic Toxicity – It is the development of toxic effects due to long-term exposure causing adverse effects on health such as decreased growth, behavioural changes, reduced reproductive growth, etc. Chronic toxicity effects manifest for several weeks, months, or years on repeated exposure.
3. Local Toxicity – Local toxicity occurs on the action of toxic substances on a specific area of contact—for example, skin, eyes, throat, mucous membrane, etc.
4. Systemic Toxicity – This type of toxicity effect is spread or distributed throughout the body. For example, the Brain is affected by the inhalation of chloroform.

Carcinogenicity and Toxicity of different Chemical Compounds

Chemicals cause beneficial and other adverse effects in animals and in humans. Besides these effects, they show toxic and carcinogenic effects too. Many chemical compounds such as TSNAs (Tobacco Specific Nitrosamines), Poly Aromatic Hydrocarbons (PAHs), carbon monoxide, benzene, etc., are carcinogenic. Let’s discuss few carcinogenic chemical compounds in detail:

1. Carcinogenicity and Toxicity of Benzene

The Department of Health and Human Services (DHHS) has determined that benzene is carcinogenic to humans. The major cause of benzene toxicity is the radiomimetic nature of benzene and its ability to induce different sites of tumours due to the formation of oxygen radicals. Long-term exposure to high levels of benzene in the atmosphere can cause leukaemia.

2. Carcinogenicity and Toxicity of Aromatic Hydrocarbons

i. Some Polycyclic Aromatic Hydrocarbons (PAHs) that have two or more fused benzene rings are toxic and possess carcinogenic properties. Such polynuclear or polyaromatic hydrocarbons are formed by incomplete combustion of organic substances such as coal, petroleum, tobacco, etc. When these polynuclear hydrocarbons enter the human body, they undergo various biochemical reactions and finally damage DNA and ultimately lead to cancer. PAHs are responsible for introducing cancer mostly in the lungs, bladder, and skin. Some examples are:

ii. Cigarette smoke contains potent carcinogens such as benzo[\({\rm{a}}\)]pyrene, nitrosamine, and benzene.

iii. Methylcholanthrene – It is a highly carcinogenic polycyclic aromatic hydrocarbon produced by the combustion of organic compounds at very high temperatures. Initially, it may cause irritation in the eyes, respiratory tract, and skin.

iv. Some other examples of carcinogenic polycyclic aromatic hydrocarbons include \(1,2\)-benzpyrene, and \(9,10\)-dimethyl\(-1,2\)-benzanthracene, etc. There is no general rule by which we can predict the carcinogenic activity of any hydrocarbons or their derivatives. However, the number and position of some groups, such as \( – {\rm{C}}{{\rm{H}}_3}, – {\rm{OH}}, – {\rm{OC}}{{\rm{H}}_3} – {\rm{CN}},\) etc., have been noticed to influence the carcinogenic activity of various polynuclear hydrocarbons.

3. Carcinogenicity and Toxicity of Hydrocarbons

Incomplete combustion of some hydrocarbons like tobacco, petroleum products, coal, etc., form polynuclear hydrocarbons. These hydrocarbons enter the human body and undergo biochemical reactions that damage the DNA and cause cancer. Examples of some carcinogenic hydrocarbons are coal tar, soot, shale oils, etc.

4. Reproductive toxicity is caused due to certain chemical substances that adversely affect sexual function and fertility in adult males and females. For example, exposure to certain radiations may cause damage to the developing fetus and can damage DNA also, causing cancer.

Summary

Cancer-causing agents are known as carcinogens. This ability to cause cancer is known as carcinogenicity. Carcinogens include chemicals like TSNAs (Tobacco Specific Nitrosamines), benzene, carbon monoxide, Polycyclic Aromatic Hydrocarbons (PAHs), gamma radiations, etc. Uncontrolled cell division or changes in DNA sequence or genetic code also called ‘mutation’, leads to the formation of ‘tumour cells. Carcinogens present in the atmosphere cause cancer, but many people are more vulnerable to this disease due to their job also.

People working in chemical laboratories, wood industry, leather industry, and tile factories are exposed to carcinogens like Asbestos and other PAHs. Ultraviolet radiations and other harmful radiations are also major causes of cancer. Apart from various factors, our lifestyle is also responsible for causing cancer to some extent, like smoking, alcohol consumption, obesity, lack of physical activities, and poor diet. Toxicity is the extent to which a chemical may cause harm to the human body. Though we cannot protect ourselves completely from carcinogenicity and toxicity of chemicals, we can take steps to protect ourselves, such as wearing gloves in vulnerable workplaces, reading labels, and not smoking.

FAQs on Carcinogenicity and Toxicity

Q.1. What are carcinogenicity and toxicity?
Ans: The ability of a cancer-causing agent (carcinogen) to cause cancer is known as carcinogenicity. These substances may include chemicals, radiations, etc. The degree or extent to which a chemical can cause damage to the living body is defined as toxicity.

Q.2. Is toxic the same as carcinogenic?
Ans: No toxic is different from carcinogenic. Cancer-causing substances are named carcinogenic substances, but toxicity is the ability to cause damage to the living body. Any medicine, drug, or even things that we consume and use daily if taken in overdose can be toxic, but all toxic substances are not carcinogens.

Q.3. What is carcinogenicity? Give an example.
Ans: The ability of a cancer-causing agent (carcinogen) to cause cancer is known as carcinogenicity. For example, tobacco is present in cigarettes, which is chemically known as TSNAs (Tobacco Specific Nitrosamines).

Q.4. What are the four types of carcinogens?
Ans: Four types of carcinogens are (i) Chemical carcinogens like hydrocarbons present in herbicides, drugs, (ii) Physical carcinogens – Radionuclides, X-rays, (iii) Oncogenes – Cancer-causing viruses, (iv) Metals like Nickel, Cadmium, etc.

Q.5. What are ten examples of carcinogens?
Ans: Ten examples of carcinogens are Formaldehyde, tobacco, Asbestos, methylcholanthrene, carbon tetrachloride, radionuclides, X-rays, oncogenic viruses, TSNAs (Tobacco Specific Nitrosamines), benzo[a]pyrene.

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