Lexikon

carcinogen is a substance or a mixture of substances which induces cancer or increases its incidence (REACH).

Tghere are other definitions, such as the UNECE (2004) "The term carcinogen denotes a chemical substance or a mixture of chemical substances which induce cancer or increase its incidence".

An third alternate definition is that carcinogenic substances are ones that "induce tumors (benign or malignant), increase their incidence or malignancy, or shorten the time of tumor occurrence when they are inhaled, injected, dermally applied, or ingested".

Carcinogens are classified according to their mode of action as genotoxic or nongenotoxic carcinogens. Genotoxic carcinogens initiate carcinogenesis by direct interaction with DNA, resulting in DNA damage or chromosomal aberrations that can be detected by genotoxicity tests (OECD, 2006).

Nongenotoxic carcinogens are agents that, at least initially, directly internact with DNA. These indirect modifications to DNA structure, amount, or function may result in altered gene expression or signal transduction (OECD, 2006).

In animal studies, most potent mutagens are also found to be carcinogenic (Maurici, et al., 2005). Substances that induce tumors in animals are considered as presumed or suspected human carcinogens until convincing evidence to the contrary is presented (UNECE, 2004).

Sources:
REACH Glossary
http://alttox.org/ttrc/toxicity-tests/carcinogenicity/
UNECE, 2004
Maurici D, Aardema M, Corvi R, et al. (2005), “Carcinogenicity”. Alt Lab Anim Vol. 33 (Suppl 1): 177−182

carcinogenic effect has a substance or a mixture of substances which induces cancer or increases its incidence and/or malignancy or shorten the time to tumour occurrence. Causing cancer may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Carcinogenic chemicals have conventionally been divided into two categories according to the presumed mode of action. Non-genotoxic modes of action include epigenetic changes, i.e., effects that do not involve alterations in DNA but that may influence gene expression, altered cell-cell communication, or other factors involved in the carcinogenic process.

Cancer is a disorder of the cells, characterized by the lack of programmed cell death. Carcinogens induces the uncontrolled, malignant division pf cells, ultimately leading to the formation of tumors. Usually DNA damage leads to programmed cell death, but if the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell. The objective of investigating the carcinogenicity of chemicals is to identify potential human carcinogens, their modes of action, and their potency. Once a chemical has been identified as a carcinogen, there is a need to elucidate the underlying mode of action, i.e. whether the chemical is directly genotoxic or not. For genotoxic carcinogens it is assumed that, unless exception, there is no discernible threshold and that any level of exposure carries a risk. For non-genotoxic carcinogens, no-effect-thresholds are assumed to exist and to be discernable. Human studies are generally not available for making a distinction between the above mentioned modes of action; and a conclusion on this, in fact, depends on the outcome of mutagenicity testing and other mechanistic studies. In addition to this, animal studies may also inform on the underlying mode of carcinogenic action.

The cancer hazard and mode of action may also be highly dependent on exposure conditions such as the route of exposure. Therefore, all relevant effect data and information on human exposure conditions are evaluated.

mutagenicity refers to the induction of permanent transmissible changes in the amount or structure of the genetic material of cells or organisms. These changes may involve a single gene or gene segment, a block of genes or chromosomes.
Alterations to the genetic material of cells may occur spontaneously or be induced as a result of exposure to ionising or ultraviolet radiation, or genotoxic substances. In principle, human exposure to substances that are mutagens may result in increased frequencies of mutations above baseline. Heritable damage to the offspring, and possibly to subsequent generations, of parents exposed to substances that are mutagens may follow if mutations are induced in parental germ cells (reproduction cells). Mutations in somatic cells (cells others than reproduction cells) may be lethal or may be transferred to daughter cells with deleterious consequences for the affected organism. There is considerable evidence of a positive correlation between the mutagenicity of substances in vivo and their carcinogenicity in long-term studies with animals. The aims of testing for mutagenicity are to assess the potential of substances to induce effects which may cause heritable damage in humans or lead to cancer.

Chemicals are defined as carcinogenic if they induce tumours, increase tumour incidence and/or malignancy or shorten the time to tumour occurrence. Carcinogenic chemicals have conventionally been divided into two categories according to the presumed mode of action. Non-genotoxic modes of action include epigenetic changes, i.e., effects that do not involve alterations in DNA but that may influence gene expression, altered cell-cell communication, or other factors involved in the carcinogenic process. The objective of investigating the carcinogenicity of chemicals is to identify potential human carcinogens, their mode(s) of action, and their potency.
Once a chemical has been identified as a carcinogen, there is a need to elucidate the underlying mode of action, i.e. whether the chemical is directly genotoxic or not. For genotoxic carcinogens it is assumed that, unless exception, there is no discernible threshold and that any level of exposure carries a risk. For non-genotoxic carcinogens, no-effect-thresholds are assumed to exist and to be discernable. Human studies are generally not available for making a distinction between the above mentioned modes of action; and a conclusion on this, in fact, depends on the outcome of mutagenicity testing and other mechanistic studies. In addition to this, animal studies may also inform on the underlying mode of carcinogenic action.
The cancer hazard and mode of action may also be highly dependent on exposure conditions such as the route of exposure. Therefore, all relevant effect data and information on human exposure conditions are evaluated.

Source: REACH