Standard IARC degrees of evidence of carcinogenicity
in humans
in experimental animals
which are the basis for the Standard IARC
Classification
Carcinogenicity in humans
Sufficient evidence of carcinogenicity:
The Working Group considers that a causal relationship has been established
between exposure to the agent, mixture or exposure circumstance and human
cancer. That is, a positive relationship has been observed between the exposure
and cancer in studies in which chance, bias and confounding could be ruled out
with reasonable confidence.
Limited evidence of carcinogenicity:
A positive association has been observed between exposure to the agent,
mixture or exposure circumstance and cancer for which a causal interpretation is
considered by the Working Group to be credible, but chance, bias or confounding
could not be ruled out with reasonable confidence.
Inadequate evidence of carcinogenicity:
The available studies are of insufficient quality, consistency or statistical
power to permit a conclusion regarding the presence or absence of a causal
association between exposure and cancer, or no data on cancer in humans are
available.
Evidence suggesting lack of carcinogenicity:
There are several adequate studies covering the full range of levels of
exposure that human beings are known to encounter, which are mutually consistent
in not showing a positive association between exposure to the agent, mixture or
exposure circumstance and any studied cancer at any observed level of exposure.
A conclusion of 'evidence suggesting lack of carcinogenicity' is inevitably
limited to the cancer sites, conditions and levels of exposure and length of
observation covered by the available studies. In addition, the possibility of a
very small risk at the levels of exposure studied can never be excluded.
In some instances, the above categories may be used to classify the degree of
evidence related to carcinogenicity in specific organs or tissues.
Carcinogenicity in experimental animals
Sufficient evidence of carcinogenicity:
The Working Group considers that a causal relationship has been established
between the agent or mixture and an increased incidence of malignant neoplasms
or of an appropriate combination of benign and malignant neoplasms in (a) two or
more species of animals or (b) in two or more independent studies in one species
carried out at different times or in different laboratories or under different
protocols.
Exceptionally, a single study in one species might be considered to provide
sufficient evidence of carcinogenicity when malignant neoplasms occur to an
unusual degree with regard to incidence, site, type of tumour or age at onset.
Limited evidence of carcinogenicity:
The data suggest a carcinogenic effect but are limited for making a definitive
evaluation because, e.g. (a) the evidence of carcinogenicity is restricted to a
single experiment; or (b) there are unresolved questions regarding the adequacy
of the design, conduct or interpretation of the study; or (c) the agent or
mixture increases the incidence only of benign neoplasms or lesions of uncertain
neoplastic potential, or of certain neoplasms which may occur spontaneously in
high incidences in certain strains.
Inadequate evidence of carcinogenicity:
The studies cannot be interpreted as showing either the presence or absence of
a carcinogenic effect because of major qualitative or quantitative limitations,
or no data on cancer in experimental animals are available.
Evidence suggesting lack of carcinogenicity:
Adequate studies involving at least two species are available which show that,
within the limits of the tests used, the agent or mixture is not carcinogenic. A
conclusion of evidence suggesting lack of carcinogenicity is inevitably limited
to the species, tumour sites and levels of exposure studied.