'Environment' in cancer causation and etiological fraction: limitations and ambiguities

The definition of environment refers to external physical conditions that may affect human health (1), but the word is used in the medical literature with different connotations, both in English and in other languages. This generates confusion in the medical literature, when the role of environment is discussed in the context of disease etiology. In particular, research on the role of environment in human carcinogenesis suffers from this ambiguity. On the one hand, environment can encompass all non-genetic factors such as diet, lifestyle and infectious agents. In this broad sense, the environment is implicated in the causation of the majority of human cancers, as has been demonstrated since the 1960s (2). Such a broad meaning of the word environment is assumed when referring to gene–environment interactions . On the other hand, environmental factors can include only the (natural or man-made) agents and circumstances encountered by humans in their daily life, upon which they have no or limited personal control. In this sense, environmental factors are restricted to air, water, soil and food pollutants, including physical pollutants such as sources of ionizing radiation. These ambiguities in the terminology and the inconsistencies in the use of the vocabulary by cancer researchers contribute to public confusion regarding the role of the environmental causes of cancer. A distinction relevant to cancer prevention may be made between factors related to personal behaviors, lifestyle (e.g. tobacco smoking and alcohol drinking), involuntary exposures, such as those linked to air, water, soil or food pollutants, and occupation. It would be preferable to abandon the term environment and to use terms such as nongenetic or modifiable determinants of disease (broad sense of environment) and pollutants (narrow sense).