Some agents within this scope of environmental, lifestyle, occupational, and radiation-related exposures have already been identified as major causes of cancer, for instance smoking and ultraviolet radiation, but further characterization of their associated risks is still needed. Ionizing radiation, for example, is a well-established carcinogen, although considerable uncertainties and divergent views remain about cancer risks at low doses.
When it comes to environmental pollutants and occupational exposures, the attributable fraction of the totality of the cancer burden is difficult to estimate but is expectected to be smaller than that from some behavioural factors or certain infections. Nevertheless, environmental or occupational risk factors are often modifiable by reducing or eliminating exposure, for instance through safety standards or technology, or the banning of chemicals. If the agents have been scientifically established as carcinogens and the political will for action exists, interventions targeted at reducing or eliminating exposure to these agents may affect a large proportion of the realistically modifiable cancer burden. For some cancers with few known risk factors, the contribution from environmental factors might be larger than currently estimated; candidates include testicular cancer and oesophageal cancer, for which the role of environmental factors is suggested by the descriptive epidemiology of geographical variation and/or by migrant studies. In addition, breast cancer, brain tumours, or cancers in childhood may have a significant contribution from the environment.
Challenges in identifying environmental and occupational risk factors are often shortcomings in the characterization of exposure. Further challenges arise in designing studies with sufficient heterogeneity in exposure distributions for ubiquitous environmental exposures, while at the same time contrasting comparable population groups in terms of other potential confounding factors. However, exposure assessment in occupational epidemiology has recently improved with the conversion of measurements at workplaces into more general exposure metrics applicable in large-scale epidemiological studies; such improvements are also necessary for environmental exposures, and advances are expected for future studies with the use of biomarkers, exposure modelling, and better accessibility of secondary data sources.