Exposure to low-level chemicals and ionizing radiation: reactive oxygen species and cellular pathways.

Reactive oxygen species (ROS), which contribute to the energy landscapes in and around cells, play numerous roles in maintaining normal cell homeostasis as components of signaling pathways. Excessively high levels of ROS, on the other hand, can lead to pronounced DNA damage and a variety of cellular responses, including cell cycle arrests, senescence, apoptosis and possibly cancer. Far less is known, however, about how supra-basal levels of ROS that can be generated in response to low doses of ionizing radiation or chemicals in the environment may bring about untoward or perhaps even beneficial cellular responses. Even so, some evidence suggests that adaptive responses that have been associated with ROS-generating stimuli can have protective effects by fundamentally altering subsequent cellular dose-response profiles to otherwise detrimental stresses. Yet, even these seemingly favorable 'adaptive' effects may have longer-term untoward consequences. Other effects that have been associated with supra-basal levels of ROS, such as enhanced states of cell proliferation, potentially could have a protective function. But again, such increases in cell growth, which may be accompanied by greater than normal ROS-mediated damage to DNA, as well may ultimately favour the expansion of cells with heritable mutations. Unfortunately, the state of the art of our current understanding of how elevated but still low-level increases in ROS that may be induced by environmental stimuli presently precluded incorporation of supra-basal ROS-associated mechanisms in predictive risk assessment models, both at the population level and at the level of individualized risk assessment.