Correlating Radiation Exposure with Health Effects
As stated above, biological effects resulting from radiation exposure are highly correlated with received radiation dose and dose rate. Acute Radiation Syndrome (ARS), or radiation sickness, is the result of whole body exposure to very high levels of radiation, usually over a short period of time. While people who suffered from ARS include survivors of the atomic bombs and first responders after the Chernobyl NPP event in 1986, populations affected by radiation release and contamination schemes similar to those seen after the Fukushima accident are much more likely to experience chronic low dose effects. The following sections therefore focus on the health effects of exposure to low dose ionizing radiation and internal contamination with radionuclides.
Low Dose Ionizing Radiation
A low dose of ionizing radiation is generally defined as an acute exposure of
<100 mGy (mSv) [20]. In the context of biology, the term “low dose” is the lowest dose of energy deposited in a single cell that results in cellular changes [21]. Interestingly, internalized radioactive materials deposited at low dose rates are not uniformly distributed at all levels of biological organization. The mechanisms of action for the biological responses induced by low doses of ionizing radiation are different from those induced by high doses. Responses estimated using linear extrapolation of high dose should be prudently interpreted since this method overestimates the real risk associated with these low dose and dose rate exposures. By and large, non-uniform distribution of low doses is less hazardous than single, acute whole-body exposures, as shown in DNA repair processes [11, 22].
Challenges lie in linking direct risk estimates for exposures at low doses. Radiation is a weak carcinogen and its effects are too small to quantify, as we are all exposed to natural background radiation at around this low level, which may mask any significant effects. Are internally deposited low dose radioactive materials more harmful than external exposures? There is no conclusive scientific evidence that shows fundamental differences between external and internal sources of radiation, or between artificial and natural radionuclides in their capacity to cause such damage. It is important to consider the location of target cells within tissues when considering doses from short-range internal emitters (e.g., alpha particles, low energy electrons) [23].
Linear-No-Threshold Model
There are conflicting schools of thought in the radiation community on stochastic health effects associated with exposures to low doses of ionizing radiation [20]. Current risk estimates and most radiation protection standards are based on the 'linear-no-threshold' (LNT) model [21]. The LNT hypothesis does not reflect the actual risk in the low-dose region, but provides a useful tool to conservatively control exposure [11]. According to this model, the effect of ionizing radiation is directly proportional to the dose, and even the smallest dose of radiation is associated with a small increase in cancer risk to humans without a threshold [24, 25]. The Biological Effects of Ionizing Radiation (BEIR) committee of the National Academy of Sciences (NAS) published a report in 2006 concluding that the available biological and biophysical data support the LNT risk model [24, 25].
In the same year, UNSCEAR issued a report citing that while the LNT hypothesis holds validity in radiation protection at low doses and low dose rates, it does not reflect the actual risk in the low dose region [12, 20, 21, 24]. In a subsequent 2012 study, UNSCEAR also concluded that there is no consensus on the impact of radiation exposure, particularly at low doses [26]. ICRP reached a conclusion similar to that of UNSCEAR and stated in their Recommendations guidance that current evidence does not support a universal threshold dose level, although a lowdose threshold is likely applicable for radiation-related cancers in certain tissue [5, 24, 27].
The French Academy of Sciences challenged the validity of the LNT model for assessing health risks at low doses [28]. The LNT model posits that carcinogenic risks remain constant in all biological reactions, regardless of dose or dose rate. The group pointed out that epidemiological studies did not show a signifi increase of cancer incidence in humans for doses ≲100 mSv. In addition, the LNT model fails to take into consideration the various biological mechanisms cells demonstrate when they are irradiated by ionizing radiation. The group concluded that the universal approach of the LNT model greatly simplifi the dose-effect relationships and may result in an overestimation of health risk at low doses since biological mechanisms and responses are different at low doses versus high doses [29, 30].
Some researchers subscribe to the once discredited hormesis concept, a hypothesis that receiving low ionizing radiation in doses just above the natural background level may induce beneficial biological responses [17]. The proponents of this hypothesis explain that a number of compensatory and reparatory mechanisms (e.g., stimulation of the immune response and DNA repair, and activation of apoptosis that eliminates damaged cells that would otherwise become cancerous) are stimulated in response to small doses of ionizing radiation [17, 31].
Stochastic effects are more likely to occur after acute exposure to internalized radionuclides than deterministic effects. At absorbed doses of ~1 Gy, deterministic effects may occur, including pneumonitis, erythema, vomiting and diarrhea, bone marrow failure, and cataracts. Some of these symptoms appear several hours after an acute absorbed dose, whereas others may take weeks or longer [4].
