3. What are the health effects of radiation?
The SCHER Opinion states
In general, radiation may induce both deterministic and stochastic health effects (Hall and Giacca, 2006). Deterministic effects of radiation include the acute health effects observed after high "radiation doses”, sometimes referred to as general "radiation sickness” which is characterized by effects of radiation on rapidly proliferating cells. Depending on the amount of the deposited energy within the tissues (often simplified as "radiation dose”) these health effects might result in the hematopoetic, the gastrointestinal, the neurovascular or the cutaneous "radiation syndrome”, or a combination of these syndromes. Deterministic effects per definition only occur above a threshold radiation dose. Examples for deterministic radiation effects are "unwanted effects” observed after radiotherapies for malignant diseases, effects seen after industrial radiation accidents (IAEA, 1996), or those observed in the Hiroshima and Nagasaki victims after the attack with nuclear weapons in World War II (Kondo, 1993; Preston et al., 2003). Exposure to DU by all conceivable exposure pathways is not expected to result in deterministic effects ("radiation sickness”) in humans.
Stochastic effects are represented by the induction of mutations by radiation, which may result in cancer. Regarding stochastic effects, a linear no-threshold (LNT) dose-response hypothesis in the low dose range is assumed. For more details on radiation doses, assessment of radiation health risks, and radiation carcinogenicity, see Annex I.
Although radiation exposure is generally assumed to be carcinogenic at all dose levels, no correlation between tumour incidence and radiation has been established at low "doses” in the range of natural radiation background. This is attributable to two factors: (1) it is difficult to obtain meaningful data from epidemiological studies where exposure is near background exposure levels, and (2) the results of such studies usually do not give statistically significant differences between exposed and unexposed groups to substantiate a health impact (Hall et al., 2009). The same problems have to be faced when trying to transfer basic principles of radiation damage mechanisms such as the so-called bystander effect (damage of non-irradiated cells by irradiated cells or mediators at very low doses) from in vitro to in vivo and estimating the real role for radiation carcinogenesis (Little, 2006; Williams, 2008). However, the low-dose linearity concept is still the accepted standard for radiation protection policies (Puskin, 2008). Recently, reviews of carcinogenicity and exposure to chemicals and radiation have questioned the non-threshold assumption (Averbeck, 2009; Clark, 1999; EU-SCHER, 2009) since there is increasing biological evidence for a potential threshold in radiation- and chemically- induced carcinogenicity.
The available information on radioactivity and its effects shows that high dose alpha radiation can cause a variety of effects in humans. The nature and the severity of these effects depend on several factors, including physicochemical form and solubility of the alpha-emitting isotope , route of entry, distribution, biological retention, and specific alpha-energy emitted. Since the specific alpha-emissions of both natural U and DU are low and the potential for internal exposures to U and DU in humans is very limited, there is no conclusive evidence on biological effects in humans by alpha-radiation from U (UNEP/UNCHS, 1999).
Potential radiological effects due to the intake of U and DU both by inhalation and by ingestion have been assessed by a variety of international expert groups in peer- reviewed reports. WHO had made a detailed assessment of potential radiation-mediated effects of both U and DU. This assessment included modelling of inhalation exposures and considered specific biokinetics of insoluble U-oxides and mixed U/Fe-oxides. This assessment concluded that potential exposures to DU will add only a negligible contribution to total U-intake. Any DU-derived radiation will remain below an effective radiation dose < 1 mSv and thus well below accepted dose-rate limits derived for radiation protection. This conclusion was confirmed by other international expert groups (Durante and Pugliese, 2002; EU-EURATOM, 2001; EURATOM, 2009; IAEA, 2003, 2009; Li et al., 2009; UNEP, 2001, 2002, 2003, 2007; UNEP/UNCHS, 1999; UNSCEAR, 1993, 2000b, a; WHO, 2001, 2003b) and SCHER agrees with this conclusion.