8. What research is still needed?
- 8.1 Human health risks
- 8.2 Environmental health risks
8.1 Human health risks
Health risk assessment determines whether a chemical (including radioactive material) may cause adverse health effects, the probability that these effects will occur, and at what level and frequency of exposure they may occur. Toxicology focuses on the identification and quantization of potential hazards by using animal studies as surrogates for humans.
Several terms frequently used and misused in risk assessment and its perception require clarification. In a discussion of the health effects of potentially toxic chemicals, the terms "hazard” and "risk” are often used with an identical meaning, although they are clearly different. Hazard defines the intrinsic toxicity of a chemical and is not identical to risk. Risk is the estimated or measured probability of injury or death resulting from exposure to a specific chemical. Risk may be described either in semi-quantitative terms such as high or low risk or in quantitative terms.
The health risks due to contact with potentially toxic chemicals are dependent on the conditions of exposure, since not only the intrinsic toxicity of a chemical determines the magnitude of the adverse effect but also the dose. The magnitude of toxic effects is the product of the intrinsic toxicity of a chemical multiplied by the dose taken up by exposed animals or humans; thus, all toxic effects are dose-dependent and even very toxic chemicals may not cause toxic effects when the dose is low. If the dose is zero, despite a very high intrinsic toxicity of a specific chemical, the toxic effect and the risk of adverse health effects will be zero. On the other hand, chemicals with low intrinsic toxicity may induce toxic effects when the dose is high and may thus pose a significant risk. In toxicological terms, risk is therefore the product of the intrinsic toxicity of a chemical and the exposure characteristics.
The US National Research Council stated that ingesting U in food and water at the naturally occurring levels will not cause cancer or other health problems in people (ATSDR, 1999; NRC, 1991), In addition, in U miners, there was "no association between exposures to uranium and lung cancer at cumulative internal dose levels lower than 200 mSv” (ATSDR, 1999; NRC, 1991). Especially for the U miners it is accepted that radon exposure is the main cancer risk factor and that smoking is the most important confounder in these studies (Harley, 2001). Based on the radiological profile of natural U and DU, radiological health hazards are also not expected. Since exposures to DU both in soldiers and in residents in areas with military use of DU could not be detected or is very low, and exposures are thus well below thresholds for chemical toxicity or accepted limits for radiological protection of the general population, health risks due to the chemical and radiological toxicity of DU are not expected. The conclusion is supported by all expert panels that were tasked with risk assessment for DU uses regarding the general population (EU-EURATOM, 2001; EURATOM, 2009; IAEA, 2003, 2009; UNEP, 2001, 2002, 2003, 2007; UNEP/UNCHS, 1999; UNSCEAR, 1993, 2000b, a; WHO, 2001).
An increased frequency of malformations in offspring from combat veterans deployed in areas where DU ammunitions were used was claimed, but could not be substantiated (McDiarmid et al., 2009; Sumanovic-Glamuzina et al., 2003). Reports on an increase in malformations in southern Iraq and/or Kuwait were not located in the scientific literature.
8.2 Environmental health risks
8.2.1 Risk for the terrestrial environment
A precise quantitative characterisation of the risk for the soil ecosystem is not simple due to the difficulty of calculating a Predicted Environmental Concentration (PEC) and to the lack of toxicological data on U and DU required for calculating a Predicted No Effect Concentration (PNEC). However, some general conclusions can be made.
The concentrations of DU measured in soil in all investigated sites (see table 7), even in locations with intensive use of DU ammunitions, are within the typical concentration range of U in soil (see table 5), with the exception of samples taken in the immediate vicinity of DU penetrators. Therefore, soil concentrations in impacted areas are of the same order of background levels of U in natural soils. As indicated above, a risk limit value of 28 mg/kg was derived by RIVM (Van de Plassche et al., 1999) for soil. It follows that potential risk to the environment is likely to occur in very limited areas, only directly in contact with DU.
8.2.2 Risk for the aquatic environment
As for soil, similar difficulties are encountered for characterizing the risk for the aquatic environment, though some toxicological data are available for aquatic organisms.
The lowest chronic toxicity values reported for U are in the 1.0 to 10 μg/L range (see section ecotoxicity). This would mean that if an assessment factor of 10 would be applied for calculating a PNEC, a value of 0.1 to 1 μg/L would result. However, as mentioned in previous opinions of the SCHER – see for example the SCHER Opinion on Copper (EU- SCHER, 2009), the standard TGD procedure for calculating a PNEC should be applied with caution to natural elements such as U, in particular if one considers that calculated values are within the range of background concentrations of U in water. The RIVM proposal for a maximum permissible addition to background levels of 1.0 μg U/L is also difficult to apply because it is not clear whether concentrations measured in the impacted areas (see table 7) represent the natural background concentrations or values modified by DU emissions.
However, it must be noted that most data reported as concentrations measured in surface water of impacted areas, except for Kuwait data, are below 1 μg U/L. Therefore, it can be concluded that a risk for the aquatic environment is unlikely to occur.
8.2.3 Risk for secondary poisoning
Uranium has been measured in plants and animals (earthworms). However, transfer factors in plants and animals are low and related to environmental concentrations. For example, in the US EPA ECOTOX Database (US-EPA, 2009), for rainbow trout, a bioconcentration factor of 37 and a BCF value of 4.2 for molluscs has been recorded. Therefore, the potential for secondary poisoning due to DU in impacted areas is low and limited to very restricted sites close to or directly in contact with ammunitions.