Working Group 1: Data Gaps for Risk Assessment of Cadmium in the Food Chain

The risk of cadmium emissions for human health through food chain transfer is the result of various steps (Fig. 1). Data gaps can be identified at each of the steps.

Fig. 1. The various steps that affect the risk assessment of cadmium emissions to soil for human health. In humans, urinary cadmium (Cd-U) is a marker for the body-burden of cadmium. Because there is considerable uncertainty in the transfer values, it is impossible to arrive at a specific value for the cadmium level in the kidney cortex, for which Cd-U is the marker.

Data gaps in step I: Predicting the mass balance of cadmium in the long term, i.e., predicting total soil cadmium as a function of time

• There is too little information on cadmium leaching; is there preferential flow down cracks and channels, leading to rapid removal from the topsoil? What happens if cadmium moves out of the plough layer but remains in the root zone? Soil acidity increases the mobility of cadmium in soils and this may be important for many tropical soils, for which there are too little data. Transport models for cadmium require verification.
  
• We need to have precise information on atmospheric deposition of cadmium for many areas and on the extent to which aerial deposition is recycled.
  
• There is uncertainty as to whether soil erosion is an important process at the continental scale for the cadmium mass balance.

Data gaps in step II: Predicting cadmium transfer to the food chain

• If fixation of cadmium varies from 0-1% per year, even a positive total cadmium mass balance can be offset by fixation, resulting in a steady state for available cadmium. However, soils vary in the ability with which they fix cadmium and the reasons for this are not well understood.
  
• Phloem loading of cadmium into plant storage organs appears to be the key factor behind varietal differences in the cadmium content of many staple food crops. If the mechanism responsible for phloem loading was understood, it could help to accelerate breeding programs aiming for lower cadmium in crops.
  
• Cd/Zn and Cd/Fe ratios should be considered and reported in crop cadmium surveys.

Data gaps in step III: Predicting dietary cadmium intake in the general population

• There are data gaps for the cadmium concentrations in food items in many regions.
  
• There is a shortage of data on the distribution of dietary cadmium intake, i.e., the upper percentiles are not well characterized.
  
• There is a need to identify sensitive groups (e.g., those with Fe, Ca deficiency) within the population and identify their dietary cadmium intake, i.e., we need 'paired data' of dietary cadmium intake and nutritional status.

Data gaps in step IV: Predicting body-burden cadmium

• What is the relative availability of Cd in food with a high Cd:Zn ratio?
  
• What is the relative availability of high cadmium food that is eaten only occasionally; is it necessary to regulate for such food stuffs?

Data gaps in step V: Predicting effects on health from absorbed cadmium

• What is the relevance to health of biomarkers for exposure to and the effect of cadmium?
  
• There is a need to monitor the Cd-U with sufficient QC/QA in the general non-smoking population, i.e., the final target system to avoid uncertainties when monitoring soil, crop, or dietary cadmium.

   

The major items on which more information is required:

1. Is there a positive soil cadmium mass balance with present inputs?
   
2. Is there a health risk for the general population from exposure to cadmium?