Cadmium: A Clandestine Threat to Food Safety

Recently, EFSA has released a set of “risk-based” food standards for Cd that, if adhered to and averaged across typical diets, would help achieve compliance with their recommended tolerable intake limit for Cd. The shift to risk-based food standards in recent decades has been a welcome development because many of older food standards were not overtly linked to tolerable intakes (toxicity is defined by the dose), or if they had been derived on this basis, the process was obscure. For food producers, these and other pre-existing food standards for Cd primarily represent a product compliance and trade risk. With a new set of food standards, it is likely that European regulators and markets will be placing a heavier emphasis on food compliance monitoring for Cd in the future.

Most dietary Cd comes from foods with elevated Cd concentrations that are consumed in significant amounts. These include cereals, vegetables, nuts, starchy roots or potatoes, and some meat products. Vegetarians have higher dietary exposures, as do regular consumers of bivalve mollusks and wild mushrooms. Smokers double their overall exposure, because tobacco contains significant Cd and the lungs are efficient at absorbing it. Together, ensuring compliance with tolerable intake limits and food standards will ensure the risk from Cd exposure from food is tolerably low.

There is Only One Problem

Most Cd in food comes from the soil where crops are grown and animals are raised. As a chemical element, Cd is present at low concentrations in all soils. However, human activities are causing soil Cd concentrations to increase, which results in increased Cd concentrations in food. While industrial activity, waste disposal, and mining can result in localized soil contamination, fertilizers and soil conditioners are the most important source of Cd in food-producing soils.

Elevated Cd concentrations are often associated with sources of phosphorus (P), an essential plant nutrient. Humanity needs to add P to soil to maintain productivity and feed an ever-growing population. P is added to soil via fertilizers, effluents, and sludge. These materials can contain Cd as an unwanted passenger that cannot easily be removed. The ultimate source of P is phosphate rock, a non-renewable resource that is mined and processed to give fertilizers (for example, superphosphate fertilizer). Geologically, Cd is co-deposited with P and phosphate rock can have over 500 mg of Cd for every kg of P. While the Cd concentration of phosphate rock varies, low-Cd sources of this mineral are mined preferentially, command a premium price, and will eventually be exhausted. In many countries like New Zealand, current P and Cd levels in many agricultural soils are now four to six times higher than their natural concentrations. Just as the majority of P in our diets could now be traced back to agricultural use of phosphate fertilizers, it is likely that most dietary Cd now also originates from this same source.

Municipal effluents and biosolids (sewage sludge) can be used effectively as P-containing soil conditioners. However, these materials also contain elevated Cd concentrations, along with other potential soil contaminants, particularly if there are industrial inflows into the sewage treatment plant.

Once added to soil, Cd binds strongly to soil particles, causing this toxic element to accumulate with each fertilizer application. Only small amounts of Cd are lost from the soil via surface runoff (with fertilizer runoff) and leaching, which is not significant until very high soil Cd concentrations are reached. The rate of Cd accumulation in soil depends on the concentration of Cd in the fertilizer. Low-cost fertilizers and effluents used by poor countries often have higher Cd concentrations, resulting in a more rapid buildup of this toxic metal in soil.