Cadmium exposures and deteriorations of cognitive abilities: estimation of a reference dose for mixture risk assessments based on a systematic review and confidence rating

To support a mixture risk assessment with a focus on developmental neurotoxicity we evaluated the strength of evidence for associations of cadmium exposures with declines in IQ by conducting a systematic review and confidence rating. We searched peer-reviewed studies published in English between 2012 and July 2021 and identified 15 eligible studies (11 prospective cohort studies, and 4 cross-sectional studies). Of the 10 studies that observed associations of cadmium exposure with child IQ declines, two achieved an overall “High (H)” confidence rating, five a “Medium to High (M/H)”, one a “Medium (M)” and two a “Low (L)” confidence rating. Five studies did not detect significant associations between cadmium exposure and reduced cognitive ability; of these, two received a “High (H)” confidence rating, two an overall rating of “Medium to High (M/H)” and one a “Medium (M)” rating. The null findings reported by the “High (H)” and Medium to High (M/H)” studies could partly be explained by low exposures to cadmium or confounding with high levels of lead. By using a one-compartment toxicokinetic model in a reverse dosimetry approach, we estimated that a daily intake of 0.2 μg/kg body weight/day corresponds to urinary cadmium levels no longer associated with cognitive declines observed in a “High (H)”-confidence study. This estimate is 1.8-fold lower than the current health-based guidance value (HBGV) for kidney toxicity of 0.36 μg/kg bodyweight/day established by the European Food Safety Authority (EFSA). Our value does not have the normative character associated with health-based guidance values and is intended only as a reasonable estimate for the purpose of mixture risk assessments. However, with cadmium exposures in Europe between 0.28 (middle bound) and up to 0.52 μg/kg bodyweight/day (95(th) percentile), our review suggests that pregnant women and children are poorly protected against neurodevelopmental effects. This warrants a revision of the current HBGV.