Hardness-Dependent EQS

For some metals (particularly cadmium), toxicity varies significantly with water hardness (calcium concentration). The EQS is therefore expressed in 5 hardness classes. For cadmium (No.6): Class 1 (very soft, <40 mg/L CaCO₃) = 0.08 μg/L; Class 2 (40–<50) = 0.08 μg/L; Class 3 (50–<100) = 0.09 μg/L; Class 4 (100–<200) = 0.15 μg/L; Class 5 (≥200 mg/L) = 0.25 μg/L. Compliance officers must classify their water body by hardness class before applying the EQS threshold.