Quantum Key Distribution (QKD) ensures theoretically unbreakable encryption by exploiting quantum mechanical principles. This work analyzes the BB84 protocol under realistic noisy channels using IBM’s Qiskit Aer simulator with depolarizing, amplitude damping and phase damping noise models. Simulations with 10,000 qubits per run show that the Quantum Bit Error Rate (QBER) increases from 0% to 12% as depolarizing noise probability reaches 0.05, reducing the Secure Key Rate (SKR) to near zero once QBER exceeds the security threshold 11%. Although depolarizing noise causes the steepest theoretical SKR decay, we observed that amplitude damping produced unexpectedly higher QBER and faster SKR collapse than predicted, due to basis-dependent asymmetries in state preparation and measurement. This deviation from analytical models highlights decoherence effects that are often underestimated in simulations. Our unique contribution is to demonstrate that amplitude damping poses a more severe practical threat to BB84 than symmetric depolarizing noise, providing critical insights for robust QKD deployments.