What factor can decrease the effectiveness of a smoke detector?

Both low battery power and excessive dust accumulation can significantly decrease the effectiveness of a smoke detector. Smoke detectors rely on a constant power supply to operate properly, and a low battery can lead to insufficient energy to function effectively, leading to potential failure in detecting smoke during a fire.

Similarly, excessive dust accumulation can obstruct the detector's sensing mechanisms, preventing it from accurately detecting smoke particles in the air. Dust can interfere with the operation of both photoelectric and ionization sensors, which are commonly found in smoke detectors.

Therefore, both factors can compromise the safety provided by smoke detectors, making it critical to regularly check and maintain batteries and ensure that detectors are clean and free from dust buildup. This is why the option indicating that both low battery power and excessive dust accumulation can reduce effectiveness is correct.