How does a carbon monoxide detector indicate the presence of this deadly gas?

• A. The digital readout flashes CO
• B. An audible alarm sounds continuously
• C. A colored LED turns red
• D. The tan spot on the detector turns gray or black

Answer: (D)

The detector uses a colorimetric indicator that is exposed to carbon monoxide. A chemical layer sits as a small tan spot on the device; when CO is present, that spot reacts and changes color, turning gray or black. This provides a direct, visible cue that CO has been detected without needing any power or electronic readouts. It’s a simple, passive method—the change in color serves as the signal. In contrast, other detectors might rely on digital readouts, audible alarms, or LEDs, but in this type of detector the indication is the visible color change of the spot.