An air purifier includes a main body having an inlet configured to receive air and a discharger configured to discharge the air. The air purifier also includes a filter disposed within the main body and configure to purify contaminates from the air. The air purifier also includes a contamination sensor disposed adjacent to the filter and integrally includes a light emitter configured to emit a light toward the filter and a light receiver configured to receive the light emitted from the light emitter.