Our Carbon Pre-Filter uses activated charcoal to absorb odors while acting as a barrier for large air debris like pet hair and dust. Depending on the air quality level of your room, it may need to be replaced more frequently than its sidekick, the True HEPA filter, so we recommend checking the pre-filter every 30 days. When its initial black color appears gray from dust and debris, it’s time to swap it out for a new one."