The answer to 'why is my air filter smells bad' is that a dirty or clogged air filter can cause unpleasant odors. As the filter becomes loaded with dust, debris, and microbial growth, it can emit musty or foul smells that get circulated through your home's air. This is a common issue that indicates it's time to replace your air filter.
The most likely causes are a buildup of dirt, dust, and mold on the filter. Over time, these contaminants can start to decompose and release unpleasant odors. Replacing the filter with a clean one will typically resolve the problem.