No, cardboard is not toxic to humans. However, if you're planning on using it in your air fryer, there are a few things you should keep in mind. First, cardboard can be a fire hazard, so it's essential to ensure the air fryer is turned off and cooled before adding any cardboard. Secondly, cardboard can absorb oil and grease, so it's essential to clean the air fryer after each use. Finally, if you're using recycled cardboard, make sure it's free of any chemicals or toxins that could be harmful to your health.