Human to human is the more common and informal way to write it. It's usually used in everyday speech and writing.
Human-to-human is a more formal and technical way to write it. It's often used in academic writing, scientific papers, and technical documentation.
Here's a breakdown:
Human to human:
* Pros: More casual and approachable.
* Cons: Might not be as professional in formal contexts.
Human-to-human:
* Pros: More professional and technical.
* Cons: Might be too formal in casual contexts.
Ultimately, the best way to write it depends on your audience and the overall tone of your writing.