Does this count as breaking the first precept? Let’s say that my face was itchy but just before I scratch it, I just remember that there might be face mites living on my face and I might be killing them. And as I’m scratching my face, I keep thinking that I might be killing the face mites. My original intention was to scratch my face but if I think I might be killing the face mites but continue to scratch my face because it’s itchy then does that count as breaking the first precept?
The first precept is:

Panatipata veramani sikkhapadam samadiyami.
I undertake the precept to refrain from destroying living creatures.

Conditions that Break the Precept
(By Ñanavara Thera)
The five factors of the first precept are:
* pano -- presence of a living being.
* panasannita -- one knows that it is a living being.
* vadhacittam -- the intention to kill.
* upakkamo -- the effort to kill.
* tena maranam -- the resulting death of that being.

All factors must be fulfilled in order to break the first precept.
If you can answer yes to all of them then the first precept is broken. If you can answer no to one of them the precept is not broken.