How To Clean a Pinhole Lens?

Hi, we are having problems that pinhole lenses are getting dirty because they are pointed through a hole in the wall towards a paying terminal that is located outdoor. Now dirt is getting into the front of the pinhole lens.

Does anyone know how to clean such lens or cleaning tools? Just blowing air isn't enough, the dirt is sticky and very hard to remove. Of course I can change lenses, but that means a lot of work remounting and adjusting the camera

pinhole lens

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