Have you tried nebulizing with and without the CS?
ie, I'd begin by establishing a control to rule out what potential causes may be behind the irritation. Such as nebulizing only with the water, then comparing this with the diluted CS for example.
That said, I'd also add that the nebulizer itself can also cause irritation based on to the quality of the atomization(or vapor).
- hope this helps
PS. 250ppm isn't very likely with traditional Colloidal Silver solutions.