If, for example, our external control sample reveals 300 hyphal fragments, and our internal samples also reveal hyphal fragments, however are less than 300. At what point do you make the call that the air quality externally is contributing to the internal air quality, in particular if the debris rating internally is less than the debris rating externally?

From A Guide to Interpreting Laboratory Reports for Mould Assessment.

If, for example, our external control sample reveals 300 hyphal fragments, and our internal samples also reveal hyphal fragments, however are less than 300. At what point do you make the call that the air quality externally is contributing to the internal air quality, in particular if the debris rating internally is less than the debris rating externally?