The concentration of an entity can be set a condition for a causal regulation.
For example, A needs to be in "high" concentration to down-regulates the quantity or the activity of B.

How can we best annotate this?
For now, I can see having the terms "high", "low" and "normal" ("normal" may not be very interesting, difficult to know what a "normal" concentration means.) concentration. But would that be enough significant? Are there any ontologies handling this type of information?