Concept:
Hard gelatin capsules contain moisture, which is necessary to maintain flexibility.
Step 1: Meaning of RH.
RH means relative humidity. It represents the amount of moisture present in air.
Step 2: Effect of low RH.
When capsules are stored at very low relative humidity, they lose moisture.
Step 3: Result of moisture loss.
Loss of moisture makes the gelatin shell dry and brittle. Brittle capsules can easily crack during handling.
Step 4: Final answer.
Therefore, hard gelatin capsules stored below \(20%\) RH become brittle and crack.
\[
\boxed{\text{Become brittle and crack}}
\]