Question

At NTP, the breakdown strength of air is

Answer 1

I guess you mean, At STP the breakdown strength of air is 21kV/cm.

Answer 2

Although air is normally an excellent insulator , when stressed by a sufficiently high voltage (an electric field of about 3×10^6 mV/m or 3kv/mm), air can begin to break down becoming partially conductive. Across relatively small gaps, breakdown voltage in air is a function of gap length time pressure. At STP, the breakdown strength of air is 21kV/cm.