Efficacious use of activated carbon beds (ACBs) as adsorbents for various types of organic vapors in respirator gas filters requires estimation of the breakthrough time. In this study, several types of activated carbon small beds acting as gas filters were preconditioned by equilibration at low relative humidity (RH = 40%) and a constant temperature of 20 degrees C. Then, we measured the breakthrough curves of 13 types of organic vapors through the beds under the same RH and temperature conditions as used for preconditioning and calculated their relative breakthrough time (RBT) with reference to cyclohexane. The ACBs comprised four types of activated carbon and had different weights (22.5-35g), yet the measured RBTs showed good approximation to the reference data, with a small number of exceptions. We confirmed that RBT can be used as an effective index at RH = 40% and 20 degrees C.