The effect of various contaminants, namely NO2, SO2, 2-butanone [methyl-ethyl-ketone (MEK)], and NH3, on the radioiodine removal efficiency of triethyl-enediamine (TEDA)-impregnated charcoal filters has been studied, and an attempt was made to characterize and quantify the weathering process of TEDA charcoal by these contaminants. The effects of the contaminants on the CH3I removal efficiency of TEDA charcoal under dry and humid conditions are described. Based on our results, the efficiency of TEDA charcoal is degraded most by NO2 and SO2, NH3 has a negligible effect, and MEK produces a mild degradation. The degree of degradation parallels the contaminant’s ability to be chemisorbed on the TEDA impregnant. The combined effect of water vapor and a contaminant on the charcoal efficiency is different for each contaminant. Nitrogen dioxide adsorbed under dry conditions is more effective in degrading the CH3I removal efficiency of the charcoal than when adsorbed under humid conditions. On the other hand, a completely opposite result is observed for SO2. The MEK contaminant behaves similarly to SO2, but the effect of humidity was less significant than for SO2. Ammonia has no effect on the efficiency of the charcoal regardless of humidity.