For effective protection from exposure to hazardous gases, a facile strategy of real-time gas monitoring is highly needed. In this study, a novel colorimetric indicator that allows the visual detection of ambient sulfur oxides (SO x ) is developed, applying copper 2-methylimidazole [Cu(mIm)] nanocrystals to a cellulose fabric. By optimization of the molar ratio of copper to mIm, the properties of the treated fabric are improved for the Cu(mIm) loading capacity, gas adsorption performance, and colorimetric response. It is revealed that the role of environmental humidity is crucial for effective adsorption and chromatic reaction of Cu(mIm) because the hydrated Cu(mIm) surface facilitates SO 2 adsorption, producing reactive species of HSO 3 − or SO 4 2−. Those reactive SO x species cause the structural transformation of Cu(mIm) and ultimately lead to a chromatic response. The chromatic reaction takes place close to the breakthrough concentration of 5 ppm, demonstrating that the Cu(mIm) fabric can be applied as a practical service-life indicator, timely signaling the replacement time of protective materials. It is anticipated that the proposed strategy offers an effective SO x monitoring solution for application in various fields that call for facile gas detection.