Image stability of unprotected prints is affected by pollutant gases in the atmosphere indoors. It is important to know the standard concentration of indoor pollutant gases, especially for estimation of the life expectancy of displayed images. Based on our measurements of indoor gas concentration using a Passive Sampler at various place for two years, we obtained proper data enabling us to determine the concentration which had to be taken into account. At the same time, we displayed several kinds of unprotected prints at the places where we measure the concentration of pollutant gases. Then we analyzed the effects of these real pollutant gases on the stability of the displayed images. In this way, we could get good correlation between actual image deterioration of inkjet prints and their exposure to ozone or other gas at various indoor places.