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 the estimation of life expectancy of displayed images. Based on our measurements of indoor gas concentrations using passive samplers at various places for two years, we obtained proper data enabling us to determine the concentrations which have to be taken into account. At the same time, we displayed several kinds of unprotected prints and measured the concentration of pollutant gases at their sites. Then we analyzed the effects of these real pollutant gases on the stability of the displayed images. In this way, we were able to get a good correlation between the actual image deterioration of inkjet prints and their exposure to ozone or other gas at various indoor places.