When replacing lithium target components in the International Fusion Materials Irradiation Facility, the lithium adhering on the surface of components will chemically react with the surrounding atmosphere. In this study, we investigate the chemical reaction of pure lithium in room temperature air, oxygen, nitrogen, and argon of various humidities. In air, oxygen, and argon, the weight gain increases with time and humidity, and no significant difference is observed in these gases. In nitrogen, except for the test with 75% relative humidity (RH), the weight gain accelerates with time, and the weight gain rate decreases with increasing RH. We attribute the weight gain increase observed in low humidity to the acceleration due to lithium nitride formation. Upon increasing the moisture content of the gas, the main lithium reaction becomes lithium hydroxide formation.