When the oxygen absorbers absorb and react with oxygen they become warm.  This indicates that the oxygen absorbers are fresh and they are working very well to absorb oxygen. Many customers look for the warmth of oxygen absorbers as it indicates that their oxygen absorbers are working. If you expose oxygen absorbers to open air, they may become hot.  However, if the oxygen absorbers are inside your stored food with small amount of air, they won't become too hot.