The clean wall condition in tokamak devices can be achieved by utilizing the active chemical property of lithium (Li). This also exposes Li to air contamination even when protected by inert gas. To determine whether air contamination affects the retention of hydrogen isotopes in Li, the desorption performances of deuterium (D2) in liquid Li with D/Li ratios of 4.1 mol.%, 2.5 mol.%, and 0.6 mol.% were compared. At a D/Li ratio as low as 0.6 mol.%, the results revealed a significant change in the desorption temperature of D2 by Li contaminants. The ratio of contaminants to D2 in Li was discovered to be important. In this regard, the influence of N2, O2, and H2O (accounting for 15 mol.% of D2) on the desorption performance of D2 in liquid Li was discussed. Both H2O and N2 preabsorbed in liquid Li could react with D2, leading to the changed desorption temperature of D2. Notably, the H2O present in Li consumed all of D2 via reaction, whereas the same amount of N2 as H2O could only react with a part of D2. However, the effect of O2 on the desorption temperature and the effect of impurities on the desorption amount of D2 were not obvious. The impacts of the findings of this work on the retention and extraction of hydrogen isotopes in Li may be further explored to prompt the application of Li in fusion devices.