The interaction of human hemoglobin with environmental and other gases and molecules is investigated using density functional theory. The investigation includes gases and molecules such as O2, N2, Ar, CO2, H2O, CO, and Cl2. Thermodynamic quantities usually include Gibbs free energy, enthalpy, and entropy. These thermodynamic quantities can be used to distinguish how much strong these molecules are bonded to hemoglobin. The interaction with the two different heme molecules, singlet and triplet states, is shown. Results show that the bonding strength differs greatly between these gases. Most of the investigated molecules remain at their monoatomic, diatomic, or triatomic structure except for O2 and Cl2 that may dissociate into two atoms attached to hemoglobin. The Gibbs free energy of interaction of these atoms and molecules reveals the toxicity of some of these gases, such as CO and Cl2.