Magnesium oxide (MgO) composite boards are valued in the construction industry because of their outstanding fire resistance, however, the long-term viability of MgO boards is affected by atmospheric humidity. This research work aims to investigate the environmental impacts and mechanical properties of four types of MgO boards from four different manufacturers under varying relative humidity (RH) levels. Three of them contained a high amount of chlorine known as magnesium oxychloride (MOC) boards, and one of them contained a high amount of sulfur known as magnesium oxysulfate (MOS) boards from their elemental composition observed by EDX. Microscopic images showed a rougher and porous surface for MOC boards, whereas smoother surface for MOS boards. MOS boards exhibited 37 % (average) lower moisture absorbency than MOC boards at 95 % RH and remained dry, while the MOC boards leaked salty water drops under the same condition. As a result, the MOS boards exhibited much better corrosion and fungal protection than the MOC boards. Mechanical properties, for example, tensile strength decreased by 45 % in MOC boards and 31 % in MOS boards at 95 % RH compared to 50 % RH. Therefore, the sourced MOS composite boards outperformed the contemporary MOC boards in all parameters.