These 10 facts about space will blow your mind. A hemithorax is one side of the chest. The thorax is the medical term for the chest area, and hemithorax indicates only one half of the chest. The plural for this medical term is hemithoraxes or hemithoraces.neumonia causes something called hemithorax opacification. This is the name for any condition that causes one hemithorax to show up dark on a radiology chart of the chest. Opaque hemithoraces can indicate pleural effusion (fluid filling the lungs), too.