Formaldehyde is an organic compound with the formula CH2O and it is the simplest aldehyde, with systematic name methanol. The aim of this investigation was to sensitize embalming laboratory users of the impending danger associated with overexposure to forty percent (40%) formaldehyde vapour (the active component of embalming fluid) on the lungs. Twenty adult male albino rats were used for this investigation. The animals were divided into five (A, B, C, D, and E) groups. Animals in group B, C, D and E were respectively exposed to forty percent (40%) formaldehyde for ten (10), fifteen (15), twenty (20) and twenty five (25) days while animals in group A which serve as control group were not exposed to forty percent (40%) formaldehyde at all. At different stages of the experiment, the animals in each of the group were decapacitated and the thoracic cage cut open to expose the lungs. The lung tissue was processed for light microscopic investigation adopting the Haematoxylin and Eosin (H and E) staining procedure. The histopathological observation in this study showed that exposure to forty percent (40%) formaldehyde induced changes in the histology of the lungs. The observed changes were duration dependent. So forty percent (40%) formaldehyde may be able to have greater marked effect at a prolonged exposure. These findings suggests that forty percent (40%) formaldehyde vapour may be remarkably toxic on the rat lung tissue and advocate for precautionary measure for human handlers while handling this chemical especially in the histopathology unit for fixation and embalming purposes.