The document discusses ventilation on board ships. It explains that ventilation is needed to avoid sweat forming on cargo and the ship's sides, reduce temperatures, expel toxic gases, and prevent oxygen depletion. Different types of cargo like hygroscopic and non-hygroscopic cargoes can be affected by a lack of ventilation. The three degrees rule and types of ventilation systems like natural, forced, surface, and thorough ventilation are described. Potential issues with natural and mechanical ventilation are also outlined.