Which class of flammable liquids must be stored in closed containers when not in use, particularly at or above their flash points?

Class I flammable liquids are categorized as highly flammable due to their low flash points, which are below 100°F (37.8°C). Because of their volatility, these liquids indeed require stringent storage measures, including being kept in closed containers when not in use, especially if they are at or above their flash points, to mitigate the risk of ignition and potential fire hazards.

Class II and III liquids are less volatile than Class I, with Class II flash points ranging from 100°F to 140°F (37.8°C to 60°C) and Class III liquids having flash points at or above 140°F (60°C). While these classes also require caution, the emphasis on closed container storage is particularly relevant for Class I due to their propensity to ignite. Storing Class II and III liquids in closed containers when not in use helps minimize vapors and prevents accidental ignition.

Class IV and V liquids typically have flash points above 200°F (93.4°C) and are not classified as flammable, thus requiring less stringent storage measures. Therefore, the answer focusing on Class II and III reflects the need for careful handling of flammable materials with lower flash points, ensuring safety by preventing vapor accumulation that can lead to ignition.