Most published LFL values are calculated at room temperature.
However, as a given mixture is heated, its flammability increases and thus the concentration required to achieve 100% LFL is less. This source of increased danger is often overlooked.
Consider the diagram above. The yellow band indicates 100% of the LFL of a sample mixture, given the degree of uncertainty in published values as previously mentioned. Note how the red area of danger becomes greater as the temperature increases. Note also the green “safe” area. The concentration that was safe at 75°F becomes dangerous at 400°F.