The lower explosive limit (LEL) is the minimal amount of concentration needed of a particular chemical to cause an explosion. The LEL is determined empirically for each pure chemical and air mixture at a given temperature. If more than one chemical is dispersed in the air, as is normally the case, then LeChatelier's mixing rule can be applied to get the cumulative LEL for the mixture. LeChatelier's mixing rule states the inverse sum of the partial volumes of each component divided by the component's LEL is the mixture's LEL.
Divide the fraction of the total volume of each component by the lower explosive limit of the component to get the partial lower explosive limit of the component.
Sum all of the partial lower explosive limits.
Take the inverse of the sum of the partial lower explosive limits to get the net lower explosive limit of the mixture.
Note that this same equation cannot be applied to the upper explosive limit.