Mortality and Lethality - What's the difference?

Mortality: is the ratio between the number of deaths from a specific disease during a defined time period and the total number of individuals in the population

Fatality (lethality): is the ratio between the number of deaths from a specific disease during a specific time period and the total number of individuals who have had the disease.

So in toxicology we can substitute the disease with the toxic exposure, and we usually survey lethality e.g. when workers die from inhaling oil fumes whean opening a hatch to a tank with oil (and lack of oxygen) then we are looking at lethality if we give the numbers per total workers opening a hatch, but we could also report the mortality if we want to convey how many individuals die compared to all individuals in the specific country.