I couldn't clearly distinguish the difference of these two words, "serious" and "severe", but got a line about it after I read one paper about hazards classification today.

To define the degree of hazards, one can classify the hazards into the following categories:

    0=MINIMAL HAZARD

    1=SLIGHT HAZARD

    2=MODERATE HAZARD

    3=SERIOUS HAZARD

    4=SEVERE HAZARD

Hope it could be of help to some researchers.