ASME Section VIII, Division 1 has special design and construction requirements for vessels that will be used in “Lethal Service.” Sounds logical and easy. Logical – yes, Easy – not so much! Section VIII mentions the term “lethal service” dozens of times. Still, it is Subsection B – Requirements pertaining to Methods of Fabrication of Pressure Vessels where the code gives us the best understanding of what ASME means by “lethal service.” But the code does not define which of OSHA’s Highly Hazardous Chemicals or EPA’s Extremely Hazardous Substances would be considered “Lethal”; that is left up to us to define.