Posted By martinw I remember reading somewhere that 100000 hours is the number, rounded slightly up, to be the total number of hours worked by a person in a lifetime eg 48 hours per week times 48 weeks times 43 years. OSHA calculate it as 100 people working 40 hours a week for 50 weeks(assuming the US employers average of only giving 2 weeks holidays per year) - so the OSHA rate is based on a larger sample and on one year, whereas the other one is on one person's working lifetime. Don't know the historical reason why different measures are used. Sorry. Martin