The "magic number" you are looking for is AFAICT between 0,05 and 0,07, i.e. it is reasonable to expect 5-7 events needing an ambulance intervention every 100 p

The "magic number" you are looking for is AFAICT between 0,05 and 0,07, i.e. it is reasonable to expect 5-7 events needing an ambulance intervention every 100 people per year. Of course this is a reference for general population, and since the workers in a factory are by definition younger and healthier than the average population and hopefully road accidents don't happen in a factory, must be reduced but 1/12 or 1/10 of that wouldn't surprise me, that would be 0.4-0.7/100 per year. As a reference a minimal requirement for ambulance service (with M.D. on board) here in Italy is to have an ambulance every 60,000 people, that would mean, with 3-4 interventions per 8 hours shift on average (2-3 hours/intervention), 365x3x8/3.5=2,502 interventions per year in theory, reducing this by 10% to consider maintenance, lost/idle time, etc. we have 2,250/60,000=0,0375 or 4 interventions every 100 people, and the number of vehicles roughly doubles considering the "simpler" ambulances without M.D. on board, fulfilling the needs. A reference data point (old) is roughly 4,000,000 interventions in the year 2005 on a population of around 60,000,000, that comes out as 0.66... So, on a "population" of 10,000 workers 40-50 ambulance calls per year wouldn't be unexpected at all, but it greatly depends on local organization, and on actually when (depending on seriousness of event) the ambulance is called, this may vary with local (state) policies and local-local (industry) ones. At least here factories of that or comparable size have an internal medical service (and almost completely equipped infirmary) and ambulance for at least first aid/first responder chores and external ambulances are usually called only in the (much more rare) cases when the worker/patient needs to be hospitalized or however transferred to a hospital for further exams or the like.