The numbers that matter
OSHA's permissible exposure limit for H2S in general industry is 20 ppm as a ceiling, with a peak of 50 ppm allowed for no more than 10 minutes per shift. NIOSH sets a recommended ceiling of 10 ppm, and the concentration considered immediately dangerous to life and health is 100 ppm. ACGIH guidance is stricter still: 1 ppm as an eight-hour average and 5 ppm short-term. Health effects can begin with prolonged exposure as low as 2 to 5 ppm, and at higher concentrations the gas deadens the sense of smell, which is why a worker cannot be trusted to smell the danger.
Alarm at 10, act at 10
OSHA's oil and gas eTool notes that industry practice uses 10 ppm as the alarm set point that warns workers and triggers evacuation. Personal monitors should be worn as close to the breathing zone as possible, on the collar or chest. Fixed monitoring, wind socks, egress routes, and alarm systems are part of the same program. OSHA's guidance for sour crude tank gauging is blunt: if air monitoring is not performed before opening the hatch, the atmosphere must be assumed IDLH, and self-contained breathing apparatus is required.
Training and the contingency plan
OSHA points operators to ANSI/ASSE Z390.1-2017 for H2S safety training programs: hazard recognition, monitor use, response to alarms, symptoms, rescue techniques, and proficiency with PPE. API RP 49, the recommended practice for drilling and well servicing operations involving hydrogen sulfide, is the industry reference for the contingency plan: gas detection, alarms, respiratory protection, and emergency procedures. Every crew member on an H2S location should know the plan before the job starts, not after the first alarm.
The monitoring log is the record that ties the program together: who was on location, what the monitors read, when alarms sounded, and how the crew responded. That log is safety paperwork in its purest form, the same discipline as the job safety analysis and the same evidence trail an audit or an investigator looks for after an event. If the H2S record is kept as a clipboard sheet that nobody reviews, an operations audit will show how thin the gap is between the written program and the field reality.