What are the OSHA limits for hydrogen sulfide?

"ppm" means parts per million. A ceiling is a level you may never go above at any moment. A time-weighted average (TWA) is the average over a whole shift, so short spikes above it can still pass.

PEL means permissible exposure limit. REL means recommended exposure limit. TLV means threshold limit value, and STEL means short-term exposure limit.

The enforceable number depends on the kind of work, not the building. OSHA sets separate limits for general industry work and for construction work, so the same ejector room can fall under a 20 ppm ceiling for one job and a 10 ppm average for another. If you aren't sure which covers your crew, plan to the stricter 10 ppm number, or ask ADOSH's consultation program. ADOSH says a consultation ends in a report to the employer, with no citations or penalties.

In Arizona, ADOSH enforces these rules. ADOSH says it enforces the federal standards at 29 CFR parts 1910, 1926 and 1928, so the table above is the Arizona number too.

The recommendations matter even though nobody is cited for them. A safety program built on NIOSH's 10 ppm ceiling or ACGIH's 1 ppm average has margin. One built on the 20 ppm ceiling has none.

### Emergency planning numbers

Exposure limits are for a normal workday. Emergency planners use a different set. NIOSH's documentation for the hydrogen sulfide IDLH lists the American Industrial Hygiene Association's Emergency Response Planning Guidelines, each for a one-hour exposure:

The three steps run from the lowest planning level to the highest. Notice that ERPG-3 lines up with NIOSH's 100 ppm IDLH.

The same NIOSH page lists older limits you may still see on labels and in old safety binders: a 10 ppm eight-hour average and a 15 ppm short-term limit from OSHA's 1989 rule. NIOSH lists those 1989 values in its appendix of limits that are not in effect. The current enforceable general industry number is the 20 ppm ceiling.

OSHA's hazards page lines these limits up against symptoms. At 2 to 5 ppm, prolonged exposure may cause nausea, tearing eyes, headaches or lost sleep. Around 20 ppm, OSHA lists fatigue, headache, irritability, poor memory and dizziness. At 100 ppm the sense of smell can shut off within 2 to 15 minutes. So a worker at the ceiling limit may already feel it, and a worker at IDLH may not smell it at all.

That is why a meter, not a nose, decides. OSHA says the air must be tested by a qualified person with the right equipment, before anyone enters and continuously during any work where hydrogen sulfide is possible. See do sewer gas detectors work for which meter reads what.

### Reading a meter against the limits

Here's how the numbers play out on a meter, as an example. A maintenance tech opens the door to an ejector room and the meter reads 12 ppm for a few minutes. That's above NIOSH's 10 ppm recommended ceiling and below OSHA's 20 ppm general industry ceiling. It's legal in the narrow sense and still a reason to back out, ventilate and fix the leak. A reading of 25 ppm is over OSHA's ceiling, allowed only as the single 10-minute peak when there is no other measurable exposure that shift, and never above 50 ppm. A reading at or near 100 ppm means get out: that is NIOSH's IDLH.

OSHA also warns that repeated exposure can cause health effects at levels a person used to tolerate without any. So "the crew is used to it" is not a safety argument.

### Respirators and the 100 ppm line

OSHA's control guidance ties respirator choice to the IDLH:

• Below 100 ppm: at least an air-purifying respirator with cartridges rated for hydrogen sulfide. A full-face respirator also protects the eyes.
• At or above 100 ppm: a full-face pressure-demand self-contained breathing apparatus (SCBA) with at least 30 minutes of service life, or a supplied-air respirator with an escape bottle.

OSHA's fact sheet adds that a half-mask respirator must be paired with tight-fitting goggles, because eye irritation becomes a serious problem at elevated levels, and that workers in areas with hydrogen sulfide must be watched for signs of overexposure.

OSHA says any respirator use requires a full respiratory protection program under 29 CFR 1910.134: selection, fit testing, medical evaluation and training. Most small property teams aren't set up for that, which is the practical reason to keep staff out of pits and vaults and bring in people who are.

OSHA's numbers protect employees on the job. They don't set a safe level for a family at home or for customers in a store, and no OSHA rule covers a homeowner. For those spaces the practical rule is simpler: a sewer smell indoors means gas is leaking past a trap, seal or vent, and it gets fixed rather than measured and tolerated. If people have symptoms, they leave first.

For commercial plumbing, the usual exposure points are sewage ejector rooms, lift station wet wells, grease interceptor vaults, cleanout pits and manholes. OSHA's gas monitor bulletin groups manholes and other enclosed spaces with permit-required confined spaces (29 CFR 1910.146), where air testing comes before entry. A pit that smells is a plumbing problem to fix, and our page on why an ejector or lift station pit smells covers the causes. If a smell is in a restroom or suite, start with sewer gas smell in a commercial building.

If anyone has symptoms or the smell is strong, the exposure limit is no longer the question. Get people out and call 911. When to evacuate for a sewer smell covers that call, and first signs of sewer gas poisoning lists the symptoms.

HQ Plumbing is licensed and insured, serves commercial buildings across the Valley and offers 24/7 emergency service with upfront pricing. We fix the vents, covers, traps and lines that let sewer gas reach your staff.