Chemical Safety Bulletins
Incident: Toxic Gas Leak/Exposure
What happened? (Fall 1996) A researcher was conducting an ongoing experiment in a fume hood using hydrogen sulfide, a toxic, flammable, irritant gas. Due to the large size of the gas cylinder, it was secured next to the hood. During the experiment the researcher noticed the characteristic "rotten egg" odor of the gas. After repeated attempts to determine the source of the gas leak the lab was vacated and Risk Management and Safety (RM&S) was contacted for assistance.
With the protection of self-contained breathing apparatus, RM&S personnel determined the cylinder valve to be the source of the leak. The leak was small but could not be stopped by tightening the valve. The cylinder was disconnected from the regulator and experimental apparatus and transported to an outdoor location where it could safely vent. After consultation with the gas supplier, it was decided to forcibly close the cylinder valve by employing an adjustable wrench and hammer. The valve was turned more than 3/4 turn to finally close. The cylinder was returned to the manufacturer.
Why did it happen?
- A toxic gas cylinder was located outside of an exhausted enclosure.
- The main valve was always left open and the gas flow controlled with the regulator.
- The cylinder had been in the lab for many years.
How can a similar occurrence be avoided?
- Hydrogen sulfide is an extremely hazardous chemical that requires prior approval (Chemical Safety Level 3 or CSL-3) by the Institutional Chemical Safety Committee (ICSC). For approval, the Minimum Requirements for Toxic/Corrosive Gas Use (PDF format) will at least be required by the ICSC, which includes keeping the cylinder inside an exhausted enclosure (e.g. fume hoods or gas cabinets) where potential leaks can be vented away from lab occupants.
High pressure portions of compressed gas systems (i.e. from the cylinder to the regulator) are the likely locations for leaks. Unless you have a walk-in fume hood or a gas cabinet, this will require ordering compressed gases in cylinders small enough to fit into a standard fume hood. Always use the smallest practical size of cylinder for your gas applications. Had this been a gas undetectable by odor or irritation (e.g. carbon monoxide), the leak would likely have gone undetected until symptoms of exposure became evident.
Hydrogen sulfide acts similarly to hydrogen cyanide by inhibiting the utilization of oxygen by the body's cells. The recommended exposure limit (TLV) for hydrogen sulfide is 10 ppm as an 8-hour average and 15 ppm as a 15-minute average. While hydrogen sulfide has the strong odor of "rotten eggs" at concentrations below 0.1 ppm (100 times less than the TLV), its odor is not considered a good warning property. Prolonged or higher exposures may rapidly deaden the sense of smell and rising concentrations may not be noticed. Irritation is not evident until exposures reach 250 ppm and exposures to as little as 500 ppm may cause immediate unconsciousness and death.
- Close main cylinder valves when compressed gas cylinders are not in use. Do not stop the gas flow from cylinders by backing off on the regulator. Even the best regulators can develop seat leaks. Use a flow control valve to regulate flow – a regulator is for pressure control.
- Hydrogen sulfide may readily cause pipes and valves to corrode or become brittle. Lines, valves, and fittings should also be inspected frequently, and receive special attention, monitoring (i.e. leak testing), and maintenance to prevent leaks.
