OH CR4P!

Ever had tap water that smells like rotten eggs? Well then you've experienced the lovely aroma of hydrogen sulfide (H₂S), a gas produced by sulfate (SO4) eating bacteria. Besides the odor, there is little consequence of household levels of H₂S; that is, besides the chlorine added to kill the bacteria.

(<-- Credit: The Fast Plumber)

In an industrial setting, there are much bigger concerns than smelly tap water. Hydrogen sulfide is toxic even in low concentrated doses, and in higher concentrations is corrosive and flammable (though combustion is not a primary safety concern because toxicity occurs at much lower levels). It threatens the safety of workers in various industries where the gas collects or is handled, including tanneries, sludge plants, sewers, dumps, oil rigs, and paper mills.

Model of the H₂S molecule -->

H₂S is considered imminently dangerous in concentrations over 100 ppm. For comparison, "eggy" well water has concetrations from 0.1-6 ppm (equivalent to mg/L). In low concentrations (20-50 ppm), H₂S is an irritant to the eyes, nose, throat, and lungs. In medium (+200 ppm) to high doses (500 ppm), it can result in shortness of breath, or even rapid or immediate unconsciousness (known as the "knockout effect"). Highly concentrated exposure is defined as acute poisoning. It can occur with no warning at all, and can be fatal if not caught in time.

One of the biggest dangers of hydrogen sulfide is that exposure to concentrations above 30 ppm causes fatigue or paralysis of the olfactory nerve in a person's nasal cavity, which results in the loss of smell. Therefore, workers bearing the risk of exposure cannot sufficiently rely on their nose and the rotten egg smell to detect dangerous levels of the gas.

Past Prevalence

There have been countless accidents involving hydrogen sulfide, many of them resulting in fatalities or severe injuries.

To just name a few within the past 15 years:

• poisoning in a Geothermal Power Plant - 1998 (forensic report pdf)

• gas leaking from an underground process sewer in a mill in Pennington, Alabama - 2002 (investigation news)

• sewer hole accident at landfill in Wisconsin - 2007 (efilmgroup report)

• hydrogen sulfide inhalation on a Virginia dairy farm - 2007 (safety report pdf)

• maintenance procedure gas incident in Lubbock, Texas - 2009 (news report)

• dry well gas poisoning at Queens, NY recycling plant - 2009 (NYtimes report)

I'll let you read about the incidents yourself, but suffice to say they are testament to the severity of toxic gas hazards and what they can amount to in situations where proper precautions are not taken.

(Credit: Mysafetysign.com -->)

Striving for Safety

Some of these H₂S accidents occurred because of failed systems or particularly abnormal environments, such as the deadly reaction at the Georgia-Pacific Naheola mill in Pennington. But many others, like the accidents in Queens and Lubbock, could have been prevented if not for a combination of poor choices and ignored standards. And while one could spend all day blaming the system or environment, the reality is that nothing should be considered inherently safe when a toxic gas may be involved. Accidents will inevitably happen, says Murphy's Law, and workers need to properly prepare for them.

Some of the most fundamental measures that can be taken include air testing and gas monitoring, either done continuously in susceptible environments or on a case-by-case basis by personnel (often using portable gas detection devices). In the Queens, NY, recycling plant accident, the sewage treatment worker was at fault for entering the reportedly clogged sewer without first checking the air quality for traces of toxic gases.

OSHA standards specify that once H₂S is detected in a space or area, it must be ventilated continually to remove the gas. In cases where the gas cannot or should not be removed, workers in contact with or entering the contaminated space must use appropriate equipment, including breathing safety equipment and rescue and communication equipment as needed.

H₂S single gas detector - (Credit: Durawear)

But as with any hazardous or potentially dangerous work, often the most valuable prevention strategy is knowledge and training. As a general rule, informed workers are more apt to take proper safety precautions, and are less likely to make poor decisions in the midst of an emergency. Specific knowledge about H₂S and its dangers could have prevented many injuries and fatalities that resulted from ill-conceived rescue attempts where the rescuers became victims, or from personnel who chose to remain after the smell of rotten eggs dissipated from their senses.

The hope is that the industries that deal with hydrogen sulfide are continually improving their safety practices and reducing risks to workers at these job-sites. The health effects and dangers of this gas, and others like it, need to be taken seriously.

Sources

Drilling Contractor - How to ensure H₂S safety on offshore rigs

OSHA - Hydrogen Sulfide Fact Sheet

Swopnet.com - Hydrogen Sulfide Kills