On Monday morning, Jan. 16, 2017, four utility workers employed by a private construction contractor responded to complaints of a sewage backup in Key Largo, Florida. One worker removed a manhole cover and descended into the 15-foot-deep drainage hole. Voice contact was lost. A second worker climbed down to help. When he also stopped responding, a third worker climbed down.

Aid was summoned. A Key Largo Volunteer Fire Department firefighter, Leonardo Moreno, could not fit through the opening with his air tank. In a desperate attempt to save the utility workers, he entered the manhole without it, and lost consciousness within seconds. Another firefighter managed to enter with an air tank, and pulled Moreno out; he was hospitalized in critical condition. But there was no such luck for Elway Gray, Louis O’Keefe and Robert Wilson. All three utility workers died from hydrogen sulfide poisoning.

This is not an isolated incident.

What is hydrogen sulfide?

Hydrogen sulfide is a colorless, toxic gas that can be created by the decomposition of organic matter such as rotting vegetation or wastewater transported in a sewer system. Ranked with cyanide for toxicity, H2S is the second-most common cause of fatal gas inhalation exposures in the workplace, after carbon monoxide. In the U.S., H2S accounts for 7.7 percent of such cases.

Why does H2S form in sewers?

H2S is formed by anaerobic processes in the thin slime layer that develops on the inside of the sewer walls as organic material in the sewage breaks down. The formation of H2S in wastewater mainly depends on:

• Flow (velocity) of sewage in pipes
• Slope of the pipe
• Ratio of wetted perimeter of the pipe wall to surface width of the stream
• Temperature of the sewage
• BOD, Biochemical Oxygen Demand
• Presence of sulfates
• pH
• Available oxygen
• Retention time in the system

Why is H2S so dangerous?

Certain characteristics of H2S, besides its general toxicity, make it particularly dangerous:

Steep exposure-response curve. The exposure-response curve, showing time to lethality as a function of concentration, is extremely steep. At 50 ppm, it is a nonfatal nuisance; at 300 ppm, death occurs after a few hours; and at 1,000 ppm, death occurs in slightly under 10 seconds.

Instant loss of consciousness. H2S has been known to cause immediate loss of consciousness at fairly low levels: estimates are as low as 250 ppm. Unconscious, the victim has no chance to flee. If the unconscious victim is rescued and removed to fresh air, the effects may be treatable. However, if the victim is working alone, or their co-worker attempts a rescue and succumbs in turn, then the duration of exposure to H2S may be prolonged and potentially fatal, even at H2S concentrations which are not normally considered as deadly.

Enclosed spaces. H2S is slightly heavier than air. For that reason, it accumulates in subsurface spaces. The wastewater industry is full of these, in the form of excavations for sewer lines, valve chambers, lift stations and sewer lines.

Soda-can effect. When an unopened soda can filled with a carbonated beverage is shaken, and then opened, the carbon dioxide dissolved in the water enters the gaseous phase with explosive effect — so explosive that the person opening the soda can is often drenched in soda. H2S is 10 times more soluble in water than carbon dioxide. A sewer main can contain huge amounts of undetected H2S dissolved in the wastewater. As a result of the “soda-can effect,” this can turn into gaseous toxic clouds when the water is disturbed, for example when pumps are activated.

No warning odor. H2S has an extremely characteristic odor similar to rotten eggs. At low to very low concentrations, humans smell and recognize the odor. Above 100 to 150 ppm, however, the neurotoxic effect known as olfactory paralysis sets in; just as concentrations become hazardous, our most important warning sign disappears.

Protecting against H2S

As it results from the decomposition of organic matter, hydrogen sulfide is a core element of our industry. There are ways to protect workers, the most important of which is to have a comprehensive confined-space entry program, supported by training for workers.

Planning before entering a confined space is critical. Pre-entry planning includes:

• Having a competent person evaluate the work site for the presence of confined spaces. Note that the term “competent person” does not refer to a character trait; in many jurisdictions, there is a specific definition of a competent person. The competent person has the knowledge and training to assess the situation for hazards, and the authority to stop work if necessary, so that engineering modifications (blocking incoming lines, adding forced ventilation, etc.) can be put in place. If the confined space contains hazards to persons who may enter, then the competent person normally classifies it as a permit-required confined space.
• Identifying the potential hazards in the confined space.
• Using engineering modifications such as source isolation or proper ventilation methods to, insofar as possible, remove or control potential hazards in the space.
• Atmospheric testing before entry, for oxygen levels, flammable and toxic substances, and stratified atmospheres. If the air in a space is not safe for workers, then it must be established whether it can be improved through ventilation or other engineering controls, such that employees can safely work in the space. It must also be determined whether entry can be permitted by persons using supplied-air respirators.
• Identifying the means of entry and exit.
• Ensuring that the required personal protective equipment is available.
• Determining rescue procedures and necessary equipment.
• Ensuring that there is a rescue plan, that everyone is familiar with the rescue plan, and that rescue equipment is available at the site.

While people are in the confined space:

• Monitoring the space for hazards, especially atmospheric hazards, must continue.
• Communication is important at all times: between workers in the confined space and those outside, but also, because there can be multiple contractors operating on a site, each with its own task.
• External attendants monitoring confined spaces must make sure that unauthorized workers do not enter them.

Equipment: The personal protective equipment necessary is determined by the nature of the confined space and its dangers. Proper training on the use and maintenance of PPE is key. Special attention should especially be paid to the regulatory requirements when respirators or a self-contained breathing apparatus are needed, as respiratory protection can require additional training or permits.

In addition to PPE, examples of equipment needed to work in confined spaces may include:

• Testing and monitoring equipment
• Communications equipment
• Lighting equipment
• Ventilating equipment
• Ladders
• Retrieval devices
• Barriers

For more information, the Occupational Safety and Health Administration (www.osha.gov) fact sheet “Confined Spaces in Construction: Sewer Systems” is an excellent starting point.