Deadly Sewer Gas
Julius Ballanco, PE
You may have missed the news story about three crew members dying while repairing a waste line aboard the Royal Caribbean ship “Monarch of the Seas.” The ship was docked at the Port of Los Angeles.
The accident report indicated the crew members were replacing a section of pipe connected to the ship's sewage system. The broken line released about 5 gallons of sewage, as well as sewer gas into the starboard propeller shaft tunnel.
While the ship provides breathing apparatus for certain repair operations, this was not considered a repair requiring such apparatus. After the medical emergency was declared, a rescue team - wearing breathing apparatus - went into the tunnel to retrieve the bodies. Los Angeles County fire officials first identified the gas that killed the workers as methane. However, after further analysis, they determined it was hydrogen sulfide. Of course, both methane and hydrogen sulfide are constituents found in sewer gas. There can be a number of other chemicals in sewer gas, as well.
Unlike methane, though, hydrogen sulfide is slightly heavier than air. Rather than rising to vent out of a space, hydrogen sulfide sinks and concentrates at lower levels.
When I heard the first report that it was methane that killed the crew members, it didn't make sense. Methane is not toxic when you breathe it. However, methane can displace the oxygen in the air, resulting in asphyxiation. Normally, when a worker dies from asphyxiation, that is how it is reported, not a death from methane gas.
When the investigation changed to death by consumption of hydrogen sulfide, that made complete sense. A brief exposure of high concentration of hydrogen sulfide can kill you; the gas is toxic. This is an important fact often missed by our industry.
What, Me Worry?You may be asking, how much hydrogen sulfide does it take to kill a person? I checked with the federal government. It reports that concentrations in excess of 500 ppm (parts per million) can be fatal. High concentrations would only take two to three breaths to kill you. Hydrogen sulfide is also flammable and explosive in a range of 4 percent to 45 percent concentration.
The government has set a workspace maximum allowable concentration of hydrogen sulfide at 20 ppm. It also reports that there appears to be no long-term effects from exposure to hydrogen sulfide.
A person that inhales a large quantity of hydrogen sulfide can pass out at levels below the lethal dosage. When they recover, there is no damage to the individual. However, long-term, continuous exposure can result in headaches, poor attention span, poor memory and poor motor skills.
If it makes you feel any better, you can begin to smell hydrogen sulfide when concentrations reach 0.5 ppb (that's parts per billion). Hence, you will know that hydrogen sulfide is present long before it reaches deadly levels.
It's not only sewer gas that produces hydrogen sulfide. Another source of the toxic gas is some poor-quality drinking water. When you smell rotten eggs in water, that is the hydrogen sulfide.
So, how worried should you be? Not too much. But, you have to keep your guard up. The crew members on the ship entered a confined space. The concentrations of hydrogen sulfide could easily reach a lethal level in such a confined space.
Of course, this is one of the reasons why the confined space rules are so important. It is not just a manhole that can result in a workman's death. A confined area in a building also can result in a high concentration of contaminants.
Whenever I mention the toxic contents of sewer gas, there are some who run around spouting off that we can't do this or can't do that because sewer gas may leak into a home and kill the residents. Leaking sewer gas in a home, for whatever reason, it not a known killer. The only reported incidents I have found are of workers, never building occupants.
The reason the occupants are much safer is because the public has come to expect a home that does not stink of sewer gas. The odor level is so low that the occupants would leave long before there ever was a toxic concentration of hydrogen sulfide.
The only problems with leaking sewer gas has been that it stinks. In very rare cases, individuals have suffered headaches.
That is not to say that sewer gas should leak into a building. It should not. It is simply that you cannot completely avoid leaking sewer gas. I don't care what we do in this profession, there is no way to prevent sewer gas from being present in every building. To do that, the first thing we would have to do is prevent sewer stoppages. As far as I know, we haven't come close to eliminating stoppages in drain lines.
When there is a back-up in a drain line, what do you think leaks out as well? Right, sewer gas! Part of that sewer gas can be hydrogen sulfide.
So use this unfortunate tragic accident as a reminder to always know your environment. Don't open a cleanout in a confined space. Realize that the simple opening of a cleanout or repair of a drain line can expose you to high concentrations of hydrogen sulfide.
Another reminder is to properly seal all drainage connections when renovating or adding to a plumbing system. Those makeshift seals of drainage connections, such as shoving crumbled newspaper into a pipe, don't work. Manufacturers have provided us with wonderful temporary seals to use for these connections. I would urge you to use them. Not only are you protecting your customer, you are protecting your employees. So be smart when it comes to sewer gas.