We have finally arrived at Part 4 of our Confined Space Atmospheric Hazards series. We have already discussed some general concepts and principles of performing atmospheric monitoring at confined space incidents, oxygen-related hazards, and explosive atmosphere hazards. In this post, we are going to focus on the two toxic gas sensors that most emergency services agencies use – carbon monoxide and hydrogen sulfide.
Carbon monoxide (CO) is an odorless, colorless, and tasteless gas produced by the process of incomplete combustion. The Immediately Dangerous to Life and Health (IDLH) level of CO is 1,200 ppm. Most four-gas detectors are set to begin alarming at the “low” level when the CO level reaches 35 ppm, which is the NIOSH Recommended Exposure Limit (REL). 35 ppm is not enough to immediately kill or incapacitate a person, but it does tell you that there are hazardous atmospheric conditions inside the confined space and that you should utilize the appropriate type of respiratory protection if entering.
Carbon monoxide’s explosive range falls between 12.5% and 74%, which is relatively wide and can be easily encountered in confined spaces. Lastly, CO’s vapor density is 0.97, which means it is just slightly “lighter than air.” This means you will possibly find CO close to the top of the confined space, but it may also be present throughout the space.
Hydrogen sulfide (H2S) is a colorless gas that has a strong smell of rotten eggs. While the gas is easily perceptible initially, it can also wipe out your sense of smell. Obviously, this is why we need to rely on our gas detectors and respiratory protection and not our sense of smell at confined space incidents. H2S is naturally produced by decaying organic matter and is commonly found in confined spaces.
The IDLH for H2S is 100 ppm, which means it is more toxic than CO. Most four-gas detectors will alarm at 10 ppm, which is the NIOSH REL for H2S. The explosive range is 4% – 44%, which makes it slightly lower, but also narrower than CO. Lastly, the vapor density of H2S is 1.19, which means it is “heavier than air” and will be found at the bottom of the confined space.
On January 16, 2017, three workers in Florida were killed and one firefighter was injured in a confined space. Workers were investigating a smell of rotten eggs reported by the people living in the neighborhood. After the first worker entered the confined space and became unconscious, two of his co-workers went in to attempt to perform a rescue and all three of them died. The firefighter also entered the confined space without an SCBA, but luckily survived. H2S and Methane were present in the confined space.
Stay safe out there. Know the hazards and know your gas detectors!
Owner / Lead Instructor
Elder Technical Rescue Services, LLC