Chapter 6: Hazardous Atmospheres
We often take the air we breath for granted. However, many gases in the environment have no color or smell, and we can not tell if the air is dangerous simply by looking at it. In excavations , these hazardous atmospheres frequently go unrecognized by workers until it’s too late. Then workers rush in to rescue their co-workers and often become the victims as well. Indeed, 60% of all workers who die from such atmospheres are the rescuers themselves.
The OSHA standard says that when working in trenches deeper than four feet that are dug in locations where hazardous atmospheres are likely to be present, atmospheric testing, ventilation, and respiratory protection, must be provided. Areas such as landfills, hazardous waste sites, chemical plants, refineries, and areas where underground storage tanks are present are all locations which may produce hazardous atmospheres. Planners who perform pre-construction site surveys should look for potential atmospheric hazards as well as the physical conditions of the area to be excavated.
Hazardous atmospheres include oxygen deficient environments, flammable/combustible/explosive environments andtoxic environments. An oxygen-deficient atmosphere means there is not enough oxygen in the space. Normal air has 20.8% oxygen. Levels below 19.5% are considered oxygen-deficient. Oxygen deficient atmospheres are dangerous and can cause unconsciousness, brain damage, and death. Flammable/combustible/explosive atmospheres contain gases or vapors in a certain concentration that can catch fire or explode if there is an ignition source. Toxic atmospheres contain gases or vapors which, if breathed in, can make you sick, or even die. Here are a few examples of the most common sources for hazardous atmospheres in excavations:
Oxygen deficient atmospheres:
In an open excavation, rain water passing over limestone, causes an acidity reaction, and in turn produces carbon dioxide. Carbon dioxide is a simple asphyxiant which replaces oxygen in the air we breath and can result in death.
Volatile organic compounds found in petroleum products can move through small spaces in soils and accumulate in excavations. This can create both a fire and toxic hazard. Buried tanks next to an excavation site are a common source of these compounds.
Another common flammable gas is Methane. Methane occurs naturally from the breakdown of organic materials, such as sewage, leaves or weeds.
Carbon monoxide from vehicles or equipment too near the excavation can accumulate and create a toxic environment for the workers.
When dealing with potential hazardous environments, early recognition is very important. Years ago, miners had to rely on canaries to tell them if the air they were breathing was hazardous. In today’s world, testing equipment for atmospheric hazards are compact and easy to use. One instrument can be purchased to detect the three most common atmospheric hazards found in excavation. The competent person understands and uses these direct reading instrument(s) that can detect the most common atmospheric hazards found in excavations. Continuous air monitoring is always a good idea because of changing conditions that can occur at a construction site.