The atmosphere in a confined space can be extremely hazardous due to the lack of air circulation. Because fresh air can’t move freely in and out of
a confined space like storage tanks, pipes, silos and manholes, a variety of hazardous conditions can be created. Deadly gases may be trapped inside, especially if the confined space is used to store or process chemicals. There may not be enough oxygen in the space to support life, or the air could be so oxygen rich that it could cause a fire or explosion if ignited. There are three types of hazardous atmospheres that confined space workers should be aware of:
- Toxic Atmospheres: The product found in a confined space can be absorbed into the walls and floors of the confined space and give off toxic gases. For example, sludge cleaned out of the inside of a tank can give off deadly hydrogen sulfide gas. Also, the work being performed in the confined space can also generate toxic fumes. Welding, cutting, sanding, scraping and cleaning can all give off toxic vapors.
- Oxygen-Deficient Atmospheres: An oxygen-deficient atmosphere has less than 19.5% available oxygen. Any confined space with less than 19.5% oxygen should not be entered without wearing a self-contained breathing apparatus. Oxygen can be removed from the atmosphere by work such as welding, cutting or brazing. Oxygen levels can also be decreased if replaced with other gases like carbon dioxide or nitrogen, or by chemical reactions inside the confined space like rusting or fermentation.
- Flammable Atmospheres: Two things make the atmosphere flammable, oxygen in the air and a flammable gas, vapor or dust in the right mixture. If a source of ignition like a spark from a tool occurs an explosion will result. An oxygen-enriched atmosphere (over 21% oxygen) will cause flammable materials like clothing and hair to burn violently. Therefore, don’t use pure oxygen to vent a confined space. Ventilate with normal air.
Never trust your senses to determine if the air in a confined space is hazardous. Many toxic gases and vapors can’t be seen or smelled. Be sure to test the atmosphere with a properly calibrated testing instrument. If testing reveals a hazardous atmosphere, be sure to take precautions like ventilating the space or ensuring workers have the proper respiratory protection. Posting Confined Space Signs will also remind workers to take proper precautions and wear the appropriate protective equipment before entering any confined space.
Source: “A Guide to Safety in Confined Spaces” by Tom Pettit and Herb Linn