Back when OSHA was writing their Permit-Required Confined Space standard (1910.146) their review of accident data indicated that most confined space deaths and injuries were caused by atmospheric hazards. And yet, still today, we see entrant, attendants, and entry supervisors; and even safety personnel, not understanding the atmospheric hazards associated with PRCSs. I hope to shed some light on these hazards and the rationale OSHA used when writing the standard with regards to the hazards found in PRCSs.
OSHA classified atmospheric hazards into three (3) categories:
- Toxic;
- Asphyxiating; and
- Flammable/explosive atmospheres
However, it should be pointed out that some chemical substances present multiple atmospheric hazards, depending on their concentration. Methane, for example, is an odorless substance that is nontoxic and is harmless at some concentrations. Methane, however, can displace all or part of the atmosphere in a PRCS1; and the hazards presented by such displacement can vary greatly, depending on the degree of displacement. With only 10% displacement, methane produces an atmosphere which, while adequate for respiration, can explode violently. By contrast, with 90% displacement, methane will not burn or explode, but it will asphyxiate an unprotected worker within about 5 minutes.
OSHA is concerned that employees could be exposed to atmospheric hazards because the employer has not properly evaluated the work operations or the conditions within the permit space. Problems can arise, for example, where an employer has not selected the necessary atmospheric test instruments or has not ensured their proper use. Problems had arisen because many of the instruments used to test the flammability of a permit space atmosphere back in the 1990’s did not identify oxygen deficient atmospheres. In fact, because some of these instruments rely on the presence of oxygen, their readings can be inaccurate in oxygen-depleted atmospheres. For example, instruments of the hot-platinum-filament type are designed to measure flammable gases and vapors in air. They depend on oxidation for their operation, and normal quantities of oxygen in the air are necessary for their correct operation. Any reduction in oxidation caused by lack of oxygen will result in a lower flammability reading. Such test instruments would indicate the absence of an explosion hazard simply because the atmosphere did not contain sufficient oxygen for combustion but would not indicate the oxygen deficiency that posed an asphyxiation risk.
On the other hand, a test performed only to determine the oxygen level might indicate that conditions are acceptable for entry without respiratory protection, despite the presence of 10% methane, an explosive level, in the atmosphere. Therefore, in the final rule, OSHA REQUIRED that employers test and monitor their entry spaces with instruments which will detect all aspects of hazardous atmospheres that may be encountered in the spaces.
OSHA presents the following examples regarding atmospheric hazards to illustrate how a relatively uncomplicated series of events can lead to workplace deaths and injuries. In each case, OSHA believes that death and injury would have been prevented if the procedures and safeguards required in this rule had been used. OSHA notes that the hazards confronted could only have been controlled effectively through the use of mechanical ventilation. OSHA recognizes that many confined space workplaces present situations which are more complex than those described in the following discussion.
a. Fatalities in ASPHYXIATING ATMOSPHERES
In its analysis of these confined space incidents, OSHA uses the term “asphyxiating atmosphere” when referring to an atmosphere which contains less than 19.5% oxygen. Oxygen levels under 19.5% are inadequate for an entrant’s respiratory needs when performing physical work, even if the space contains no toxic materials.
There are many potential causes of asphyxiating atmospheres. For example, the oxygen in a space may have been absorbed by materials, such as activated charcoal, or consumed by chemical reaction, such as the rusting of a vessel or container. In another situation, the original atmosphere in the space may intentionally have been wholly or partly inerted using such gases as helium, nitrogen, argon, or carbon dioxide. Victims of asphyxiation often are unaware of their predicament until they are incapable of saving themselves or even calling for help.
Three (3) incidents involving fatalities in asphyxiating atmospheres were discussed in the preamble of the NPRM (54 FR 24083). In addition, OSHA has received information during the rulemaking that further documents the hazards of exposure to asphyxiating atmospheres in permit-required confined spaces.
Example #1. A worker at a Texas steel mill was assigned the task of clearing a blockage at the No. 2 degasser vessel DUST COLLECTOR. He entered the vessel through an access manhole and proceeded to clear the obstruction. A coworker, assigned to assist, left the area to locate an electrical receptacle. About 10 or 15 minutes later, the coworker returned and found the worker who had entered the vessel unconscious. The coworker was able to remove the unconscious man and called for assistance. Unfortunately, the worker died. An oxygen test showed a level of 10% oxygen in the vessel. (The coworker was not injured.)
Example #2. A steel worker was asphyxiated when he entered a tank in the reagent storage building. There were no witnesses to the incident, but, since the tank had been used for the transport of nitrogen, it was assumed that the atmosphere within the tank was oxygen deficient.
b. Fatalities in TOXIC ATMOSPHERES
The term “toxic atmospheres” refers to atmospheres containing gases, vapors or fumes known to have poisonous physiological effects. The toxic effect is independent of the oxygen concentration. The most commonly encountered toxic gases are carbon monoxide and hydrogen sulfide.
Some toxic atmospheres may have severe harmful effects which may not manifest until years after exposure, while others may kill quickly. Some can produce both immediate and delayed effects. For example, while CARBON DISULFIDE at low concentrations may exhibit no immediate sign of exposure, it can cause permanent and cumulative brain damage as a result of repeated “harmless” exposures. At higher concentrations, it can kill quickly.
Two (2) incidents involving fatalities in toxic atmospheres were discussed in the preamble of the NPRM (54 FR 24083, 24084). In addition, OSHA has received information during the rulemaking that further documents the hazards of exposure to toxic atmospheres in permit-required confined spaces.
Example #1. A worker in Maryland entered a 6500 GALLON TANK TRAILER to finish cleaning the inside. He had with him a bucket containing about a gallon of a cleaning solvent (identified in the accident abstract only as “Niagara Trex 1900 Presol”). In only five to seven minutes the employee passed out and fell to the tank bottom. There was no ventilation, respirator or safety harness with lifeline provided. The outside “standby man” only checked the employee periodically (every three to five minutes). When the outside man discovered the unconscious employee, he attempted a rescue (without benefit of any protective equipment for himself) but was unsuccessful. He left the tank and called emergency personnel. The unconscious employee was rescued by emergency personnel and immediately transported to a hospital, where he was declared dead.
Example #2. An employee of a zinc refinery was working in a zinc DUST CONDENSER when he collapsed. Another employee donned a self-contained breathing apparatus (SCBA) and attempted to enter the condenser to rescue the downed employee. He was not able to fit through the portal wearing the SCBA, so he removed it, handed it to another employee and then entered the condenser. He planned to have the other employee hand the SCBA to him through the portal, re-don it and then continue with the rescue. He collapsed and fell into the condenser before he could re-don the SCBA. The first employee was declared dead at the scene; the would-be rescuer died two days later. The toxic air contaminant was later determined to be CARBON MONOXIDE.
c. Fatalities due to FLAMMABLE/EXPLOSIVE ATMOSPHERES
OSHA considers an atmosphere to pose a serious fire or explosion hazard if a flammable gas or vapor is present at a concentration greater than 10% of its lower flammable limit or if a combustible dust is present at a concentration greater than or equal to its lower flammable limit. This category of hazardous atmospheres includes atmospheres containing gases such as methane or acetylene; vapors of solvents or fuel such as carbon disulfide, gasoline, kerosene, or toluene; or combustible dusts, such as coal or grain dusts.
An incident involving five fatalities in flammable or explosive atmospheres was discussed in the preamble of the NPRM (54 FR 24084). In addition, OSHA has received information during the rulemaking that further documents the hazards of exposures to flammable or explosive atmospheres in permit-required confined spaces.
Example. An employee of a trailer service company entered a 8500 GALLON CARGO TANK to weld a leak on the interior wall of the tanker. Despite the presence of strong fumes of lacquer thinner (the material previously carried in the tanker) the welder decided to proceed with the repairs even though the written company safety policy required the use of an explosion meter at that point. When he began welding, an explosion occurred. The employee was removed from the tank and taken to a nearby hospital, where he was declared dead by the attending physician.
1 Methane is lighter than air when both are at the same temperature (the normal case), and the configuration of some confined spaces can trap accumulating methane at “ceiling” level. On the other hand, in the unlikely event that liquified methane is released into the atmosphere of a confined space, the methane released would be heavier than air and would displace the air from the “ground” level up.
Source: PRCS Preamble
