By Gary Morris
A 21-year-old worker was cleaning and repairing a drain line in a wastewater holding tank when he collapsed and fell face down into six inches of water at the bottom of the tank. Sulfuric acid was used to unclog a floor drain leading into the holding tank. A second worker attempted a rescue and was also overcome and collapsed. The first worker was pronounced dead at the scene and the second worker died two weeks later. Cause of death was attributed to asphyxiation by methane gas. Sulfuric acid vapors may have also contributed to the cause of death.
This scenario is typical in many confined space-related fatalities. Either the space wasn't initially monitored to ensure the air was safe for entry, or the work conducted in the space rendered the space unsafe. One of the critical elements of the Occupational Safety and Health Administration's (OSHA) Confined Space Standard is atmospheric monitoring. Effective confined space management programs begin with formal atmospheric monitoring procedures. It's important that employers understand the various issues surrounding atmospheric monitoring, including regulatory requirements and instrument specific instructions.
InstrumentationAtmospheric monitoring is conducted to evaluate hazards of a permitted space and to verify that acceptable entry conditions exist for that space. In order to select and properly use atmospheric monitoring instrumentation, the user should be aware of what kind of hazards they may expect during confined space entry. Once the hazards are known, the user can select the instrument and monitoring features that best meet the needs of the worksite. Potential sources of hazardous atmospheres that may be found in water treatment facilities include a lack of oxygen or the presence of hazardous gases/vapors such as hydrogen sulfide or combustible gases. It's recommended that employers contact several instrument representatives for demonstrations prior to selecting an instrument.Since the data obtained from testing is only as good as the instrument used, calibration of the instrument is a critical link in hazard evaluation. Some instruments are advertised as "low maintenance," requiring only annual or biennial calibration. However, users should be aware that infrequent calibration of their instruments might not ensure the accuracy of the device during subsequent use. There is no guarantee that the sensors and electrical components of the instrument will remain stable over a 30-day or 12-month period.
In addition to periodic calibration, users should also perform a functional test before each use by zeroing the instrument in uncontaminated air and then introducing a calibration gas of known concentration. This will demonstrate that the sensors respond to the test gas and the instrument alarms function properly. Please note that only certified calibration gases that are within their expiration date should be used. In any event, always consult with the manufacturers' technical representatives to determine the most pro-active instrument maintenance/calibration recommendations.
When and Where to TestOSHA establishes atmospheric testing requirements in 29 CFR 1910.146. Hazards must be evaluated in the following order: oxygen content; flammable gases and vapors; and potential toxic air contaminants. Be sure to test all areas of the space, as stratification can occur. The space should be initially tested by dropping the probe (with an extension) into the hole. Once the entry area of the space is cleared, then the other areas of the space can be monitored by walking the instrument through the space. Periodic testing of the space during entry may be necessary, depending on the results of initial tests and/or the nature of the work to be conducted during entry.If a flammable gas or vapor was detected during initial entry, or if decreased oxygen was noted, periodic testing would be required to ensure that conditions in the space remain safe for occupancy. Likewise, if the work to be conducted in the space could adversely affect the air in the space, periodic testing would be required. For such conditions, personal, continuous monitoring is strongly recommended.
Is 19.5 percent Oxygen A Safe Level for Entry?Most confined space guidance documents define oxygen deficient atmospheres as less than 19.5 percent. This definition is derived from 1910.146, which lists as one of the definitions of a hazardous atmosphere as atmospheric oxygen concentration below 19.5 percent. However, the language in the standard does not say that 19.5 percent is safe; rather, it says that levels below 19.5 percent may be hazardous.As such, employers usually permit entry into spaces as long as the oxygen concentration is between 19.5 percent and 23.5 percent. Caution should be taken in using 19.5 percent as the acceptable level for entry. Since the normal percentage of oxygen in ambient air is approximately 20.8 percent, any reading higher or lower than 20.8 percent should be viewed as a warning and subsequently investigated. Such readings may indicate that there could be something other than oxygen in the space, and that the agent may further affect the percentage of oxygen during the entry period. Oxygen levels decrease when something has entered a space that has the ability to displace oxygen. That something could be another gas or a toxic contaminant such as hydrogen sulfide or an organic solvent.
ConclusionFormalized atmospheric monitoring procedures are an integral component of an effective confined space entry program. Written standard operating procedures (SOPs) should be developed for instrument care, calibration, use, and interpretation and all employees involved with the program should be trained on the SOPs. Cutting back on time and resources needed to ensure the accuracy of atmospheric monitoring instrumentation does not justify the risk to entry personnel. WW/About the Author: Gary Morris, MS, CIH, CSP, is an independent industrial hygiene and safety consultant, specializing in OSHA compliance, ergonomics, and noise and chemical exposure assessments. He can be reached at (301) 865-0101, or by email at [email protected]
