In the OSHA Standard, Permit Required Confined Space Entry (29 CFR 1910.146), Appendix B, Procedures for Atmospheric Testing, there are broad recommendations for sampling confined spaces. Topics include testing stratified atmospheres, duration of testing, and verification testing. There is no detailed, nor step- by step guidance in the standard. OSHA expects that "technically qualified professionals" will be collecting and evaluating the data. A useful discussion of sampling can be found in the preamble to the standard which can be obtained directly from OSHA's Office of Publications at 202-219-4667.
There are standard procedures that represent good practice and should always be applied. Testing should be completed for oxygen before testing for combustible gases because if the oxygen level is low, the combustible gas meter may not operate properly. The absence of solvent vapors in concentrations high enough to register on a combustible gas meter does not mean that the solvent vapors aren't in concentrations sufficiently high to cause deleterious health effects. The order for sampling is to look at oxygen levels first, then combustible gases, and then toxic agents. This is a requirement of the OSHA standard under paragraph (d)(5)(ii).
Sampling must take place at the level that workers will enter. There have been fatalities after air sampling because the samples weren't collected at the lowest level workers would enter. Good confined space sampling equipment comes with probes that can be lowered into the space. It is important to note that hand-operated aspirator bulb equipment requires much effort to purge a lengthy hose and draw a proper sample from a confined space.
It is absolutely critical to examine the limitation of real-time instruments used to monitor for toxic agents. Many of these units are not specific for the agents listed but may have interferences. These should be noted in the manual. Battery life may also pose problems in a confined space. Additional batteries are important, particularly for remote sites. It is crucial to determine which agents may be present. Hydrogen sulfide and carbon monoxide detector cells are the most common on real-time instruments.
OSHA carefully reviewed the ANSI standards Z117.1-1977 and Z117.1-1989 and considered both to be too heavily weighted towards air sampling rather than considering other risks in confined space entry, such as engulfment or being crushed by equipment. These documents may prove an excellent source on air sampling information, however, particularly the 1989 version which replaced the earlier one.
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Last modified: 04/18/96 08:50:32