Detecting IDLH Concentrations

29 CFR 1910.120 identified immediately dangerous to life or health (IDLH) as "an atmospheric concentration of any toxic, corrosive, or asphyxiant substance that poses an immediate threat to life or would interfere with an individual's ability to escape from a dangerous atmosphere." In this definition, it is important to note the word "concentration." At what concentration is a substance considered IDLH? Determining the IDLH concentration is one of the main aspects of risk identification.

29 CFR 1910.120(c)(5)(iii) states that employers should use direct-reading instruments to determine IDLH concentrations. As you learned earlier, direct-reading instruments give real-time (or near real-time) measurements, which allow employers to determine if an area is IDLH. These devices come in many forms, each with a specific monitoring purpose. Direct readings are often supplemented with the more precise results of "full-shift" monitoring and laboratory analysis.

Remember: Risk identification is ongoing. Employers determine IDLH concentrations at several points:

• Initial entry inspection (preliminary survey)
• Detailed survey inspection (upon entering the site)
• Periodic monitoring

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