Measuring Exposure Levels

Combustible chemicals are those whose vapors or gases ignite when mixed with oxygen. The flammable range is a percentage of the total volume of air called the Lower Flammable Limit (LFL) to the Upper Flammable Limit (UFL) or LFL-UFL. Industry expresses this combustible range as the Lower Explosive Limit (LEL) to the Upper Explosive Limit (UEL). For the purpose of this course, we will use the industry standard.

The LEL is expressed as a percentage in the air that will ignite vapors or gases. OSHA has set the safe entry level at 10% of the LEL; any level above 10% should generate an audible alarm on atmospheric monitoring devices. Ensure that your entrant completes active monitoring of a confined space before and during entry.

Recommended Exposure Limits (RELs) were compiled by NIOSH and are recommended levels, thereby limiting their use to chemicals not covered by OSHA. RELs can be found in the NIOSH Pocket Guide to Chemical Hazards, which we will discuss later on in this course.

Exposure levels for toxins can be determined by utilizing the permissible exposure limit (PEL). The PEL is exchangeable with the threshold limit value (TLV), but for the purpose of this course we will refer to this limit as the PEL. The PEL levels are based on average exposure over an eight-hour workday. The PEL is used to determine the acceptable level of respiratory equipment, set up safety zones to restrict personnel, and prepare site safety plans.

Further PEL information can be found in OSHA 29 CFR 1910.1000.

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