Different from a shift average
IDLH values address acute danger and the ability to escape. Occupational exposure limits serve other objectives and periods. A concentration below IDLH can still be unacceptable for ordinary work. These references must not be interchanged or used to cancel one another out.
Escape is part of the hazard
Severe irritation, disorientation or respiratory injury may prevent leaving before serious harm occurs. Asking only whether a substance is lethal at a particular concentration is therefore insufficient. Effects impairing vision, mobility or the ability to act during an emergency also matter.
Respiratory protection and unknown atmospheres
Selection for an IDLH atmosphere requires appropriate atmosphere-supplying equipment and a specialist programme; a conventional filter does not make entry safe. Oxygen deficiency and unknown concentrations need specific assessment. Procedures, training, communications and planned rescue must exist, without improvised entry to assist another person.
Thirty minutes is not permission to work
The escape period considered in the historical derivation of IDLH references is not time available to spend working. In hazardous conditions, the aim is to leave exposure immediately following the established plan. A supposed permissible dose is not calculated by multiplying the IDLH value by minutes spent inside.
Preventive interpretation example
In a hypothetical example, a failure releases gas at an unknown concentration. A previous routine measurement below IDLH does not describe the current emergency. Access is restricted and the prepared team acts according to procedure, with atmosphere assessment and suitable equipment. An absence of smell does not demonstrate safety either.
Check the value and version
NIOSH maintains values derived using different criteria and explains how to identify updates. Substance, physical state, units and current documentation are checked. Reports must make clear that a NIOSH technical reference is being used, without automatically turning it into a universal legal limit or a substitute for full scenario assessment.
