To provide standardized recommendations for the emergency department (ED) response to chemical, biological, radiological, and nuclear (CBRN) events by combining the human factors/ergonomics method of hierarchical task analysis with the theoretical framework for Work as Imagined versus Work as Done.

Document analyses were used to represent CBRN response operational procedures. Semi-structured interviews using scenario cards were carried out with 57 first receivers (ED staff) to represent CBRN practice at 2 acute hospitals in England.

Variability existed in general organizational responsibilities associated with the CBRN response. Variability was further evident in top level CBRN tasks and CBRN phases at both EDs. Operational procedures focused on tasks such as documentation, checking, and timing. CBRN practice focused on patient needs through assessment, treatment, and diagnosis.

The findings provide top-down and bottom-up insights to enhance the ED CBRN response through standardization. The standardized CBRN action card template embeds the choice approach to standardization. The standardized CBRN framework implements the streamlined categorization of CBRN phases. Work as Imagined versus Work as Done is a useful theoretical framework to unpack a complex sociotechnical system, and hierarchical task analysis is an effective system mapping tool in health care.