Exertional heat illness (EHI) presents significant risks for National Guard (NG) disaster response teams, especially when they are performing operations in impermeable personal protective equipment (PPE). Impermeable PPE does not allow passage of air or fluids either from the outside or inside of the equipment. While EHI prevention and management strategies are well documented, these strategies do not account for the additional heat-related risks NG teams confront when responding to disasters requiring PPE that protects against any hazards. NG personnel who wear the full gamut of impermeable PPE (including Trek coveralls and respirators) experience core body temperature increase as a result of builtup body heat or accumulated perspiration. We conducted a qualitative descriptive study using thematic analysis with three focus groups to identify EHI-related factors during disaster response operations that require PPE. We organized focus group data into phases of disaster response operation: pre-event, event, and post-event to reflect four conceptual groups: human (host), agent (energy transfer), environmental, and workplace/social conditions. Participants identified 12 themes covering the 3 phases and situated in the 4 conceptual groups. Results of this study serve as an evidence-based foundation for enhancing pre-event, event, and post-event assessments administered by NG medical personnel and can be applied to other professionals who are required to wear PPE.