On April 22, 2016, the Normandy Regional Health Agency (ARS) received reports of health events, including headaches and nausea, among staff members at a private social services facility in Le Havre. The condition of those experiencing symptoms did not require specific medical treatment. These health events had been occurring primarily during meetings in a room within the building since July 2015. Epidemiological investigations and assessments of workplace quality of life related to these health events confirmed the presence of an unexplained mass illness. Environmental investigations identified that a ventilation malfunction, combined with the layout of the premises, may have triggered the onset of symptoms. These symptoms may have been exacerbated by the context of anxiety and confusion caused by the failure to address this environmental issue and the prolonged time it took to resolve it. A coordination task force was established by Cire Normandie, involving management and team members. The task force’s first meeting served to validate the findings, share information and insights from each participant on the topics discussed, and identify appropriate management measures. Ultimately, the information presented during these meetings helped establish a dialogue among stakeholders and reassure those affected based on concrete evidence.