On Friday, February 24, 2014, the prefecture’s offices reported to the Nord-Pas-de-Calais Regional Health Agency an incident involving the perception of foul odors and mucosal irritation, which affected some of the staff at a public service agency employing 59 people. The main objectives of the investigation were, on the one hand, to precisely describe the symptoms and, on the other hand, to identify the cause. To achieve these objectives, three steps were implemented: Inspection of the premises: - conduct an initial assessment of the building’s condition; - identify a source of an environmental contaminant; - investigate the presence of social tensions.A measurement campaign: - to objectively confirm the presence of an environmental contaminant; - to identify the source of the contaminant release. An epidemiological investigation and clinical assessment: - to objectively determine the number of cases and the temporal and spatial dynamics of their onset; - to precisely describe the symptoms; - to investigate the existence of social tensions. The hypothesis that may explain the occurrence of this episode is a sewer maintenance operation that may have led to the release of gas from the sewer system. During the measurement campaign, ammonia was detected in moderate quantities, but it is possible that this was merely an indicator of a single, intense release of gas from the sewer system that triggered the initial symptoms. This phenomenon was exacerbated by poor indoor air quality due to insufficient air exchange and the general dilapidation of the premises. Furthermore, a probable herd effect linked to staff fears may have combined with these factors, leading to an increase in reports of mild, nonspecific symptoms by staff members and thus amplifying the incident. Finally, the epidemiological investigation showed that more than half of the staff was affected, but the clinical symptoms remained mild. Their nonspecific nature did not allow them to be correlated with an identified etiology. The actions taken in the wake of this event (cleaning and refurbishment of the premises, improvement of ventilation) and consultation with all levels of management via the Health and Safety Committee (CHSCT) allowed for the reopening of the premises in June 2014 without any further symptoms being reported.