In response to the sudden onset of irritative symptoms affecting a group of people in June 2013 at a public education and research institution in Paris that was under construction, an investigation was launched by the Île-de-France Regional Intervention Unit (Cire) of Santé publique France. The investigation followed the protocol established for the diagnosis and management of unexplained mass illness events (InVS, 2010), in collaboration with the Regional Health Agency, the poison control center, the administration, and the Health and Safety Committee (CHSCT) of the affected institution. In October 2013, following the investigation, employees working in another part of the building reported chronic irritative symptoms associated with the occurrence of odors and were included in the study population. The coordination team, composed of the facility’s technical advisors and members of the Health and Safety Committee (CHSCT), was involved in developing the investigation tools, conducting site visits, sharing information, and discussing etiological hypotheses. The investigation confirmed two collective episodes of irritative symptoms, most likely resulting from different multifactorial phenomena. The first episode, which affected one construction worker and at least 21 staff members between June and August 2013, may have been the result of acute exposure to volatile hydrocarbons contained in a glycerophthalic paint applied to pipes in the building’s basement. The layout of this area and its poor ventilation may have contributed to the dispersion and stagnation of these vapors on the upper floors. For one of the affected teams, signs of discomfort at work were addressed with specific care. The second incident, which affected 12 people between September and November 2013, was characterized by the random occurrence of irritating odors and symptoms among different teams. It had begun as early as 2012. The chronic nature of the complaints and associated odor nuisances, without any particular difficulties identified at the organizational level or regarding working conditions, pointed to an unknown and uncontrolled intermittent chemical exposure. Several environmental investigations conducted jointly by the Scientific and Technical Center for Building and the City of Paris’s Hygiene Laboratory led to the hypothesis that staff were exposed to secondary chemical reactions occurring in the laboratory exhaust ducts before the air was released onto the roof. The first health incident, which was limited in duration, did not require any specific actions regarding the building. However, for the second incident, a technical intervention aimed at improving the dispersion of atmospheric emissions on the roof was carried out.