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Introduction: Heavy rainfall in May 2016 caused large-scale flooding of the Seine and its tributaries. Analysis of this unusual event showed that it could recur on an even larger scale. The sanitary consequences were less frequently assessed in this analysis, particularly the risk of accidental collective carbon monoxide (CO) poisoning caused by the use of combustion engine drainage pumps. Methods: We retrospectively reviewed all cases of acute accidental carbon monoxide exposure observed in the Ile-de-France region, related to the use of drainage pumps in spring and summer 2016 and notified to the Ile-de-France CO poisoning surveillance network. Results: Five events were identified, including 45 people exposed to carbon monoxide. Thirty-four of these people were poisoned, 5 were not poisoned and insufficient data were available for 6 people. Three people showed signs of severity and 2 were treated by hyperbaric oxygen therapy. The other poisoned individuals were managed in hospital and treated by oxygen therapy. All were cured. Discussion: Collective CO poisonings are common sanitary events during flooding and can be potentially severe. They can occur during the event or over the following days. Preventive measures may help to reduce the risk of CO poisoning, such as increased awareness among professionals, better information of individuals who rent these types of devices or even the use of CO detectors during their use.