In a southeast Moscow apartment, a resident reported an unknown substance affecting several people on the evening of August 14. About 40 residents were evacuated, and three sought medical attention, according to authorities and local reports.
Three residents on Octyabrskaya Street called for help after falling ill. Ambulances arrived, but the patients and several others nearby appeared unwell. The entire entrance was evacuated to the street as a precaution, a law enforcement source told TASS. Mosgortrans organized two buses to provide temporary shelter for the evacuees.
Two people were later described as injured and stabilized in hospital. The two women who were poisoned were listed in moderate condition, with non life-threatening symptoms. A male family member who initially sought care declined hospital treatment. Upon arrival, responders found the family and their dog convulsing, and the apartment smelled of a chemical odor. Officials later said reports of health workers being injured were unconfirmed. The five floor building residents were relocated to a temporary facility near a cultural center named after IM Astakhov. By around 01:40, residents were allowed to return home.
Initial versions suggested a gas leak at the site. Regional health and safety authorities moved in to inspect, conduct laboratory analyses, and perform instrumental tests to determine causes. On-duty units from emergency services and fire rescue centers supported the response, along with city utilities. After the first air samples were collected, authorities reported no hazardous concentrations in the air.
The city’s municipal services complex posted on its Telegram channel that there was no evidence of a network gas leak, challenging early media reports. The MOSGAZ laboratory also confirmed no network gas was detected. Officials noted that entrances were checked with gas analyzers, samples were collected, and tactical ventilation measures were applied as needed. Fire and rescue specialists and Mosgaz remained on scene, and the city’s operation center coordinated the response.
Nevertheless, some sources cited by news agencies claimed that carbon dioxide levels in the building were well above normal limits, with reports suggesting a fourteenfold increase.
Several competing theories emerged about the cause of the incident. Some media outlets reported that investigators found unusually high levels of hydrogen cyanide, a chemical compound present in some plants and used in various industrial processes. Exposure to hydrogen cyanide can occur through industrial accidents or unsafe handling of certain chemicals. Other accounts, shared on messaging channels, proposed methane buildup in the sewage system as a potential factor, noting that gas may accumulate in pipes and that water seals in the sewer system typically mitigate such risks.
As officials continued to assess the scene, there was no consensus on a single explanation. The evolving narrative reflected the complexity of rapid on-site investigations where multiple pathways are considered. Inquiries from regional health authorities, emergency services, and environmental surveillance teams remained ongoing to determine whether an indoor gas source, a chemical release, or another mechanism caused the symptoms observed in residents. Reports from TASS, RIA Novosti, and other outlets highlighted the importance of thorough testing and verification before drawing final conclusions. Attribution for some early details was provided to those agencies, while other statements reflected ongoing procedural updates from the city and regional authorities.