A mysterious gas-like odor that spread across parts of the Greek capital in May, triggering evacuations and widespread public concern, was most likely caused by emissions from a liquefied petroleum gas (LPG) vessel operating off the coast near Athens, according to the most comprehensive investigation conducted so far into the incident.
Researchers at AtmoHUB, an atmospheric monitoring and forecasting unit of the National Observatory of Athens, concluded that a maritime area southeast of Salamis Island, a major shipping hub near the Greek capital, was the most probable source of the smell that drifted across southern Athens and parts of the city center on May 19.
The findings help explain an incident that puzzled residents and authorities for weeks. Despite emergency responses and widespread concern on the day of the event, no official explanation had previously been established for the odor, which many residents described as resembling a gas leak.
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The investigation analyzed 164 reports submitted by citizens through Greece’s Civil Protection service and combined them with meteorological data from the European Centre for Medium-Range Weather Forecasts. Researchers then used high-resolution atmospheric models to reconstruct the movement of air masses and trace the odor back to its likely point of origin.
The analysis indicates that the smell most likely originated from waters southeast of Salamis and was linked to a vessel operating in the area. Investigators consider an LPG carrier among the most plausible sources.
The substance responsible for the odor is believed to have been propane or butane, both commonly transported as liquefied petroleum gas. Although these gases are naturally odorless, suppliers routinely add sulfur-based compounds known as mercaptans to enable the detection of leaks. These additives create the distinctive smell typically associated with gas.
The conclusion is consistent with available environmental data. Air-quality monitoring stations, which track regulated pollutants, did not detect unusual concentrations of other contaminants. Researchers also ruled out natural gas leaks and naturally occurring environmental sources as likely explanations.
Authorities stressed that propane and butane are not considered toxic at the concentrations believed to have been involved in the incident. Nevertheless, the event caused significant public alarm and highlighted the challenges of identifying the source of unusual atmospheric phenomena in a densely populated metropolitan area.
On the morning of May 19, reports of the odor spread rapidly through southern Athens and the city center, prompting emergency inspections and the evacuation of several buildings as residents feared a potentially dangerous gas leak.
Researchers also examined the possibility that the odor originated from Psytalia, the island that hosts Athens’ main wastewater treatment facility. While prevailing winds on the day could theoretically have carried odors toward the city, the available evidence suggests that scenario is considerably less likely than the maritime-source hypothesis.
The investigation has also exposed weaknesses in Greece’s emergency-response capabilities. Scientists noted that if air samples had been collected while the phenomenon was occurring, authorities could have identified the substance with far greater certainty rather than relying primarily on atmospheric modeling and dispersion simulations after the fact.
The incident has prompted calls for the creation of a rapid-response protocol that would allow authorities to immediately collect air samples and conduct field measurements whenever similar events occur.
