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A strong chemical smell that spread across several districts of Bratislava this week came partly from the Slovnaft refinery, where maintenance work led to the release of vapours containing benzene and toluene, company representatives and local authorities have confirmed.

Residents in Ružinov, Vrakuňa and Podunajské Biskupice began reporting the odour earlier in the week, describing it as gas-like and unpleasant. The smell was strongest on Wednesday evening, according to local reports.

Slovnaft spokesperson Anton Molnár acknowledged that “some smell did come from us, both yesterday and the day before”, explaining that the refinery had been carrying out planned maintenance. “We announced this in advance, and such works can be accompanied by odours,” he told Slovak daily Denník N.

Benzene and toluene levels spiked briefly

Data from Slovnaft’s air quality station in Vlčie hrdlo recorded short-term spikes in the concentration of benzene and toluene around 19:00–20:00 on Wednesday. Benzene levels briefly rose from 3 to 217 micrograms per cubic metre, while toluene increased from 5 to 54 micrograms, according to the refinery’s publicly available app Sused Slovnaft.

Molnár said the company had taken measures to reduce the emissions and that levels “subsequently returned to standard values”. He stressed that the process was “planned, controlled and safe”.

Ružinov mayor Martin Chren linked the smell to Slovnaft but said there was no cause for alarm. “Slovnaft assured us that apart from the unpleasant odour, there should be no health risks,” he said, noting that the short-term spike would not exceed yearly safety limits.

Environmental experts said that the leaked substances pose a risk only with long-term exposure. Benzene, a by-product of oil refining, is a known carcinogen, while toluene is toxic and highly flammable but typically causes discomfort rather than acute harm at low concentrations.

Work at wastewater plant may have added to odour

The Slovak Hydrometeorological Institute (SHMÚ) confirmed elevated levels of several pollutants but said it could not yet determine the precise source without further data, such as operational logs or dispersion modelling.

Slovnaft later suggested that part of the odour detected in the city might also have come from ongoing repair work at the Petržalka wastewater treatment plant, operated by Bratislavská vodárenská spoločnosť (BVS). The water company said maintenance on a sludge tank could temporarily produce smells, though these were unlikely to spread more than 500 metres from the site.

This week’s incident is the second this year in which Bratislava residents have complained of a mysterious odour. A similar episode in February was later linked to tetrahydrothiophene, a harmless chemical used to scent natural gas.