At 8 a.m. concentration was 126.3 micrograms per square metre.
"We have never had such high concentrations of hydrogen sulfide. This is worrisome," said Peter Tot Djerdj of a civil initiative for clean air in Slavonski Brod, adding that past experience indicated that such concentrations were the result of accidents in the refinery in Bosanski Brod across the Sava river in Bosnia and Herzegovina.
He said the refinery had not responded to queries about what was going on.
Slavonski Brod Mayor Mirko Duspara said the city's authorities had sent a strong-worded protest letter to Bosnian Serb entity President Milorad Dodik and Prime Minister Zeljka Cvijanovic as well as urging Croatian President Ivo Josipovic and Prime Minister Zoran Milanovic "to protect citizens' health."
Duspara said no talks held on this ecological problem had yielded results.
City authorities reported that between January 15 and March 28 they had sent 34 memos to the environment ministry with information about the excessive air pollution in Slavonski Brod.