Zenica is an industrial town that produces steel and where many homes are heated by burning coal. That sometimes leads to significant air pollution.
Samir Lemes of the citizens association Eco Forum said the concentration of sulfur-dioxide in Zenica's air reached 1.400 micrograms per cubic meter Tuesday, far above the acceptable concentration of 350 micrograms.
Authorities ordered factories to stop production if they have no alternative to burning coal. Families were told to begin burning wood to heat their homes.
The cities of Tuzla and Sarajevo were suffering heavy fog, partly the result of air pollution, but no orders about the burning of coal were given.