Some 30 people, including several children, have been taken to hospital after too-high levels of carbon monoxide were recorded in their homes in Rotterdam Zuid.
The fire brigade was alerted around midnight. A number of people had been complaining of headaches for several days, Nos television said.
People were taken to hospital by ambulances but eventually a bus was used as the number of people with complaints increased.
The source of the carbon monoxide was traced to a café heating boiler, which was next to a broken window opening on to the apartment complex hall.
People were allowed to return home after the fire brigade gave the all-clear at around 03.00 hours.
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