A hydrogen fluoride leak on the cargo ship Mia Summer II at the Deurganck Dock in the Port of Waasland in Antwerp led to 155 people undergoing medical checks, local authorities in Beveren-Kruibeke-Zwijndrecht said on Wednesday morning.
Those examined were mainly port workers and crew members, most of them from the Philippines, who are believed to have inhaled fumes from the corrosive substance.
Authorities said 28 people showed more serious symptoms and remain under observation in hospital.
One of them is being treated in intensive care, mayor Marc Van de Vijver said.
Most of those with minor injuries have already been discharged from hospital.
No local residents were affected by the fumes, the mayor added.
Tests carried out at some distance from the vessel found no dangerous concentrations of the chemical.
The release of vapour has eased since Tuesday evening, and specialist teams are now working to seal the leaking container before removing it safely.
Shipping traffic in the affected area remains suspended while the operation continues.
