25 firefighters battled and fought through an acidic vapour at King George Dock in the early hours of Tuesday morning after a tank containing 580 tonnes of hydrochloric acid started to leak.
Associated British Ports (ABP), who run the site, said air quality had returned to safe levels by 6pm, and that all those who reported feeling unwell had been discharged from hospital.
Residents had been warned to shut their doors and windows because of the vapour cloud.
Work continued on the site throughout the night, with lorries transporting the hydrochloric acid away from the port.
Normal service was expected to resume with the exclusion zone lifted by late Tuesday evening.