Police spokesman Ajith Rohana said 72 factory workers and nearby residents were taken to hospital earlier today after gas -- thought to be ammonia -- leaked from the plant.
"Some of them are still in hospital eight hours after the incident was reported," Rohana told AFP.
"The initial investigations suggest there had been an ammonia gas leak from the plant," he said.
The Central Environmental Authority (CEA) said it immediately shut down the factory in the suburb of Piliyandala on the outskirts of Colombo pending further investigations.
Industrial accidents are rare in Sri Lanka but there is growing concern about factories near residential neighbourhoods following a recent groundwater contamination by a rubber glove plant just outside Colombo.