Residents living within a 2- to 5-kilometre radius of burnt-out Ming Dih Chemical Co in Bang Phli district of Samut Prakan have been allowed to return home.

The Pollution Control Department (PCD) earlier confirmed air quality in the area to be at a safe level.

Provincial governor Wanchai Kongkasem said on Wednesday that all areas within that radius of the factory had been examined to ensure safety.

The management team handling residual chemicals that contaminated a 1-km area immediately around the factory is also in talks with the provincial command centre, PCD, Department of Disaster Prevention and Mitigation (DDPM), and oil and petrol company PTT.

Mr Wanchai said the team voiced concern over residual amounts of an industrial chemical, styrene monomer, stored in 2,000-tonne tanks that have yet to be cleared following the blaze. Residual chemicals in surrounding areas must also be dealt with, he added.

Since the fire, the PCD has been working with the Ministry of Natural Resources and Environment to assess environmental impacts. Because the contaminated water can be absorbed into soil, Environment Minister Varawut Silpa-archa instructed the Department of Groundwater Resources to expedite quality checks on water sources both above ground and underground within a 12-km radius of the factory.

Mr Varawut also said the PCD and provincial governor were in talks to determine whether to use organic peroxide to spray the styrene to prevent further fires.