Eleven workers were taken to hospital after lime was "inadvertently released" inside Dublin's new incinerator plant on Wednesday night.
Staff at the Dublin Waste-to-Energy plant complained of breathing problems, blurred vision and nausea, according to Irish broadcaster, RTÉ.
Nine were discharged from hospital, but two were kept in overnight.
The plant's operators said the lime was "contained within the building and did not escape into the environment".
A spokesperson for Dublin Waste-to-Energy Ltd said the company was "investigating the incident thoroughly".
It is also co-operating with investigations by the Environmental Protection Agency and Health and Safety Authority.
The plant at Poolbeg started to incinerate waste for the first time last Thursday in what its operators described as its "important 'first fire' milestone".
The operators said the safety of its employees and contractors was of "utmost importance".
"Late on Wednesday night, a small amount of lime was inadvertently released inside the flue gas treatment area during the commissioning and testing of the Dublin Waste to Energy plant at Ringsend," their statement said.
"At the time, there were a number of workers in an adjacent area. As a precaution, eleven workers were sent to St Vincent's Hospital nearby for medical evaluation."
The statement added: "We are investigating to understand what exactly occurred, but it appears from preliminary investigation that the release of the lime was due to a problem with a door seal in the fabric filter bag house."
The facility was developed with the aim of reducing Dublin's reliance on exporting waster or sending it to landfill.
The operators have said the project will divert up to 600,000 tonnes of waste from landfill and export, while generating electricity "for at least 80,000 homes".