Fire crews were called to an industrial estate near York after chemical waste drums began leaking poisonous gas.
Fire engines from York, Harrogate and Ripon were sent to the Marston Moor Business Park in Tockwith on Thursday.
The area was cordoned off and a trailer carrying 26 200-litre drums of chemical waste was moved to a safe area at the edge of the business park.
The substance was identified as antimony slag which can produce a toxic and flammable gas on contact with air.
The fire service made the drums safe and said the owners had been left to arrange specialist removal of the load.