Fire investigators are trying to identify what materials have been burned in a blaze at a recycling plant in Prescot, Merseyside, which handles hazardous waste.
The fire began at Remondis on Carr Lane at about 06:00 GMT on Sunday.
Fire crews have found "containers filled with oily rags" at the warehouse, Merseyside Fire and Rescue Service (MFRS) said.
The Environment Agency said it had not been necessary to monitor air quality.
Residents were advised to keep doors and windows closed. Those with pre-existing health conditions are urged to "keep medicine close".
The site holds an environmental permit for the treatment, storage, transfer and disposal of hazardous waste.
Firefighters are damping down and the smoke had "significantly reduced" following "good progress", MFRS added.
The Environment Agency said: "We are working with partner agencies including Merseyside Police, Public Health England and Merseyside Fire and Rescue Service to ensure the risk to people and the environment is minimised."
A Knowsley Council spokesperson said: "The type of waste handled is publicly available, and there was no threat to the public's health.''
A total of 40 business premises were evacuated.
A cordon remains in place from the roundabout at Tesco down to Huyton Lane. Motorists are asked to avoid Carr Lane.
Three engines and specialist vehicles remain at the scene of the fire. At its height, 12 engines attended.