Ten people have been arrested in connection with the suspected illegal treatment, dumping and trading of hazardous waste including asbestos, the Environment Agency said.
A 60-strong team from the Environment Agency, backed by Avon and Somerset Police, carried out the UK's largest ever hazardous waste raid on several sites, in an investigation into alleged toxic waste offences by an Avonmouth-based company.
The Health Protection Agency has been closely involved because of the nature of the waste which includes asbestos and heavy metals, and will continue to assess any potential risk to public health, the Environment Agency said.
The 10 people arrested have been bailed pending further inquiries to a date in January.
Andy Higham, head of the EA's national environment crime teams, said: "The Environment Agency targets gravely negligent organisations and organised criminals who risk damaging health, livelihoods and the environment."
He added that the agency took waste crime extremely seriously.