Emergency services have cordoned off a residential street following the discovery of an "unknown substance".
Firefighters, specialist paramedics and police officers are at the scene in Brading Court, Ingleby Barwick, near Stockton.
Cleveland Police confirmed it was supporting Northumbria Police with an ongoing investigation following the discovery of the substance.
A spokeswoman said the inquiry was separate to the recent corrosive liquid attack in Clapham, south London, linked to suspect Abdul Ezedi from Newcastle.
The spokeswoman added: “The local community will see an increased police presence in the area, as officers who are experts in handling substances visit the property.
"The public should be reassured there is no risk to the community’s safety.”
She said the incident was not terror-related.
Cleveland Fire Brigade said it was at the scene to support the force in its investigations.
Ingleby Barwick resident John Buck - who has lived in the area for 15 years - said: "It's a shock to see something like this happening right on your doorstep.
"They've got the green chemical suits on, something serious is going on."
James Nevison said he was "not concerned at all" due to the presence of emergency services in the area, saying all appeared "calm and safe".
A spokesman for the North East Ambulance Service said specialist paramedics were on stand-by.
He said: “We are supporting our police colleagues as part of an ongoing investigation at an address in the Ingleby Barwick area of Stockton.
"We have dispatched two resources from our Hazardous Area Response Team to be available on standby if required.”
Hazardous Area Response Teams are trained to provide the ambulance response to high risk and complex emergency situations.