Police officers, firefighters and paramedics arrived at Lupton Road Lowedges, shortly after 8.20am and sealed off part of the street.
The cordon is now centred around a house where an occupant is reported to have ingested cyanide.
Residents claim the occupant was taken to the Northern General Hospital.
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South Yorkshire Fire and Rescue said two appliances - from Lowedges and Rotherham - were sent to the scene at 8.20am but no other details have yet been released.
A fire service incident commander is at the scene overseeing developments.
A spokeswoman said the operation is being led by South Yorkshire Police.
One resident said specialist 'chemical teams' are at the scene.
Another said: "It looks like a war zone."