Two PSNI officers were taken to hospital after suffering the effects of a "noxious substance" which they encountered while attending a 999 call last night.
Two others also suffered ill-effects but did not need hospital treatment.
Some residents in the area also complained of symptoms but were not badly affected.
It happened in the Ravenhill Reach Court area of east Belfast.
Police were responding to an unrelated emergency call when they encountered what they thought was a gas leak in a communal area of a building.
Some people were evacuated and a community centre was opened to accommodate affected families.
This morning police said the Northern Ireland Ambulance Service and Fire and Rescue Service also attended.
"It has since been established that this was not a gas leak and at this time we believe that some kind of noxious substance was released inside the building."
Residents were able to return to their homes after several hours.
