A building in Farnborough has been evacuated after a man fell ill upon opening an envelope.
Police and fire crews were called to Alexandra Road at around midday on Friday.
Local residents were warned to keep their windows closed while the authorities dealt with the incident.
A statement from Hampshire Constabulary said: “We can confirm we were called to a commercial property in Alexandra Road, Farnborough, at 11.52am to a report of a man feeling ill after opening an envelope.
“We have attended the scene with our colleagues from Hampshire Fire and Rescue Service.
"As a precaution, the building has been evacuated while we establish the exact nature of this incident.
“As is routine, we have advised people in neighbouring properties to close their windows while we deal with this incident.” A spokesman for South East Coast Ambulance Service (SECAmb) said: "It was recorded that a man had suffered the effects of potentially inhaling some kind of fume.
"It has been categorised as a burn as he said his hands felt like they were burning.
"He's opened an envelope and that's happened."
One SECAmb car, one ambulance and hazardous materials paramedics attended the scene.
The man was fully conscious and taken to Frimley Park Hospital, but had been suffering from breathing problems.
Adam Jacobi works in Alexandra Road and he said at around 3pm: "Everything has gone now.
"I only know details from the guys in office next to us as we are one further on.
"[There was a] package delivered to the office and something within it got onto one of their staff who then fell ill."
Matt Attfield, 43, from Aldershot, works in JD Printing in Alexandra Road and he said: “Basically I was busy at work and noticed on the road just opposite a policeman talking to a man outside the building.
“It all followed on from there - police, ambulance, fire engines and everything all turned up pretty quickly.
“I went across and a fireman said to stay away and I didn’t see exactly what was going on, it all lasted about three hours.
“We were just standing there and didn’t get told anything.
“It all died pretty quickly and the man was escorted away to hospital by someone in a clear suit.
“I mean it looked like a very serious incident, it was a bit worrying and I have never seen anything like it.”