Staffordshire Ambulance Service declared a major incident after a leak of titanium tetrachloride at the Tamworth Heat Treatment factory at 1045GMT on Tuesday.
The chemical, which is used in the heat treatment of metals, turns to hazardous hydrogen chloride when in contact with the air.
One Darwell worker, who did not wish to be named, said as he was taken to hospital for a check-up: "A number of the lads have been badly affected.
"They are suffering from tightness of the chest, slight nausea and coughing.
"We all came out coughing."
The industrial estate in the Stoneydelph area of the town was not near any homes.
Oxygen therapy
The fire service said there had been no further contamination and no danger outside the immediate confines of the factory.
The incident was brought under control by 1530 (GMT).
Divisional Officer Shaun Irwin, of Staffordshire Fire and Rescue Service, said: "Whilst the company reacted quickly and correctly by absorbing it with granules and a neutralising alkaline solution, a small quantity of gas escaped from the unit through the ventilation system and was inhaled by workers at a neighbouring factory."
Thirty five people were taken to hospital and 10 were treated with oxygen therapy.
Their injuries have been described as serious but not life threatening.
Small amount leaked
Staffordshire fire spokesman Paul McCafferey said the amount of chemical leaked was only the size of a small carton of juice.
But it had reacted with moisture to form a gas that was sucked into Darwell Fabrications through its ventilation system.
The injured were treated at the Good Hope Hospital, Sutton Coldfield, and the George Eliot Hospital, Nuneaton.
The fire service, which sent five crews including a specialist chemical unit to the scene, is working with Environment Agency officials to investigate the leak.