The Wakefield Health Authority said yesterday that it was carrying out follow-up checks on victims who were on the Hickson and Welch site when the blast caused a huge fireball to spread across part of the plant.
Seven other individuals, who live nearby but who may not have been involved in the chemical incident, also have similar symptoms. None of the casualties has been admitted to hospital.
The blast, on Monday afternoon, claimed three victims: two workmen caught in the fireball, and a temporary office worker, Sara Atkinson, 18, who was asphyxiated and suffered a heart attack.
She was air-lifted to hospital but died on Wednesday after a life-support system was switched off.
Fourteen other people were hurt in the blast and three are still in the regional burns unit of Pinderfields Hospital in Wakefield.
John Hopson, 42, a process technician and Neil Gaffeney, also 42, a manufacturing controller and married with one child, were said yesterday to be in a critical condition.
Terrance Douthwaite, 55, a team leader at the plant, married with one child, was said to be poorly but stable.
All the West Yorkshire firefighters who dealt with the blast were allowed home from hospital yesterday.
They had been suffering from vomiting and diarrhoea, and their fingers and lips had turned blue.
Dr Sally Pearson, consultant in public health and medicine at the Wakefield Health Authority, said yesterday that seven residents had reported symptoms of skin and eye irritation, breathing difficulties and gastric problems to their doctors.
She said: 'They were not on the site and we are not sure whether they are connected but we are assuming they could be and are following them up. 'There seems at this stage to be a possibility of two causes: smoke, which may or may not have been toxic, or a separate food source.'
A spokeswoman for Hickson and Welch said yesterday that investigations were continuing into the cause of the sudden ignition at the plant.
She said that one of the men involved in the routine maintenance work when the blast happened has said that he saw a small flame near a still they were cleaning. This ignited and caused the fireball, which then ran across a carpark and struck an office building before vanishing.
Health and safety executive officials are now constructing a model in an attempt to discover precisely what happened. Their report will be published in two months' time.
The blast happened during a routine maintenance operation as nitrotoluene was being moved from a still.
Nitrotoluene is used to make dyes, pharmaceuticals and fertilisers and is highly flammable. The explosion happened at lunchtime when most of the company's employees were not in the plant.Reuse content