A Police Complaints Authority inquiry has confirmed the spray was used on Mark Bell, 26, while he was alone in the loft of the family home, where his body was found hanging a short time later.
"He was in a loft, he doesn't appear to have been going anywhere, or to have been a threat or danger, so why was there a need to use CS spray, which is intended as a last resort?" said Jenny Wilmot, policy officer for MIND, the mental health charity.
A second PCA inquiry has being carried out on the case of another mentally ill man seriously injured when jumping through a window after he too had been sprayed with CS.
The investigations come amid growing concern about the use of CS spray on mentally ill people.
The first legal challenge to the use of CS on the mentally ill is already under way: a 28-year old Cambridge man claims he was assaulted with CS in his home and that its use denied him his constitutional rights as a patient.
Yesterday the British Association of Social Workers (BASW) urged the Government to provide new guidelines on use of the spray with the mentally ill.
A PCA report on Mr Bell has been completed and is expected to confirm Humberside police used CS at his home in Scunthorpe two months ago. Earlier in the day he had been treated at hospital because of concerns about his health. It is understood he discharged himself against advice.
When police arrived at his home he retreated to the loft, where spray was used.
Mr Bell, a father of three, was found asphyxiated in the loft.
The cause of death is thought to have been hanging; an inquest has yet to be held.
In the second case, the PCA has been investigating how a 28-year-old Hebden Bridge man was critically injured in a fall from a fourth-storey window soon after being sprayed with CS. Attempts were being made to take him to hospital and there was a social worker in attendance. The man is believed to have thrown himself through a window.
A West Yorkshire Police spokesman said yesterday: "We conducted a full inquiry and we have submitted a final report to the PCA and we are now awaiting a response. We were asked to attend by the psychiatric service to help transfer the man to hospital. CS spray was used during the incident."
In Cambridge, a 28-year-old man is suing police for alleged assault with a CS spray and for violating his rights as a patient. It is the first case of its kind.
He is suing for assault and exemplary damages for police violation of his constitutional rights as a patient being detained under the Mental Health Act. It is claimed he was sprayed in his home in the presence of his family and a social worker, who also suffered the burning after- effects of the spray, as he was about to be taken to hospital.
An added problem with the use of CS sprays on the mentally ill is the risk of dangerous interactions with the cocktails of drugs that many mental ill people are taking.
Although CS is regularly used on the mentally ill, no research has been done on the reaction of CS chemicals with the powerful anti-psychotic medication and its effect on behaviour. MIND wants an immediate halt to the use of the spray on mentally ill people and is especially concerned about its use when patients are being taken to hospital.
A PCA spokesman said it had received the final report on the man who died in Scunthorpe and was examining it prior to giving it to the coroner.
"The Hebden Bridge investigation has also been completed and we are examining that too. In both cases CS spray was involved."