He said the spraying had created sufficient vapour to pass under the adjoining door and poison the couple.
Earlier, the three-day inquest heard from toxicology expert Professor Robert Chilcott.
He told the hearing in less developed countries the pesticide Lambda is sometimes diluted with the substance, dichloromethane, which causes the body to metabolise or ingest carbon monoxide.
The hearing also heard from Home Office pathologist Dr Charles Wilson, who told the hearing the Coopers' hotel room had not been secured and it would be "inconceivable" such measures would not be taken in the event of a double death in a hotel in the UK.
The inquest, five years after the deaths, also heard of multiple, repeated attempts to obtain more documents and information from the authorities in Egypt despite numerous requests from the Foreign Office.
Coroner Dr Adeley said Mr Cooper's illness and death was rapid, but described the medical treatment provided for Mrs Cooper as "utterly insufficient" after she was taken to a clinic in the hotel before an ambulance was called, creating a delay of four hours before she got to hospital.
Jatinder Paul, a lawyer from Irwin Mitchell, representing the family, added: "To find out that the couple died from a pesticide that had been sprayed in an adjoining room, leading to carbon monoxide poisoning, is shocking and deeply concerning.
"It's now vital that lessons are learned to ensure a tragedy like this doesn't happen to future holidaymakers."
