The HSE's National Director of Health Protection has said that all remaining houses on the Gleann Riada housing estate in Longford are unsafe to live in unless urgent assessments are carried out.
Dr Kevin Kelleher was speaking after a meeting with residents in Longford this afternoon.
He said the HSE did not have the power to evacuate the remaining 79 houses in the estate.
However, he said there was clear evidence of the effect of carbon monoxide, methane and hydrosulfite in the area and the gases from the sewers were dangerous.
Dr Kelleher repeated his warning that residents should not light open fires in their sitting rooms.
He said he has been told by residents they were suffering from symptoms, such as headaches and nausea, and there was an urgent need for each resident to have an assessment carried out on their home.
Dr Kelleher said he will pass on the information to Longford County Council.
In a statement, Longford County Council said it is constrained in what it can say due to legal proceedings undertaken by some of the residents.
However, it said all residents should ensure adequate ventilation in the house, do not light open fires, install carbon monoxide alarms in each house and maintain water traps in all toilets.
It said the council will continue to work closely with the HSE in relation to this issue.
Meanwhile, the HSE primary care centre for Co Longford has been located on a site adjacent to the Gleann Riada estate.
The developer behind the primary care centre, Frank Kelly, told RTÉ News that even though the centre is built on the same floodplain as Gleann Riada, he is "100%" satisfied there will be no issues with gases or smells.
Mr Kelly said he has replaced the entire sewerage pipe network in the building.
He said that even though the primary care centre would use the same mains network as Gleann Riada, he was certain there would be no adverse effect on the residents who were experiencing noxious smells and gases.
Mr Kelly said Gleann Riada had been developed by a separate company.
The HSE began an investigation into levels of noxious gases in the area after an explosion at a house in the estate.
Engineers have blamed it on serious faults discovered in the foundations of the sewers, which have resulted in high levels of methane and carbon monoxide.
The initial explosion in the estate blew out a front window of one house and caused major internal structural damage.
After a second incident on Friday, an engineer representing over 30 residents called for the development to be evacuated.