Gas Boiler

Having any boiler inside the house is not harmful per se. The danger is if the room where the boiler is situated is not correctly ventilated.
We have a gas boiler and the fumes ventialate into a chimney leaing to the roof of the house. There is also a ventilation panel in the door of the room where the boiler is situated.
So long as the installation and ventilation are correct, there is no question that any one of Gas, Electric or Oil is 'better' than the other two.
Think about it; is having a Gas cooker inside the kitchen harmful?