Normally gas rings and gas ovens burn efficiently with a constant blue flame so the problem of CO is a negligible one.
Dell's right that, as houses become more air tight, ventilation is more critical, not just to burn gas efficiently in the open but also to prevent condensation [and then mould] just from breathing, washing, bathing and drying things inside. If you have a chimney in your rooms then, by rule of thumb, you should have enough natural ventilation drawing air through the other rooms. An extract ducted to outside should remove any CO effectively- the extracted air has to be replaced which in existing houses occurs naturally through the building anyway.
Evryone should adequately ventilate rooms regardless of the airtightness of the construction, doors and windows.... it's best to open a window but you need to ventilate at high and low level to clear a room efficiently so ideally a vertically hung window or ideally two windows on opposite sides of the house/flat to create a good through draught , a bathroom window will vent the moist air after a bath or shower in less than a minute depending on the difference between the inside and outside temperatures- which saves having an inefficient fan toiling away, high up tight in one corner where the airflow is ineffective, running for 20 minutes or so... but then all the cost and benefit of heating the room go out the window!
In new houses I try to get people to install heat recovery ventilation which runs constantly [about the cost of an 11W light... so about 0.25kWh per day] extracting warm moist air from bathrooms and kitchens passing it over a heat exchanger to warm the incoming ducted fresh air to each habitable room [you can boost the extract rate in really damp areas].... you can recover up to 90% of the heat otherwise lost and the positive pressure in the habitable rooms naturally draws to the negative pressure, where the extract ducts are, taking out CO2, CO, moisture and cooking smells]. But, most of us can't do that even if we want to because we live in places where it's not feasible to fit the ducts and vents.
