I'm aware that there is little risk of CO/CO2 poisoning in typical house rooms in the US, even after long periods (A few hours, maybe a day) in a particular room. How is this accomplished, and what safeguards, if any ensure that there isn't a buildup of CO/CO2 in a room just due to typical human breathing?
If there is no oil/gas/wood burning/ heating system in the near, the natural air change through house hulls is enough to prevent a dangerous concentration. If the house is built with a very air tight hull (e.g. passive houses) , a ventilation system provides the air exchange. Air exchange via fully opened window every few hours helps also to avoid high CO2/CO levels. In general, not high Carbon(di)oxide levels , but fine dust from candles/smoke or aerosols from cleaning agents/moebels etc. or asbestos from building materials used in the past (radiator wall coatings, cement flooring, pipe insulation, PVC floor or tile glue, Eternit Wall covers etc. if sawed/grinded/broken/drilled) are the biggest threat for the health.
We exhale approx 5% co2 with each breath in a sealed room with enough o2 you would start breathing faster and get a raging headache if the levels co2 level goes two high. If there is a source for co and the levels are elevated first a headache and at higher levels your blood can not exchange o2 so you go to sleep and die. Safeguards for co would to install a detector many smoke detectors now have co detectors and are required in my state. As far as co2 I have only known it to be a problem in rebreather diving your home usually has enough air flow to prevent problems, if it is a very "tight" home with no make up air crack a window open to allow some fresh air or have a fresh air intake added to the furnace. Running a bathroom exhaust fan or over the range exhaust fan sucks fresh air in the home so that could be helpful but usually movement through the home is enough.