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.
Just open the windows daily to aerate every room, nothing compares with fresh air.