An interesting observation @SueW. The levels you observed of up to 1100ppm are not considered hazardous to health. Although twice that of fresh air urban environments typically also have elevated levels of CO2 due to motor vehicles, gas usage, etc
A higher urban level of CO2 may be associated with greater motor vehicle exhaust or similar pollution. It’s not the CO2 that is the hazard. It’s the other stuff that may be produced along side it that is the hazard. A high CO2 level is not proof any of the nasties are present. It is a simplistic indicator there may be elevated levels of pollutants present.
The occupational safe level for CO2 over 8hrs is a TWA (time weighted average) of 5,000ppm. This is equivalent to 0.5% by volume. In itself CO2 is not considered toxic. It’s an inert gas under normal conditions, even used as a shielding gas with high temperatures in welding steel.
The effects of higher levels of CO2 are most commonly associated with less oxygen in the air. It’s also generally considered safe to have oxygen levels as low as 19.5% compared to the norm of 20.9%.
CO2 is heavier than air and will displace oxygen. You do need a lot of CO2 to accumulate for this to occur. Much more than 0.5%.
Assuming you are not burning candles or doing anything else to consume the oxygen in the house it would appear you are well within the accepted safe levels for CO2. However another gas, Carbon monoxide (CO) with an 8hr hazardous TWA of just 30ppm is often the big silent killer. CO actually enters the blood stream and blocks the transfer of oxygen around the body. It’s effects are not instantly reversed by simply moving to fresh air. Kero or gas heaters, wood fires, etc can be deadly in poorly ventilated spaces.
With rising CO2 in a perfectly sealed environment there is a long term risk of asphyxiating, typically due to the lack of oxygen. It might take days for the scenario to develop.
One source suggests the average adult (no heavy exertion) breaths in approx 11m3 of air a day and consumes 4-5%, or approx one quarter of the oxygen in the air in each breath. Note there is 21% O2 to start with. And many times 11m3 of air in a small unit, or typical bedroom. Not typically a risk for a short period, but certainly a real risk over days or weeks.