A few comments as scientist:
Having a central (monitored) fire and CO2 alarm saves lives.
I hate to nitpick, but others are reading and they might get confused. You should actually be worried about carbon monoxide (CO)... you don't really have to worry about carbon dioxide (CO2).
Carbon monoxide is a by-product from incomplete combustion of fuel. It's colorless, odorless... and it will make you sleepy at an intermediate level... but it will kill you at high levels.
(an aside, you really don't have to worry about CO if you don't have any combustion in your home (gas furnace, gas stove, gas clothes dryer). For example, a house with all electrical appliances has very little chance of having issues with CO (especially if there is a detached garage).
Carbon dioxide is the product of complete combustion. We exhale carbon dioxide when we breathe. Carbon dioxide is relatively harmless (although it is a major contributor to global climate change). Carbon dioxide is also a useful surrogate for how much (or how little) ventilation there is in a room. For example, the carbon dioxide levels in a school classroom increase significantly when classes are in session. I mention this because during the COVID pandemic... there has been a lot of focus on schools and classrooms (and other crowded, indoor spaces) on carbon dioxide levels. There is nothing directly concerning about high carbon dioxide levels in a room, except that these rooms would also be more likely of a place for COVID to spread (i.e., better ventilation = lower CO2 levels = less likely to get COVID).
As a former handyman who worked for a rental management company (i.e., about 20% of my job was making sure that rental units that we managed had proper smoke and CO detectors):
Having a central (monitored) fire and CO2 alarm saves lives.
You should, in theory, change the batteries in your smoke and CO detectors twice per year.
Personally, I recommend getting the units that do not have batteries to change at all. They have an internal lithium battery that lasts 10 years, which is the life of the detector itself. This is another safety tip: replace all smoke and CO detectors that are older than 10 years.
Finally, and this is more obscure but it's 100% true (I promise!).
The type of smoke detector that you get is VERY IMPORTANT. There are two types currently on the market: photoelectric and ionization. YOU WANT PHOTOELECTRIC. YOU DO NOT WANT IONIZATION.
Ionization smoke detectors are an older technology... they are the ones that are super easy to trigger when you burn your toast. This is actually about half of the problem with these detectors... they are easily triggered and people disconnect them. Meaning, they are left without any smoke detector at all. The other problem with ionization detectors is that they are not quickly triggered by actual fires (i.e,. they are better at detecting your toast burning than your house being on fire). According to the data that I have seen, the survival rate of people in homes with ionization detectors is about 50% when there is a major house fire.
In contrast, photoelectric detectors are not easily triggered by nuisance events like burned toast. They are also much better at detecting real fires than ionization detectors. The data that I have seen is that the survival rate in homes with photoelectric detectors is about 95% when there is a major house fire.
And the really good news... ionization detectors and photoelectric detectors are about the same price. Another tip: there are combination detectors that use both technologies. These are more expensive (because they're effectively two-in-one detectors) but they do not perform better in genuine house fires than photoelectric-only detectors. So, I don't recommend these (i.e., more expensive but not better).