The CO content is extremely low if the system is up-to-snuff and burning efficiently & cleanly. A high CO content would be an indication that something was wrong with the boiler causing incomplete combustion.
The primary combustion products from the burning methane with atmospheric oxygen (CH4 + O2) are CO2 & H2O, neither of which is very dangerous to humans unless highly concentrated. There are smaller amounts of NOx, (since N2 is the primary component of the air, and not easily separated from the air) but it's not in concentrations high enough to be concerned about. The vast majority of the N2 present in the air entering the combustion chamber is not at all involved in the combustion, and is still present in the exhaust as N2. By contrast the majority (but not all) of the O2 present in the combustion air combines with the CH4 (releasing chemical energy in that reaction as heat), and the O2 content in the exhaust is pretty low. Unless it's being vented into a very confined space it's not particularly dangerous, since the dilution with outdoor air is very high.
To lower the amount of H2O in the exhaust, adjust the programmed "outdoor reset" curve to the lowest water temperature on the system that still keeps up with the space heating load. Avoid using overnight temperature setbacks on the thermostat, since recovery ramps from setback require higher water temps in the system. Condensing as much of the moisture in the boiler's heat exchanger as possible has a very positive impact on efficiency (it's the very point of designing condensing boilers), but the strong secondary effect is that the amount of moisture blowing out the vent pipe to form those annoying view-obscuring clouds is also dramatically reduced. When the return water from the radiators/baseboards entering the boiler is above ~125F/52C almost all of the water formed in the combustion goes into cloud formation, whereas when that water is below 110F/43C at least half of it condenses out inside the boiler, delivering the "heat of vaporization" to the heating system water rather than condensing when it hits the cold outdoor air, ticking off your neighbor.
Consult the manual (or ask the installer, if they seem competent on the subject) on how to adjust the outdoor reset temperature curve for the Cadet series boilers.