You should first check:
- any combustion appliances in your home such as stove, heaters, dryer, etc.
- Is your garage an attached garage? Is the door to this garage tight, fire-rated and self-closing?
- Are there any smokers in your home?
There is so much to look at in the case of carbon monoxide. It's a sneaky, odorless killer.
Check combustion appliances
The best way to ensure minimal (if any) exposure is to check all appliances that are not electric for proper combustion. Better yet, upgrade to all sealed combustion equipment.
- If using a wood stove, ensure the draft is correct for that appliance.
- Use a direct-vent ("out of the envelope") hood fan over the stove while cooking or baking to reduce the amount of carbon monoxide spilled into your home's "envelope."
Check if there are any spaces or areas that will or can leak from the garage to the home.
- If so, have these areas air-sealed by a professional who understands safe air sealing.
- I highly recommend a BPI-certified analyst or envelope professional to check the tightness of the home.
Get a monitor
Also, I recommend a carbon monoxide monitor -- preferably hard-wired with a battery backup -- installed outside of sleeping areas and near garage entry to the home.
If anyone smokes, have them smoke outdoors.
Check, check, and double-check
It seems like a lot to think about, but any combustion appliance is capable of allowing significant amounts of carbon monoxide into the envelope.
Imagine a home that is sealed tightly and an appliance spilling carbon monoxide. It has devastating results.
Check, check, and double-check.