I am in no way a medic, nor can I give you any advice.
What I know from our training, is that Carboxyhemoglobinemia occurs MOST OF THE TIME from carbon monoxide poisoning. You can read more about carbon monoxide poisoning in the forum called "Carbon Monoxide". In addition, smoking cigarettes was reported to increase carboxyhemoglobin levels by an average of 5% per pack of cigarettes smoked per day.
Remember that chronic poisoning is also possible due to long exposure to low levels of CO. Check http://en.wikipedia.org/wiki/Carbon_monoxide_poisoning.
In the house, CO can come from a car running in a garage, connected to the house; or anything with a flame (furnace, water heater, gas stove, wood burning stove, gasoline powered tools (leaf blower, lawn mower), etc... Exposure typically occurs when equipment is used in building, or semi enclosed space.
Being in an idle vehicle for a long time with a broken exhaust pipe could do it also.
As you can see, there is many possibilities. I would recommend to check the CO detectors in your house.
You can buy a personal CO detector (they are kind of expensive) and run it next to the furnace, water heater, stove (if it's gas), fire place.. and look at the reading you get...
Here are a couple of links to Personal CO monitors:
This is one we use in our fire dept:
Good luck !