I have used Sterno for years. As an open flame heat source it is about as safe as you can get. Since it is a gel it is far less prone to spillage than liquid fuel. Awhile back I emailed the company and specifically asked them about CO emissions. Here is the exchange:

"I am trying to determine whether or not Sterno puts out any appreciable amount of Carbon Monoxide (CO) while burning. I am putting some into my 72 hour kits. I understand adequate ventilation is necessary, but I specifically need to know about carbon monoxide emissions."

"Thank you for your inquiry. Sterno produces only CO2 (Carbon Dioxide) and H2O (Water) as a result of burning. It was designed to be safe indoors. As it is a fire-producing product, however, we would advise that children and pets be kept away to prevent accidental burns.
The Sterno Brand Team "

I would definately keep the dogs away from it, and I would not let it burn while sleeping and keep a window cracked as it will deplete oxygen. I keep 4 large cans in my truck kit. Each one will burn for 3 hours. No mixing or measuring; just pop the lid and light. They will boil water but can take up to 20 minutes to do so. All in all it is a very useful addition to a truck kit.