Grill-makers, grill-sellers, charcoal manufacturers and firefighters all will tell you in no uncertain terms not to grill with charcoal indoors. So, why not in the garage? There's that big roll up door for ventilation. You could get a couple of fans or something and grill in the garage out of the rain, right? Well, not really!
Carbon monoxide, or CO, forms due to incomplete combustion of the briquets in a charcoal grill. Large amounts of CO are produced by charcoal fires. Carbon monoxide replaces oxygen in the lungs and can quickly suffocate you. Even a partially enclosed space can contain carbon monoxide gas at dangerous levels.
Fire departments and grill manufacturers recommend that grills be operated more than 10 feet from any building or flammable structure. Grills give off considerable heat and garage ceilings aren't high enough for the heat from the grill to dissipate safely. Sparks and heat can set the ceiling on fire.
Residual heat in cooling charcoal still produces life-threatening levels of carbon monoxide. Even in a semi-open space, without an adequate chimney system and even with one, when you open the grill top to cook, the grill can emit enough CO gas to accumulate in quiet corners of the garage.
Carbon monoxide is not irritating, is odorless, colorless and invisible. The gas itself is lighter than air and easily collects at the ceiling, displacing oxygen from the top down, where your head is located. You don't notice that your lungs are absorbing carbon monoxide until it's too late.
Carbon monoxide poisoning manifests as headache, dizziness, ringing ears, vomiting, weakness and heart palpitations. Children react far more quickly than adults. Oxygen should be administered as quickly as possible.