I don't think carbon monoxide is a bad method, the charcoal in a car didn't work for me though. If I remember right the gas settles to the lowest area, some tent deaths only occurred to occupants who were faced down. I think in my case the vehicles was just too large (suburban). I didn't tape up the vents although all were closed and don't let air in so I didn't see that it would matter. Maybe they are putting something in charcoal to absorb carbon monoxide? (I don't know). Years ago they used to say spray the coals with water to increase the ppm too 500,000 which was done. It was a very peaceful sleep after 12 beers and half a bottle of bourbon, no dreams. When I woke up in the morning some things were odd, the clock which never worked was working on the dash.