Its not safe or healthy, not only for the co but for the fuel bi products you are inhaling, freshening up the air is a joke, I have an uncle with an 8" open hole in his back because 2 weeks before thanksgiving they removed a few of his ribs, a lung, a little of the other, and a few other organs due to cancer, he always did stuff like this, washed his hands with gasoline, actually use a bullet heater for 15 years in his shop with a wood stove, he would come in, in the morning, fire up the bullet until the wood got warm, he used to also swing them doors open a few times to get the "fumes' out, what a joke..
Cancer is serious, being careful what we breath in, let soak into our skin, our mouth, nose, eyes, should be taken seriously..
And if you are doing it to save money, it is a no brainer.
Get an oil furnace, brand new air temps with becket guns on them are $1000, a power venter, some wiring, and a 50 ga drum to hold the oil if you dont want to buy a tank.... That is 84-86% efficient, and you can heat the building with the doors closed, lol.. Them units are 100% efficient, then you get rid of 50% of the heat to "freshen up the air", it makes no sense, and being in the business and seeing the effects of irresponsible BS first hand gets me passionate about it...
I really hate when guys say "I did this for years", well a lot of people smoked cigs for years, some never got cancer, or got hit by cars before the cancer could get them, BUT THE BULK of them got to lose their jaw bones, and miss seeing their kids grow up or grand kids be born, I know of a bunch..
PS0 its not just cancer, them combustion by products will destroy your heart too...