Portable heaters.

The problem with any fuel heater, ANY. Kero, LP or natural gas

ALL produce "products of combustion" and lots of water, so if they are UNvented this all goes right into the occupied space

Worse, if you are doing something like bodywork or cleaning parts, there is a fire / explosion hazard, and also the chemicals you are using with your car mix and burn with the fuel (drafts right into the heater) and that to put it simply amplifies the effects of the chemicals. Cleaning agents, thinners, body putty, whatever it might be. You think headaches? THAT is headaches!!.

Water from combustion can be a problem. It will condense on colder objects, and will cause rust on anything unfinished.

The age old warning still applies "well ventilated space" for combustion heaters.

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Infrared / radiant heaters. There IS a minor danger with those. Since they ARE radiant, sometimes people (who are cold) move them closer to get warmed, or set them someplace that they radiate say, "right into" the paint. In other words you need to be careful where you set them, else overheat / damage something nearby.

I MUCH prefer old school "fan" convection heaters. They are much safer, especially if then have a TIP SWITCH, something you should check for when buying.