I'm sure people won't believe it when I say it's clean but...
I've used Propane torpedo heaters for many many years in my garage and hangar. I also have a high sensitivity (not for home use but for RV trailer use) CO monitor that I keep right near the workbench. I only use Kerosene for heat, and within seconds of firing up the heater (I have 115K BTU, 110K BTU, 155K BTU, and now a 140K-180K BUT version in various hangars and garages) they are making lots of hot air. Not much for any smell either, when warmed up. If you let them run out of fuel, they will huff and puff during their last gasps, and make plenty of CO for a minute...but if you avoid that, they burn very clean. Also, short cycling them where they turn on and off a lot, will increase CO a bit. So you don't necessarily WANT to buy the biggest and most expensive one you can. I'm in WI, and worked on 2 airplanes thru winters when it was sub zero. If you're anywhere in the lower 2/3 of the country, you won't need a very big heater for a normal garage.
Now, one time I got cheap and stupid and tried to run diesel in them, since they run fine on diesel, but I ended up with a headache and it was very noticeably worse than kerosene.
My latest one I just bought a couple weeks ago because the local Natural Gas company here decided to up my zero usage meter reading rate to about $35/mo...and that's without using any natural gas. That got me irked enough at spending hundreds a year to heat a hangar that I only need heated a few days a winter that I decided to get another torpedo and just do it that way.
The only real drawbacks are that Kerosene isn't easy to find sometimes, and the heaters make some noise. But depending on your work area, it may or may not be a big deal. Make sure to get a CO meter, and if you have CO readings climbing up, make sure you have adequate ventilation. My garage leaked enough that I didn't need to add more.