Gas fridges and ammonia
Laundress, the gas absorbtion refrigers use ammonia as the refrigerant. And, some of the older ones used hydrogen gas as a carrier to make the ammonia circulate. Both hydrogen and ammonia are flammable, so if a line burst while the fridge was operating, there would be the chance of an explosion. However, as far as I know, no gas fridges ever used carbon monoxide as a refrigerant. The CO hazard most come from the burner itself and the fact that in most installations, it was not vented -- the same hazard you'd get from an unvented gas space heater.
My maternal grandmother had a Servel gas fridge that she used for years and years without problems. She was a bit paranoid about electricity. In the summer, whenever she went out of town, she'd pull the main fuse from the fuse box! Since the fridge was gas (and the house had no air conditioning), she only had the minor inconvenience of resetting clocks when she got back. She probably would have done this in the winter too, except for the fact that the floor furnace for some reason was equipped with a 110V thermostat instead of a millivolt stat.
Ammonia isn't toxic per se, but high concentrations can burn the insides of your lungs. Oddly, though, it is possible to acquire a tolerance to it. I experienced this first-hand at a summer job I had when I was a teenager. I ran the print shop in an architect's office. One of my responsibilities was making blue-line copies of original drawings on a machine that used a diazo process. This process uses concentrated ammonia as a developer. The machine had a big tank of liquid anhydrous ammonia connected to it, and it made its own ammonia water solution. And it leaked ammonia like a sieve. After a few weeks, I got to where I couldn't smell it. One afternoon I was running a big batch of drawings, and the receptionist came back to see what I was doing. She was holding her nose and had a hanky over her mouth. She asked me to shut it off for a while -- the smell had driven all the other employees out of the building! And I didn't notice anything! After that, whenever I had a large batch of drawings to do, I had to set up a fan at the back door to blow the gas out, and take breaks periodically. It bothered me because I could never tell how bad it had gotten, and sometimes I'd forget to turn the ammonia off after a run.