So, I open my freezer door tonight and there are these white clouds of vapor wafting about. Never happened before. Can’t figure out if it’s water vapor (then why never before,) or if it means there is an ammonia/nitrogen dioxide situation?
Usually happens to mine when humidity rises, summertime…YMMV.
OK, but it’s been humid before and this never happened.
Also, some plastic stuff had accidentally been blocking some of the internal vents in the freezer, wonder if that does something?
Where would the ammonia be from? Or the nitrogen dioxide? Do you have an industrial sized freezer? I mean warehouse sized, not restaurant or store sized. If you had ammonia leaking, you’d not be able to stay in the room long before it became lethal. source: my ammonia first responder training.
Modern household refrigerants are 1,1,1,2-Tetrafluoroethane which
I’d pin the source of the vapor on humidity.
Agreed with others, it’s just from the humidity. The cold air is pouring out and the humidity is condensing out of the air. It’s very common. As for why you haven’t seen it before, it could be anything from you not noticing it, to the humidity wasn’t as high (in your house) to the exact conditions weren’t met.
Also, as poppasan said, it’s not ammonia or nitrogen dioxide. It’s going to be R-134a or R-12 or something. Further, there’s probably only a few ounces in it. Less than you’d even find in a car. Those big bottles you can buy at the auto shop, there’s probably half that much in it. If you had a leak big enough to create any visible fog, your freezer would be warm by now.
I don’t think I’ve ever heard of NO2 as a refrigerant, ammonia is on HUGE systems. My walk in cooler holds 30# of refrigerant and it’s just a ‘regular’ one. NU22b, it was getting really expensive to dump r-22 in there. The only ammonia ones I know of are large enough that the last time someone was concerned about a leak, the police evacuated a 1 mile radius.
OK, I asked because this link said that depletion of ammonia causes ammonia and nitrogen dioxide and water to react together to form a white mist. I use a normal household freezer.
Dude, do you worry about everything being dangerous?
Yeah, that could cause your freezer to keep pushing colder and colder temps vs stopping at the normal cold temp. I just had it happen in the fridge when we packed too much stuff in. Frozen eggs, frozen cucumbers and my milk was a shake.
Boys and Girls, this is what happens when you cut back science education.
^This.
The OP needs to follow the immortal advice of Leslie Glass and “take a Valium like a normal person.”
A friend of mine complains sometimes about her Vitamin V deficiency.
Folks do tend to live longer that way. 
Everyone’s all humidity this and water vapor that but in less than a week there’ll be a post from the OP going “There is no Velocity, only ZUUL.”