They do use liquid Nitrogen in there, but only air will come out with combined oxygen.
No, sorry. It is not liquid nitrogen and that is not how these cans of "dusting gas" work. A quick look at the can contents (or a simple Google search) will show that these cans most likely contain difluoroethane or something chemically similar. It costs a lot of money to cool and compress nitrogen into liquid form (-196°C) and the results would be damaging cold once disbursed. And these cans do NOT disburse or mix contents to form "air". It is not breathable so care should be taken not in ingest it while using. Sadly, because cans of spray paint are harder for kids to buy, these cans of compressed dusting gas are some times abused by "huffers" to get high.
Because of that, when referring to these cans, the terms "compressed air" or "canned air" or the like is being discouraged and you will typically see instead cans labeled as Dusting Gas, Compressed Gas Duster or Dust Remover - not "air". Difluoroethane is used because it is cheap, and it turns into a liquid form suitable for canning at a much higher temperature than nitrogen or other gases. If you get liquid nitrogen on your fingers, they will be severely damaged almost immediately.
And you should avoid tilting the can when in use to minimize spewing the liquid form of the gas. The liquid form is not as effective at cleaning out dust for one, but because it is very cold, it can cause microfractures if sprayed on still very warm components due to rapid contraction of the matter. A rare event, but possible if a hot device is cooled too rapidly.
Many shops and technicians use air compressors. These are perfectly safe as long as the pressure does not exceed 90psi, and most importantly, as long as an in-line moisture and particulate filter, such as one of these
is used. Regardless if you use a can of compressed dusting gas, or an air compressor or even a vacuum cleaner, immobilize the fans when cleaning. The compressed air or gas can cause the fans to spin beyond design limits and damage the bearings. You can buy bags of wooden Popsicle or glue application sticks at most hobby and crafts stores. I use these to hold the fans still while blasting the blades and surrounding vents.
While you can use a vacuum cleaner, extra care must be taken to prevent ESD damage. Air and dust particles banging into the nozzle end while zooming past creates extremely high static electricity potentials which WILL destroy ESD sensitive devices such as CPUs and memory modules. So if using a vacuum, which I do if raining outside, I wrap my hand around the nozzle and extend my forefinger out front planting it on bare metal of the case to discharge and prevent static build up. I keep it planted at all times. I then use a soft brush stir up the dust and push it to the vacuum nozzle.