Good discussion.

Someone mention SCBA earlier.

I'm retired from the fire department, and so have a lot of experience with SCBA (Self Contained Breathing Apparatus). These are the units that are somewhat similar to the scuba gear that divers wear. Basically there are two types found in general use - demand units (no longer sold), and pressure-demand units.

In both cases the firefighter wears a full-face mask which is connected to a tank of compressed air worn on his back. There are 30-minute tanks and there are 60-minute tanks, as determined by the pressure that the tank can be charged to,, and the capacity of the tank.

The old-stype demand masks have a very simple pressure regulator that, when one inhaled (and thereby created a negative pressure in the mask) caused the regulator to open and allow fresh air to flow from the bottle into the mask. There was also an exhalation valve that, when one exhaled (and thereby created a positive pressure inside the mask) opened the exhalation valve to allow ones exhaled air to be exhausted into the general atmosphere.

The newer style are pressure-demand masks, with the principle difference being that a slight positiove pressure is maintained inside the facemask. Presuming that the firefighter is operating in what is known as an IDLH environment (Immediately Dangerous to Life and Health) charged with toxic gasses, the slight positive pressue inside the mask meant that, if there were any leaks in the facepiece or a less-than-perfect seal between the firefighter's face and the facemask, the slight pressure inside the mask would prevent contamination of the atmosphere inside the mask. The old demand mask, requiring one to pull a negative pressure inside the mask, would tend to pull the outside toxic environment inside the mask to be inhaled.

Now, having said all that, I will tell you that it is not easy to breathe in one of these things while doing any sort of strenuous work. The newer pressure-demand units are a little better in that you don't have to "suck" inside the mask to inhale, but in both types you have to push your exhalations out. And those 30-minute and 60-minute ratings.....don't know where they came up with those. Maybe if you wore the thing sitting in an easy chair, I guess, but doing any kind of work? No way. I personally sucked two 30-minute bottles down in 15 minutes flat at a fire once.

Also, these things are very heavy. A pressure tank that can hold 2200 PSI (30-minute) or 4500 PSI (60-minute) has to be made strong, even if it is carbon fiber, or spun aluminum. Heck, when Ii started on the job the bottles were steel! And one still has the backpack harness, regulator, hoses, etc. to lug around.

Another expense with these things that make them impractical for the average joe is where to get them filled - you just can't hook 'em up to the air hose at the filling station. It's gotta be a compressor system rated for breathing air. A scuba shop would probably be able to do it, though.

But then there's hydrostatic testing of the tank, though.

And this is a system that provides clean air under pressure from a tank.

I can't imagine having to pull air through a filtration system with lung power alone.

And are the filters really effective? (I guess even if they're only partially effective they're better than nothing, though.)

Maybe some sort of mechanical ventilation system that forces air through a filter? But then how does one power the system?

Look at the pictures of the Chinese riding around on their cruisers, though, wearing those paper surgical masks.