I'm writing a story in which a character has to make it through an area filled with aerosol poison...or something along those lines. He starts out in a hospital area, with access to equipment. Is it feasible that he could just put on an oxygen mask and be adequately protected? I think hospital oxygen masks are just to supply EXTRA oxygen, and not keep other stuff out. So...wanting to make sure this plot point doesn't have any leaks (pun intended ;-)
...on which kind of mask your character finds, and if they know the difference.
A standard mask used to administer oxygen, air and anesthetics:
A standard mask used to administer only oxygen:
If you character knows what to look for (and where in a hospital to find one), the first will do well enough, if properly sized and worn. This kind of mask is designed to work with the coaxial gas delivery / evacuation tubing that's attached to an anesthesia machine. The hose attaches to the opening at the top, a rubber head-band attaches to the four prongs, holding the mask on the face. At the bottom is a soft plastic balloon that surrounds the nose and mouth.
The second mask is designed to provide "blow-by" oxygen. This mask is a slightly harder plastic whose rubber band attachment is quite flimsy and weak (by design). This kind of mask can be found almost everywhere in a hospital and is designed to hook up either to the central O2 system or to any portable O2 tank. This kind of mask rests lightly on the face and is, by design, very leaky.
Obviously, the second mask will admit aerosol poison along with oxygen or air from the tank. Ungood, that. This could, however, be the cause for several hectic MacGyver moments as your character realises that the mask isn't going to be quite up to the task. This is why God invented Tegaderm film dressings. Just glom a bunch of those all over the face and mask, taking care to seal up the O2 delivery tube and Bob's your uncle.
Should your character obtain the first kind of mask, they'll need a "converter" to go from the O2 tank's small gauge hose to the large opening. Part of the anesthesia supplies is an ambu bag which will do the conversion. Still, this mask isn't perfect! These masks are designed to keep anesthetic gasses inside the circuit (OR staff headaches from leaking anesthetics are not uncommon); but they're not designed for long term use and they certainly aren't designed to keep poisons out. Even when strapped to a person's face, they often leak, especially at the bridge of the nose. Your character will have to carry the O2 tank in one hand and jam the mask to their face with the other.
The tank can provide 15 lpm maximum, which should be more than enough to get your character through the Zone of Yuck. Using a standard hospital E tank, your character should be good for about 45 minutes or so at maximum flow. Tanks are notorious being not quite full...so, make sure they check the gauge before grabbing a random tank!
Reality Check = PASS!
Even so, it will be a harrowing and exciting venture with a very high risk of failure. For example, do note that the mask won't protect the eyes, and they, being mucus membranes, are susceptible to noxious fumes.
Yes, it can work but it depends a great deal on the "aerosol poison" you are dealing with.
It needs to be something like chlorine gas. Chlorine works by physically damaging the respiratory tract and pulmonary system when inhaled in relatively large doses. The oxygen mask doesn't need to seal perfectly to protect the wearer and the lack of eye protection will leave the person needing to flush out their eyes when they get clear but should cause no permanent harm. You are also clear of the danger once you get past the poison fumes and into fresh air.
With other airborne poisons such as most nerve toxins or blood toxins an oxygen mask is not going to be adequate. Breathing in even a small amount could cause harm and the oxygen masks you are talking about cannot be completely sealed from outside air. An oxygen bottle contains pure oxygen, it is not a SCUBA tank filled with compressed air. Breathing 100% oxygen from a bottle is better than breathing poison gas but pure oxygen can cause be just as harmful if the person breathes too much of it or too long.
Also consider that many airborne blood or nerve toxins can be absorbed into the bloodstream through the eyes or even through exposed skin. For these types of toxins you need a full mask that protects the entire face and a full protective suit. (It is possible that such protective equipment is stored somewhere in the hospital as hazmat equipment.)
Also, some aerosol poisons are intended to be easy to clear, such as the stuff used when fumigating a house. Other poisons are intended to be longer lasting. They are sprayed as an aerosol mist of tiny droplets that stick to things, including the person and their clothes. These droplets will continue to evaporate and off-gas toxic fumes long after leaving the area where they are deployed. This means that the person needs to keep the gas mask on while following a strict decontamination protocol, stripping off and discarding their clothing and showering off any residue that may be on their skin.
(Slight) frame challenge,
At an high exercise level we need 3.5 litres per minute (average person):
This would indicate that if the person were to leave the mask on, given that they would only be able to take shallow breaths, lest they breath in the outside poison air, they'd not be able to get up, walk and in the stressful situation function.
They'd either need to:
Up the flow rate on the regulator and take lots and lots of shallow breaths - the dead-air space in their trachea would make this impossible to sustain and they'd gasp for air and get poisoned.
What they would need to do is rip the tube out of the mask, shove it in their mouth and turn the flow-rate up on the cylinder's regulator.
If they need to, then, replacing the mask and doing the inpatient-shuffle along the hall would disguise their escape.
TLDR: Fit any medical oxygen mask, then throw a plastic bag over your head. Done.
I paint with 2-part (A and B part) Imron-style paints. The two molecules link up in a conga line of ABABABABABABABABA, forming an extremely long molecule. The magic B-part is isocyanate. When it's not grabbing A-parts, it likes to grab white cells. This can really mess up your immune system. It has very poor penetration through skin, but extremely good penetration through lung membrane.
If you brush or roll the stuff, the isocyanate stays on the brush or roller. But if you spray it, now it's aerosolized. So the painter is in the same pickle as your hero.
The manufacturer recommends protective gear with externally supplied air. It keeps positive pressure in the mask. The positive pressure assures no aerosolized material can get into the mask.
That's what your hero needs. Medical masks aren't designed to do that, but the external oxygen or air tank will provide positive pressure if only he could come up with a painter style mask/hood. So the hero needs to throw a plastic bag over his head and tie it at the neck with a competent but imperfect neck seal. Once the bag inflates, the mixed inhaled/exhaled air will leak out the neck seal. This will create the same positive pressure that the painter enjoys.
Assuming he doesn't have access to bottled air, the oxygen and his exhaled CO2 will dilute and expel the nitrogen in the air. He will be breathing a mix of oxygen and CO2. Which won't be pleasant! Pure oxygen isn't healthy, but the presence of CO2 creates the sensation of being unable to breathe, filling him with a desire to tear the bag off his head. The concentration of CO2 will be decided by how fast he runs the oxygen supply, which in turn will decide how long it lasts.
This isn't sustainable for hours, but could get him through a tough area.
If he also has access to nitrogen, (liquid nitrogen, used to preserve samples or cool the CT scanner's magnets)... he could make himself a perfectly pleasant cocktail that he could breathe indefinitely, at the cost of additional weight to carry.
As others have answered, it's physically possible.
However, you should note that breathing pure O2 is toxic.
I can't find clear figures for 1 bar of O2, but 0.5bar O2 causes irreversible damage in 16h. I'd expect 1 bar to be much faster (not just half the time).
Your character would need to be breathing it at just over 1bar, as he won't be able to generate negative pressure.
Also even in the short term, pure O2 causes hormonal responses – you should google for more info on this – which is why it's not used for resuscitations any more. There's also issues with the low CO2 levels in the blood.
Someone with actual medical knowledge can weigh in and expand this.