I was riding the subway and noticed an educational video on how to act in case of a fire hazard.

Looking back, it's common knowledge to use a wet paper towel in case of close areas in the presence of smoke.

But what exactly is the reason for having to wet a paper towel? Is it water - being the polar molecule it is - attracting molecules or the oxygen content dissolved in water helping us survive longer?

  • $\begingroup$ I'm going to guess that a wet paper towel will just shred if you try to breathe through it. $\endgroup$
    – Dan
    Commented Dec 27, 2021 at 1:53

2 Answers 2


Smoke isn't what you might think it is. What is referred to in layman terms as "smoke" or "fumes" or various similar terms is actually a mix of two distinct components:

  • solid airborne particles or liquid droplets (aerosols)
  • actual gasses (vapours)

Much of what you see in visible smoke (maybe all of it, actually) is solid particles. The only time I can think of being able to see gasses might be your breathe during the winter time and dry ice, and I'm pretty sure that's actually not vapours but just ice crystals which are solid particles.

Passing the air through a soaked material before you breathe it causes the solid particles to be removed from the air because they adhere to the liquid in the cloth via surface tension tension. It's the same as how rain removes dust from the air or how you use a wet cloth to wipe dust from a table. A good dry filter, like a HEPA filter, will do the same thing. So would a cloth soaked in something like mineral/baby oil.

But this type of mechanical filtering (or any type of mechanical filtering) won't protect you from the vapours. For a similar method to protect you from the vapours, something in the cloth must react with the vapour. The best example of this is breathing through a urine soaked cloth would protect you from mustard gas in World War 1.

And it obviously won't protect you from lack of oxygen.

This is why industrial respirators have both particulate filter cartridges and chemical vapour cartridges. And that's why there's many types of chemical vapour cartridges targeted for different substances but only only one or two particulate filter cartridges varying in particulate size. And why they say they are not for use in oxygen deficient environments.

  • $\begingroup$ Thanks a lot. This answered some of the extra questions I had. I never knew some respirators even had chemical vapour cartridges. $\endgroup$ Commented Dec 29, 2021 at 12:11

I'm no expert in this field, but my guess is that smoke is carbon monoxide. So it's one part solid, one part gas. My guess is that the wet cloth prevents the solids from coming through (the carbon). I'm not saying that you breathe in clean oxygen, but perhaps it filters the air enough that you can survive just a bit longer. As for why a dry cloth won't work, paper towels and even wash cloths are actually porous, the holes are just too small to see. Perhaps when wet, these pores become clogged with water, trapping the carbon in it. This would definitely become polluted, so eventually you'll breathe in toxic gases again, but it will (hopefully) last long enough for you to escape the smoke. That being said, the best way to avoid breathing smoke is to get under it, as the molecules are lighter than air so they rise up. That's why they recommend crawling when in a house fire, etc.


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