Singapore is having an episode of its regular hazy season because of smoke from forest fires. Some people have been using surgical masks like the the one shown at the bottom of this post, presumably for health reasons.

I know the mask has to stop unwanted particles from getting through. But at the same time, it has to let in enough air to breathe. I understand how it could work to keep people from getting sick - after all, bacteria are much larger than air molecules, so you could have little holes that let air through but keep bacteria out.

But aren't smoke particles too small to be stopped in this way? I think of smoke as a gas (it certainly looks that way, e.g., coming off a candle or a piece of burning incense). Are smoke particles as small as the molecules that I need to breathe in to survive? If they are, how can this mask differentiate between, say, oxygen and smoke?

surgical mask

  • 1
    $\begingroup$ Not all the components of smoke are gases; gases do not form "particles". $\endgroup$
    – DJohnM
    Jun 19, 2013 at 3:13
  • $\begingroup$ So? can you enlighten me with an answer? $\endgroup$
    – Saif60
    Jun 19, 2013 at 3:34
  • 1
    $\begingroup$ Aren't surgical masks designed more to keep stuff in (the wearer's respiratory tract) than to keep stuff out? $\endgroup$ Jun 19, 2013 at 4:51
  • $\begingroup$ @ChrisWhite thanks for editing the question. It certainly looks better now. $\endgroup$
    – Saif60
    Jun 19, 2013 at 5:11
  • $\begingroup$ Urban aerosols are a mix of gases and particles like soot, some of these are sub-micron in size. Also consider that these face masks (I used to wear them when ill when I lived in Tokyo) do not entirely cover the mouth and nose. $\endgroup$
    – user24901
    Jun 19, 2013 at 7:42

2 Answers 2


You are right, these masks are almost useless as a protection against urban aerosols. With swine flu, there was a lot of discussion (example) that even the best masks cannot catch virus particles which are only 100 nm in size. The usual surgical masks are even less effective - they will hardly block anything smaller that 1 micron.

Now, urban aerosols have several size modes: most numerous are just 10-50 nm in size, although most of the mass will be in large 1-10 micron particles (this is the soot/dust that you can see).

The largest particles are blocked by the mask - but they are also filtered by your nose. The smallest particles - below 200 nm - that are considered much more dangerous because they can reach your lungs and even enter the bloodstream. The mask will also not help against nitrogen dioxide - the major component of urban pollution.


Many air inhaled are leaked in, skipping the filter.

The boundaries of the mask do not fit faces, there are crevices. The air leak is huge. Let's say there are 20% air leak, then 20% virus remained in the air inhaled.

You can test the air leak by pressing the crevices of the mask and inhale, a difference will be felt.

In a surgical mask, the misfitting is remarkable.

I read a line in Internet: "Previous studies shown majority of (N95) masks fit less than 40% of the test population..."

So I thought the fitting is much more important than the filter, unfortunately the fitting problem is still unsolved.


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