Do oxygen and carbon dioxide move with brownian motion? And, if so, do they get trapped by N95 masks?

Health professionals are experts agree: we should be wearing masks all day when in public during the COVID-19 pandemic, and there are no serious health risks to wearing N95 respirators long-term, unless we have difficulty breathing (eg due to a disability or during strenuous exercise).

That said, I recently learned that N95 masks filter:

  1. At least 95% of particles that are 0.3 microns
  2. More than 95% of particles that are larger than 0.3 microns (this is intuitive)
  3. More than 95% of particles that are smaller than 0.3 microns (due to Brownian motion)

The following chart visualizes this, showing that the minimum fractional collection efficiency is at 0.3 microns. Particles that are both larger and smaller than 0.3 microns are, in fact, both easier to filter.

chart showing that the fractional collection efficiency is lowest at 0.3 microns, and higher for particles both larger and smaller than 0.3 microns source: wikipedia

And, at the same time, the common argument for why masks are safe is that oxygen and carbon dioxide particles are too small to be restricted. Here's some facts about the particles in question:

  1. The coronavirus is approximately 0.06 - 0.14 microns in size alone [3]
  2. Coronavirus is usually attached to other particles making it effectively a larger particle around 1 micron in size or larger [4]
  3. Oxygen gas (O2) is 0.000299 microns [5]
  4. Carbon Dioxide (CO2) is 0.00065 microns [6]

So, the question is: do O2 (0.000299 microns) and CO2 (0.00065 microns) travel with brownian motion? And, as such, are >95% of their particles prevented from moving through surgical N95 masks?


From what I can tell, yes, both oxygen and carbon dioxide gas particles will follow brownian dynamics as they move through the air on both sides of an N95 mask.

And, yes, studies (on people wearing N95 masks, not on the mask itself--which more directly answers the concern) have shown that carbon dioxide does get trapped in the mask and builds-up over time. (Roberge, et al., 2010) This has, in fact, shown a "significant increase" in transcutaneous CO2 levels in nurses wearing the masks for the greater part of 12-hour shifts, resulting in nurses complaining of "nausea, headache, light headedness, visual difficulties, shortness of breath, palpitations, confusion, and difficulty communicating" (Rebmann, et al., 2013)

However, in both studies there was no decrease in transcutaneous O2 levels due to N95 masks. Nor were there any other issues such as blood pressure. And the elevated CO2 levels did not reach the clinical definition of hypercapnia (defined as an arterial CO2 level ≥ 45 mmHg).

So, from what I can tell, it appears that wearing a mask all-day during mildly strenuous exercise (eg constant walking around a hospital) may cause minor discomfort (eg a headache), but it will not cause serious health issues.

And, certainly, the benefits of wearing a mask still appear to far outweigh the risks in the average person during the COVID-19 pandemic.


Rebmann, T., Carrico, R., & Wang, J. (2013). Physiologic and other effects and compliance with long-term respirator use among medical intensive care unit nurses. American journal of infection control, 41(12), 1218-1223. doi: 10.1016/j.ajic.2013.02.017 pmcid: PMC7132714

Roberge, R. J., Coca, A., Williams, W. J., Powell, J. B., & Palmiero, A. J. (2010). Physiological impact of the N95 filtering facepiece respirator on healthcare workers. Respiratory care, 55(5), 569-577. pmid: 20420727

  • This answer doesn't address the question exactly, and I do hope that someone can find a chart of the fractional collection efficiency of N95 masks for particles approaching 0.0001 microns in size. – Michael Altfield Sep 22 '20 at 10:16
  • Welcome to MedicalSciences.SE. When citing articles, can you please cite full reference information so that the articles can be easily found if the links break? I have helped you out with this one – Chris Rogers Sep 22 '20 at 13:24

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