Risk from contamination of your face mask?

In the early months of the pandemic, the general public was advised that the benefits of masking might be outweighed by the risks of transferring virus from your mask to your nose or eyes while putting the mask on or taking it off.   Now nearly all countries in the world require masks to be worn indoors (except in bars and restaurants), but the risk of mask contamination is still held out as a major consideration.  Thus, you should not wear masks that are contaminated, and you should clean your hands before and after handling your mask.  This is certainly good advice you should follow whenever you can, but sometimes we have awkward situations and need to make some practical decisions.  So how large is the self-contamination risk under typical conditions, relative to the risk of using, say, a poorer mask?

A simplified model of mask self-contamination

Consider typical non-medical conditions.  You are making a trip to a store, or on a bus, and nobody has sneezed on your mask, your hands, or a surface on which you placed the mask.  I’m talking about the contamination of the mask that occurs because you are inhaling air, and your mask has done its job by catching most of the particles carried by that air.

Suppose that over some period of time, you inhale (with no mask) 10,000 viruses carried by droplets and aerosols with a wide range of sizes.   Mask efficiencies vary, but you might expect that with a mask, you will inhale only 1000 viruses. Nearly all will land on the lining of your respiratory system – this is your dose.

You now have a mask with 9000 viruses embedded in the filtration material.  How many of these viruses would be transferred from the mask to the mucus membranes of your eye or your nose?  As far as I know, there have been no quantitative studies of this problem, and the theoretical risk of self-contamination has been supported primarily by studies that show that used masks contain detectable amounts of virus .

The transfer of viruses from the mask to your mucus membranes would normally have 3 stages.

  1. Your fingers must touch the contaminated portions of the mask.  Considering typical mask and finger sizes, I guess you contact 20% of the mask area with your fingers.
  2. For contacted areas of the mask, your fingers will pick up only a fraction of the particles embedded through the thickness of the mask.  These particles in the mask adhere to the filter material at least as strongly, say as pencil graphite adheres to paper, which does not brush off easily onto your fingers.  Let’s assume 3% of the particles in the contacted area transfer to your fingers based on fomite transfer efficiency tests.
  3. The contaminated portions of your fingers need to contact your mucus membranes.  This isn’t always going to happen, and even when it does, transfer will not be 100%.  Assume 30% transfer probability for this last stage.

Multiplying the efficiencies for these 3 stages, we expect your dose will be increased by only 16 viruses by mask contamination, compared to 1000 viruses coming from mask inefficiency, or 10,000 if you skip the mask altogether.  The risk of mask contamination is reduced further when the natural decay of the virus is considered.  On cardboard, the half-life of SARS CoV2 is about 3.5 hours.

The absolute numbers in these calculations mean nothing, and the transfer model has huge uncertainty, but it looks as if the risk of self contamination is far, far lower than the risk of using a mask that leaks, and trivial compared to the risk of not wearing a mask at all.   Again, this assumes the mask and your hands are not grossly contaminated, but maintaining hand hygiene is an important regardless of your masking practices.