Fomite transmission

Hand hygiene and surface cleaning are the major interventions aimed at preventing transmission by “fomites”, a term born in the dark days of medicine to describe transfer of pathogens from surfaces via touch to receptor areas of the body.  The extent to which fomites can cause infections depends on amount of viable pathogen on the surface and the efficiency with which the pathogens can be transferred to your finger.

Surface loading of SARS-CoV2 is very hard to estimate!  How does the virus get on a surface?  Without getting too graphic, there are probably two basic routes:

  1. Expiratory droplets and aerosols (from speaking, coughing etc) settle to the ground (or any surface).  Mask wearing would block this route.
  2. The infected person sneezes on their hand, then wipes their hand on a door knob (or variations on this theme).

The amount of pathogen in route 2 could be very high, and concentrated where some “victim” might touch, so there seems to be a very good case for cleaning high-touch surfaces frequently.

Transfer efficiency from different types of surfaces to fingers has been studied, long before COVID-19.  Lopez et al (2013) measured the transfer efficiency of a variety of viable pathogens from many types of surfaces for humid and dry conditions.  A simplified version of their Table 3 is shown below, focusing on the viral transfer, rather than the bacterial transfer.  In these experiments, the contaminated surface area was similar to the contact area with the finger.  For the transfer of pathogens from a mask, the most relevant case would be the high-humidity porous surface, resulting in transfer efficiencies less than 3%.

Surface Low Humidity (15-32%RH) High Humidity (40-65%RH)
Glass and acrylic 19-22% 67-80%
Metal, ceramic, laminate 5-10% 30-64%
Porous (cloth or paper) 0.03 to 0.5% 0.3-2.3%