what determines virus durability on various surfaces?

This is more broadly applicable than just the coronavirus, so I’m putting it in GQ.

The news has reported a range of periods of viability for the coronavirus on various surfaces. Anywhere from a couple of hours adrift on the wind, to a few days on some solid surfaces.

So why does viability of a virus vary depending on what surface we’re talking about (cardboard, countertop, metal doorknob, etc.)?

The environment and the type of surface that fomites (small droplets that the virus is transmitted in) reside on definitely has an impact upon the persistence of the virion.

Virions (the encapsulated form of the virus that is actually transmitted from organism to organism) need a fairly moist environment to survive. An environment that is low humidity or which absorbs moisture reduces the time before the capsid—the protective and molecularly interactive protein container around the genomic material that makes the virus biogenically active—starts to break down and exposes the delicate DNA or RNA inside, which itself has no means to induce a host cell to replicate it. Some viruses also have lipid coat referred to as viral envelope which aids in avoiding stimulating an immune response; although it is another layer these are generally very sensitive to desiccation rendering these virions less persistent in the environment.

In general, virions will remain active longer on non-porous substances such as metal, plastic, and sealed wood (up to several days and potentially as long as a month although some metals such as copper and silver have an antimicrobial and antiviral effect) and are less likely to survive in porous substances that will absorb water such as cloth and unsealed wood (usually just a couple of days or less). They also break down in the presence of ultraviolet light, although waving your SteriPen around in the air won’t do you much good; intense UV-C light which irradiates all exposed surfaces for tens of seconds is necessary to break down virions. Since the Earth’s protective ozone layer filters out UV-C light, intermittent exposure to sunlight does not assure sanitation though there is epidemiological evidence that the longer duration of sunlight in summer months correlates to less viral transmission overall.

Virions spread by aerosol vector will generally disperse and contact the ground or other horizontal surfaces, which is why maintaining an adequate distance provides some prophylactic effect, but the 6 ft/2 m range currently being recommended is really pretty arbitrary; if you are in a confined space with someone actively shedding the environment it could easily travel to fill the volume. BTW, this is a serious probably for space travel; the gravity environment of the Earth provides a certain latent protection against the spread of viruses and pathogenic bacteria which is not present in a free-fall or reduced gravity environment, and this is a serious consideration for long-term habitability in space or colonies on a low gravity body.

Carl Zimmer’s A Planet of Viruses is a very accessible introduction to virology and at barely over a hundred pages can be read in an afternoon. However, depending on your current state of mind you may not want to read it, as it doesn’t offer a lot of comfort about our ability to respond to viral pandemics of a more virulent nature.