UKH

I am curious to get an estimate of the rough size of the virus i.e. diameter and number of atoms it contains.  I can't find an authoritative source which gives these figures  but I did find a reference giving a diameter between 80-160 nanometres (which, being double, seems rather a wide range) and another saying 200 million atoms. I have done some figuring considering the typical diameter of atoms, bond lengths etc and I can't reconcile these figures. So one or other or both are potentially wrong; alternatively my calculations could easily be wrong. Virology is not my field. Can anyone on here enlighten me please regarding the size of this virus and the scale of its atomic make-up or point me to a suitable reference? 

If I pack all the atoms at 1.54 Å separations (carbon-carbon single bond length) I have a volume of (200x10^6 atoms * (1.54 x 10^-10)^3 m^3/atom) giving a total volume of 7.3x10^(-22) m^3.

The radius of a sphere of this volume is about 59 nm.

But there's no way the atoms are packed that densely; the molecules of life have a lot of space in them - they need it to be able to bend, flex and move in their role as machinery,.  Think about how high density a tight-packed atomic form such as diamond or a metal is compared to typical living things.

People can argue the toss about if a virus is alive or not, but it's made out of the same sorts of stuff as life, so that's good enough for my point.

So I don't have a problem reconciling a noddy estimate of 59 nm with the 80 - 160 nm range you gave.  It's a good back-of-the-envelope sanity check on the numbers you give;  it comes out in the right ball pack, and off to the side we expect.   

"2019-nCoV also has enveloped virions that measure approximately 50–200 nm in diameter with a single positive-sense RNA genome."  from this paper in the Lancet in February: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7135076/

That in turn cites another paper but that one is not open access so I haven't linked it here.

As wintertree said, the atoms that make up the virus will have a lot of space in them - for example proteins form complex shapes that can only pack together in certain ways, and the RNA genome will be a long chain that may fold into different conformations depending on various factors.  Variability in size is also expected. This may be normal biological variation and/or may depend on where individual virions were in their lifecycle when measurements were made.

I'd say that was accurate enough for most purposes!

The wide size range is because coronavirus lipid envelopes do not have a nice, regular shape: Think potato rather than golf ball!

That envelope contains the ca. 30 kb RNA genome wrapped around a protein scaffold, a bunch of other structural proteins and enzymes in varying numbers, and whatever cytoplasmic fluid (and the gunk swimming around in it) of the host cell that got pinched off together with the virion during budding.

This is different from, say, picornaviruses, which lack envelopes and are almost cystalline footballs! We even use empty virus capsids of that type tagged with fluorescent proteins to standardize molecular brightness in single molecule microscopy: We know that each assembled particle will contain exactly 12 fluorescent subunits.

CB

Thank you all for your various contributions. Very interesting and informative . It seems I needed to make more allowances in my sums for the biological nature of the beast .

So, what you are saying is that most masks that people wear wouldn't prevent it passing through?

The virus tends to be in droplets in your breath and fewer of the droplets get through the masks.

Something I learnt from UKC - it's not an airborne virus because medical definition of airborne means virus in the air. It is not virus in the air. For this virus it is droplets containing virus in the air. The tiny droplets are huge (a thousand or millions of times heavier) compared to the virus. The bigger droplets containing more virus are more likely to be blocked by a mask.

I think the definition is much more to do with the transmission mechanism. In which case, it is predominantly airborne (in an aerosol), and secondarily surface transfer.

This is to distinguish it from other transmission mechanisms, such as waterborne, body fluid transfer, or animal vector (bites, stings, etc).

Size of Covid virus? That would’ve made my childhood somewhat confusing.....

7.33

youtube.com/watch?v=tA6zp9L44GA&

I would say that a ~100 million atoms was quite a good estimate for a virus with a size of ~100 nm. Atoms are ~1 Angstrom in diameter, so about 1000 times smaller than the whole virus. Cubing that ratio gives about a billion. Knocking that down for a more realistic atomic spacing (say 1.6 A) and allowing for free volume (not enormous) - does indeed bring the estimate down to a couple of hundred million atoms.

Actually I think it's droplets below a certain size count as airborne so I was probably wrong (and may still be!). Small droplets may not contain enough to infect. Larger droplets more likely to be blocked by mask.

The virus is carried in water droplets.

Masks protect against transmission by large droplets which are large enough to be blocked by a mask provided it is worn correctly. Without a mask, droplet transmission is a big risk to people within ~1metre. Over larger distances, the concentration of larger droplets rapidly drops off as they fall to the ground due to gravity.

"Airborne transmission" occurs over larger distances when the virus is carried by much smaller droplets (~5nm), commonly referred to as aerosols. This would not be prevented by most masks. While there is evidence that the virus can be detected in aerosols over larger distances, the number of virus particles are low and this does not appear to be a significant transmission pathway.

Masks do have a significant effect at reducing the distance that larger droplets travel, which appears to be the main path of transmission.

I think you may have meant ~5 um (micrometres, I can't find the mu symbol)  rather than nanometres. 5nm is way smaller than the covid virus. In fact 5nm is about 5 glucose molecules end to end. 

I won't be spotting them on my birdwatching trips then - even with my Zeiss binoculars.

Filters work in a more complex way than a simple sieve so pore size does not directly correlate to the size of particle that can pass through. Inertial mass effect means larger particles/droplets just get stuck on the filter media, so multiple layers increase the chance of any one particle getting stuck. The interception effect is similar but relates to the convoluted route that particles must take to get through the filter so the particles get stuck on the media trying to get round corners. So again layers help slow the speed on the gas passing though and increases this effect. Most of the effect of a filter is trapping the particles on the filter medium rather than just sieving based on particle size.

Added to that, the filters are made of a wicking material: Any liquid droplets touching or settling on the paper or cotton fibers will be absorbed into the material, trapping viruses travelling in these droplets on the filter matrix.

CB

I have done quite a bit of relevant reading and study both during lockdown and, coincidentally, in the year prior to that. The 80 to 160nm range is familiar to me as is the 50 to 200nm range. Some are also using 100 or 120nm as typical sizes but it seems like the 80 to 160nm range has some acceptance. 

What is important to understand is that none of the masks are designed to filter viruses as such. Generally, the boundary between the virus world and bacterium world is around 300nm and it is this measurement that is used as a standard for the filtration of the masks. So these are masks designed for filtering out bacteria.

Viruses exist in an airstream as aerosols, that are light enough to remain suspended, and droplets, which are 5 microns or larger and deemed to be subject to ballistic pathways. So both aerosols and droplets are larger than a single virus and very likely to be trapped by 300nm mask filter. Masks may filter not only through a physical barrier but also electrostatically. 

None of these filtration properties, or the low breathing resistance requirement of the EN standards, are possessed by masks run up by your auntie on her sewing machine, or by masks quickly thrown together in a profiteering sweat-shop in the middle of a pandemic.

However, in a non-clinical environment, ANY device that takes some of the momentum out of exhaled air is helpful. It has the effect of reducing the 2m physical distancing requirement. It should be remembered that although your favourite buff will do this job of taking some of the momentum out of your exhaled air, it is soaked in virus particles if you are unknowingly one of the millions of asymptomatic infected persons: wash it out every night. 

Masks that meet the standards fail to contain exhaled particles if the wearer sneezes or coughs. Studies have shown that these events result in airstreams to the side and rearwards for several hundred millimetres. Improvised masks in the same style as surgical masks will be worse because they do not pass the breathing resistance requirements. Wearing a buff will not have this effect but some with a low level of filtering will emit significant particles forwards, although normal cough precautions will be effective.