[id2000 (OP)]

I have read reports in reputable newspapers that some European countries are asking citizens to minimise outdoor activities such as walking, running & cycling.

Surely there is no risk of contracting an infection while exercising in the open air providing one takes care to avoid contact with others?

[evan_au]

I heard of some measurements by the US NIS that measured the half-life of infectious viral particles on various surfaces.

They found that half of the virus was still infectious after an hour when created in the form of an aerosol (virus particles floating in the air).

So when you are out walking or cycling, you are potentially breathing in virus particles left by someone who passed that way an hour before.
- If there is a breeze, you may be breathing in virus particles expelled by a sneeze from an infected person a mile away.

Of course, the outdoors has a higher level of UV light from the Sun than you would find in an indoor lab.
- UV would tend to inactivate virus particles faster
- And the outdoors has a lot more volume in which to spread out the virus particles, so you are less likely to breathe in an active virus than if you spent an hour in a small room with an infected person.
- But this only works if there are no infected people outdoors; later in the pandemic, when there are many infected people, if they are all outdoors, you are better off inside by yourself.

[id2000 (OP)]

They found that half of the virus was still infectious after an hour when created in the form of an aerosol (virus particles floating in the air).

So when you are out walking or cycling, you are potentially breathing in virus particles left by someone who passed that way an hour before.

That is quite scary and something I did not realise.

On a more positive note I have found this on the Public Health England website:

You can also go for a walk or exercise outdoors if you stay more than 2 metres from others.

The page was last updated on 20th March.


From reading around it seems that there is a view that the virus is not normally distributed in aerosol form. However, there seems to be particular concern about some medical and dental procedures (e.g. scaling).

So do you think it would be fair to say that going for a walk outdoors in a quiet area and keeping a distance of at least 2 metres is very low risk even if not zero risk?

[Colin2B]

From reading around it seems that there is a view that the virus is not normally distributed in aerosol form.

I don’t know where you read that, but it is not the current scientific view eg CDC
Main infection route is considered to be aerosols caused by people coughing or sneezing etc, close contact eg hugging, etc hence the 2m rule. Pick up from surfaces is not thought to be the main route, but it’s worth taking precautions as it is easy to touch a contaminated surface and then eyes, nose or mouth.
The tests which were done on the virus showed that it was still viable after 3hrs in still air, however in moving air it quickly settles on a surface - which might be you.

So do you think it would be fair to say that going for a walk outdoors in a quiet area and keeping a distance of at least 2 metres is very low risk even if not zero risk?

There is unlikely to be zero risk. On a still day with someone coughing and sneezing nearby, you could be at risk, but if there is a breeze and you are upwind then your risk would be low.

[id2000 (OP)]

From reading around it seems that there is a view that the virus is not normally distributed in aerosol form.

I don’t know where you read that, but it is not the current scientific view eg CDC

I don't think I am allowed to post a link but I found this on a science website called STAT - it was written by Sharon Begley who seems to be a respected science journalist and author.


The weight of the evidence suggests that the new coronavirus can exist as an aerosol — a physics term meaning a liquid or solid (the virus) suspended in a gas (like air) — only under very limited conditions, and that this transmission route is not driving the pandemic. But “limited” conditions does not mean “no” conditions, underlining the need for health care workers to have high levels of personal protection, especially when doing procedures such as intubation that have the greatest chance of creating coronavirus aerosols. “I think the answer will be, aerosolization occurs rarely but not never,” said microbiologist and physician Stanley Perlman of the University of Iowa. “You have to distinguish between what’s possible and what’s actually happening.”

and

If virus particles, probably on droplets of mucus or saliva, can be suspended in air for more than a few seconds, as the measles virus can, then anyone passing through that pathogenic cloud could become infected.

There are strong reasons to doubt that the new coronavirus has anything close to that capability.

“If it could easily exist as an aerosol, we would be seeing much greater levels of transmission,” said epidemiologist Michael LeVasseur of Drexel University. “And we would be seeing a different pattern in who’s getting infected. With droplet spread, it’s mostly to close contacts. But if a virus easily exists as an aerosol, you could get it from people you share an elevator with.”



The article was dated 16th March so I guess the thinking could have moved on in the last week.

[Colin2B]

The article was dated 16th March so I guess the thinking could have moved on in the last week.

Probably.  Tests show it was suspended in still air for up to 3 hrs (they cut off the test at this point, probably thinking they wouldn’t find any) and as @evan_au says, it was still viable after 1hr. However, there could be differences in how different people are using the term aerosol and the figures are for still air, in a lift you do get air movement. Even so, I wouldn’t want to share a lift with someone who coughed!
Certainly CDC advice at moment is that aerosol/droplet transmission is main mechanism.
Also, remember R₀=3 is a very high transmission rate.