[magnus]

The CDC has updated its guidelines on close contact to include numerous encounters that add up to more than 15 minutes.

This after a Vermont prison guard got infected without prolonged contact.

https://www.cnn.com/2020/10/21/health/cd...index.html

He was wearing a microfiber mask and goggles.  Prisoners were not all wearing masks and were asymptomatic at the time.

Just reinforces how little we know for certain about this virus. =(

[magnus]

The CDC has updated its guidelines on close contact to include numerous encounters that add up to more than 15 minutes.

This after a Vermont prison guard got infected without prolonged contact.

https://www.cnn.com/2020/10/21/health/cd...index.html

He was wearing a microfiber mask and goggles.  Prisoners were not all wearing masks and were asymptomatic at the time.

Just reinforces how little we know for certain about this virus. =(

[fullmetal]

Seems typical of hospital healthcare workers who lapse in their PPE or precautions, even for a single patient exam.  Close quarters is no good.

[fullmetal]

Seems typical of hospital healthcare workers who lapse in their PPE or precautions, even for a single patient exam.  Close quarters is no good.

[DocSavage87]

This seems fairly obvious to me and has been something I've been saying since the beginning.  It's cumulative exposure time, and you also have to take into account the environment that can be accumulating virus if enclosed and not ventilated.

Likewise, the fact that six feet social distancing can be further elaborated is obvious.  The public message has to balance complexity that reflects our current understanding and simplicity that the general public can understand and handle.

[DocSavage87]

This seems fairly obvious to me and has been something I've been saying since the beginning.  It's cumulative exposure time, and you also have to take into account the environment that can be accumulating virus if enclosed and not ventilated.

Likewise, the fact that six feet social distancing can be further elaborated is obvious.  The public message has to balance complexity that reflects our current understanding and simplicity that the general public can understand and handle.

[OutsiderFan]

As my nurse friend said last week. So much of what we must do is just common sense. Unfortunately, it isn't as common as we think or maybe wish to believe.

[OutsiderFan]

As my nurse friend said last week. So much of what we must do is just common sense. Unfortunately, it isn't as common as we think or maybe wish to believe.

[GoodGrief]

Does anyone have an opinion if the total exposure “over 24 hours” time frame is legitimate? Is the idea that if there’s not enough virus to take hold within 24 hours that it will die out in the body?

[GoodGrief]

Does anyone have an opinion if the total exposure “over 24 hours” time frame is legitimate? Is the idea that if there’s not enough virus to take hold within 24 hours that it will die out in the body?

[Farm93]

Does anyone have an opinion if the total exposure “over 24 hours” time frame is legitimate? Is the idea that if there’s not enough virus to take hold within 24 hours that it will die out in the body?

I feel for the CDC here.   What an impossible task.

They have to do enough to scare citizens into sensible action while not saying too much that might scare them into panic.
That'd be hard enough.
Then they have to say it in a way that doesn't offend the federal official living in the WH.

In recent days in SoCal I have seen so many people walk without masks in parking lots passing all kinds of folks while they walk to a store or business.   Only as they get to the front door of a store or business do they quickly put on a mask as they open the door.

Not sure if they are merely motivated to comply with store policies, or if they now believe that masks are only needed indoors because they heard that from people in power.   Doesn't matter really, COVID is going to thrive in the USA over the next 4 months. 

#1) Strive for 0 minutes of exposure each day.
#2) Wear a mask every time you leave your home/apartment. 
#3) Avoid indoor and outdoor group events of any size.

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   

OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

[Farm93]

Does anyone have an opinion if the total exposure “over 24 hours” time frame is legitimate? Is the idea that if there’s not enough virus to take hold within 24 hours that it will die out in the body?

I feel for the CDC here.   What an impossible task.

They have to do enough to scare citizens into sensible action while not saying too much that might scare them into panic.
That'd be hard enough.
Then they have to say it in a way that doesn't offend the federal official living in the WH.

In recent days in SoCal I have seen so many people walk without masks in parking lots passing all kinds of folks while they walk to a store or business.   Only as they get to the front door of a store or business do they quickly put on a mask as they open the door.

Not sure if they are merely motivated to comply with store policies, or if they now believe that masks are only needed indoors because they heard that from people in power.   Doesn't matter really, COVID is going to thrive in the USA over the next 4 months. 

#1) Strive for 0 minutes of exposure each day.
#2) Wear a mask every time you leave your home/apartment. 
#3) Avoid indoor and outdoor group events of any size.

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   

OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

[dabigv13]

I think passing somebody in a parking lot without a mask would be extremely low risk, unless they coughed right in your face.

It is a tricky balance, because it's all about a threshold of risk based on incomplete information.

Consistency and simplicity are the key things. But the risk is a spectrum, and you have to make arbitrary decisions, like the 6 foot rule, that are only loosely based on science. 

I don't think there is anything special about 24 hours. If you see a covid positive patient for 5 minutes on three consecutive days, are you more or less at risk than 15 minutes in one go? I don't think that is knowable at this time.

[dabigv13]

I think passing somebody in a parking lot without a mask would be extremely low risk, unless they coughed right in your face.

It is a tricky balance, because it's all about a threshold of risk based on incomplete information.

Consistency and simplicity are the key things. But the risk is a spectrum, and you have to make arbitrary decisions, like the 6 foot rule, that are only loosely based on science. 

I don't think there is anything special about 24 hours. If you see a covid positive patient for 5 minutes on three consecutive days, are you more or less at risk than 15 minutes in one go? I don't think that is knowable at this time.

[82 Card]

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   


OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

Although Germany's infection rates are significantly lower than in the US, the rates are jumping. Using Worldometers numbers, on the last 3 days (including today),  German new cases per 100K has been 8.5, 12.5 and 14.9. On the same days, the US has had 19.0, 19.5 and 22.6. 

Germany's deaths per million have been much lower than US. On the last three days, German Covid death rate per million was 0.668, 0.525 and 0.537. In the US, the Covid death rate per million was 2.900, 3.731 and 2.964.

Note: This post was originally done an hour before the end of the day (GMT). It has been updated to reflect more numbers that came in at end of day and corrections to the previous two days that were also posted late in the day. Unfortunately, today turned out to be the fourth highest day for new cases in US, only 4675 less than the record set on July 24.

[82 Card]

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   


OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

Although Germany's infection rates are significantly lower than in the US, the rates are jumping. Using Worldometers numbers, on the last 3 days (including today),  German new cases per 100K has been 8.5, 12.5 and 14.9. On the same days, the US has had 19.0, 19.5 and 22.6. 

Germany's deaths per million have been much lower than US. On the last three days, German Covid death rate per million was 0.668, 0.525 and 0.537. In the US, the Covid death rate per million was 2.900, 3.731 and 2.964.

Note: This post was originally done an hour before the end of the day (GMT). It has been updated to reflect more numbers that came in at end of day and corrections to the previous two days that were also posted late in the day. Unfortunately, today turned out to be the fourth highest day for new cases in US, only 4675 less than the record set on July 24.

[Goose]

I think passing somebody in a parking lot without a mask would be extremely low risk, unless they coughed right in your face.

It is a tricky balance, because it's all about a threshold of risk based on incomplete information.

Consistency and simplicity are the key things. But the risk is a spectrum, and you have to make arbitrary decisions, like the 6 foot rule, that are only loosely based on science. 

I don't think there is anything special about 24 hours. If you see a covid positive patient for 5 minutes on three consecutive days, are you more or less at risk than 15 minutes in one go? I don't think that is knowable at this time.

I really like this post because it points out the uncertainties involved in almost all of the mitigation strategies we are using. In many instances, we just don't know. All the recommendations involve arbitrary decisions and trade-offs. How much better is 7 feet than 6 feet? Nobody really knows, although there are models (that may or may not be any good) that attempt to tell us that. The public hates uncertainty. However, asserting certainty when it actually doesn't exist eventually destroys credibility. The public health secor has a reall tough job here.

[Goose]

I think passing somebody in a parking lot without a mask would be extremely low risk, unless they coughed right in your face.

It is a tricky balance, because it's all about a threshold of risk based on incomplete information.

Consistency and simplicity are the key things. But the risk is a spectrum, and you have to make arbitrary decisions, like the 6 foot rule, that are only loosely based on science. 

I don't think there is anything special about 24 hours. If you see a covid positive patient for 5 minutes on three consecutive days, are you more or less at risk than 15 minutes in one go? I don't think that is knowable at this time.

I really like this post because it points out the uncertainties involved in almost all of the mitigation strategies we are using. In many instances, we just don't know. All the recommendations involve arbitrary decisions and trade-offs. How much better is 7 feet than 6 feet? Nobody really knows, although there are models (that may or may not be any good) that attempt to tell us that. The public hates uncertainty. However, asserting certainty when it actually doesn't exist eventually destroys credibility. The public health secor has a reall tough job here.

[Farm93]

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   


OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

Although Germany's infection rates are significantly lower than in the US, the rates are jumping. Using Worldometers numbers, on the last 3 days (including today),  German new cases per 100K has been 8.5, 12.5 and 14.9. On the same days, the US has had 19.0, 19.5 and 22.6. 

Germany's deaths per million have been much lower than US. On the last three days, German Covid death rate per million was 0.668, 0.525 and 0.537. In the US, the Covid death rate per million was 2.900, 3.731 and 2.964.

Note: This post was originally done an hour before the end of the day (GMT). It has been updated to reflect more numbers that came in at end of day and corrections to the previous two days that were also posted late in the day. Unfortunately, today turned out to be the fourth highest day for new cases in US, only 4675  less than the record set on July 24.

And remember - October in Germany is rather cold. 
There are a lot of places in the USA that won't see coronavirus friendly conditions like that until November with numbers emerging in December.
Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.
Thanksgiving traditions probably will appear in the USA numbers in December.
Therefore, better to measure fall/winter success relative to another place by March 31 2021 rather than day to day.

[Farm93]

If we do all of that we might be able to get to a place where our infection and fatality rates for the next 6 months are almost as good as Germany's rates.   


OK, we probably will not be as good as Germany, but hate to say let's try in this wave to beat Italy or France.   :(

Although Germany's infection rates are significantly lower than in the US, the rates are jumping. Using Worldometers numbers, on the last 3 days (including today),  German new cases per 100K has been 8.5, 12.5 and 14.9. On the same days, the US has had 19.0, 19.5 and 22.6. 

Germany's deaths per million have been much lower than US. On the last three days, German Covid death rate per million was 0.668, 0.525 and 0.537. In the US, the Covid death rate per million was 2.900, 3.731 and 2.964.

Note: This post was originally done an hour before the end of the day (GMT). It has been updated to reflect more numbers that came in at end of day and corrections to the previous two days that were also posted late in the day. Unfortunately, today turned out to be the fourth highest day for new cases in US, only 4675  less than the record set on July 24.

And remember - October in Germany is rather cold. 
There are a lot of places in the USA that won't see coronavirus friendly conditions like that until November with numbers emerging in December.
Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.
Thanksgiving traditions probably will appear in the USA numbers in December.
Therefore, better to measure fall/winter success relative to another place by March 31 2021 rather than day to day.

[oldalum]

Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.

Oktoberfest 2020 was officially canceled, but I don't know what actually went on anyway.

[oldalum]

Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.

Oktoberfest 2020 was officially canceled, but I don't know what actually went on anyway.

[BostonCard]

A couple of numbers:

The infectious dose of SARS-CoV (the original SARS virus) is estimated at 300 or so viral particles (this is typically represented as an ID50, or the dose that causes infection in 50% of people) https://onlinelibrary.wiley.com/doi/full...10.01427.x.  The infectious dose of SARS-CoV-2 which causes COVID-19 is probably similar.

Normal talking can generate up to 3000 1-micron particles per minute in exhaled breath,4 and each particle could contain more than 250 virions, which means that a single minute of speaking potentially generates more than 750,000 virions. (link, it is in the second letter).  Obviously, the longer the exposure, the more particles are generated.

Figuring out how many of those viruses end up being inhaled by the exposed individual is complicated and is affected by a whole host of issues, including mask-wearing, distance, confinement, ventilation, etc.

Risk = exposure x time.

BC

[BostonCard]

A couple of numbers:

The infectious dose of SARS-CoV (the original SARS virus) is estimated at 300 or so viral particles (this is typically represented as an ID50, or the dose that causes infection in 50% of people) https://onlinelibrary.wiley.com/doi/full...10.01427.x.  The infectious dose of SARS-CoV-2 which causes COVID-19 is probably similar.

Normal talking can generate up to 3000 1-micron particles per minute in exhaled breath,4 and each particle could contain more than 250 virions, which means that a single minute of speaking potentially generates more than 750,000 virions. (link, it is in the second letter).  Obviously, the longer the exposure, the more particles are generated.

Figuring out how many of those viruses end up being inhaled by the exposed individual is complicated and is affected by a whole host of issues, including mask-wearing, distance, confinement, ventilation, etc.

Risk = exposure x time.

BC

[M T]

A couple of numbers:

The infectious dose of SARS-CoV (the original SARS virus) is estimated at 300 or so viral particles (this is typically represented as an ID50, or the dose that causes infection in 50% of people) ... The infectious dose of SARS-CoV-2 which causes COVID-19 is probably similar.

Normal talking can generate up to 3000 1-micron particles per minute in exhaled breath,4 and each particle could contain more than 250 virions, which means that a single minute of speaking potentially generates more than 750,000 virions. (link, it is in the second letter).  ...

Figuring out how many of those viruses end up being inhaled by the exposed individual is complicated and is affected by a whole host of issues, including mask-wearing, distance, confinement, ventilation, etc.

With these numbers, inhaling one of the 1 micron particles, puts you close to the ID50 dose.   Inhale two particles and you're well over that dose.   (And, of course, the exhaled particles have a wide spectrum of sizes, and I presume a wide spread of viral particles.)   1 micron particles are well within the aerosol range (typically described as < 5 microns) and in the range to float around in air for a long time.

I believe these numbers are misrepresentative at best.  Specifically, I don't see the "3000 1-micron particles per minute in exhaled breath" as stated in the Brosseau(et al) letter to NEJM.  The reference in the letter is to a study by Asadi. As I understand it, he was looking for aerosolizable particles emitted during vocalizations.  I presume the letter authors' "3000 1-micron particles per minute" would come from the paper's slightly less than 50 particles per second.  This number is the largest measured value for the loudest continuous "ahhhh" sound (NOT "normal talking").  (I will also note that Asadi indicates that the instrument intentionally did not measure every particle.)   If you look at Figure 5A, you will see that the average loud speech (reading) measured less than 6 particles/second (<360 per minute;).  There is a high outlier at 17 particles/second (~ 1000/minute) for someone termed a "speech superemitter".

The letter didn't specify a reference for the 250 virons per micron-sized droplet.

The letter's numbers aside, the Asadi study shows that loud talking (~5.3 particles/sec) results in 75% more particles that intermediate speech (~3.0p/s), and 4x quiet speech(~1.2).  Even quiet speech dwarfs mouth breathing (which appears to be ~0.1particles/sec) and nose breathing (too low for me to estimate).  Unfortunately, he didn't measure yelling (as in an argument, or at a sporting event).

If anyone is interested, I put together a HTML file (with Javascript) that simulates someone emitting particles at a specified rate in a specified room, with another person inhaling the particles for a specified period, to get a concept of how all these numbers work together to give a dose to the 2nd person.
For example, if you are in a 10'x10'x8' room with an infected person who talks quietly for 15 minutes (1.2 aerosol particles/sec @ 250 virons/particle), with air exchanges at the low end recommended for a household (0.35/hour), and neither has a mask, you will inhale about 580 virons.  If the infected person is quiet, you'll get maybe 50 virons.


I don't know about others, but when I've been around people that have drunk too much, they've typically been overly loud.  Here's one more reason to avoid anyone in that state.   (I would also say that a trip to the library might be one of the safest places to go.;)

From the Asadi article:

Chao et al. used an interferometric imaging technique to obtain the size distribution of particles larger than 2 μm and found that counting aloud from 1 to 100 releases at least 6 times as many particles as an individual cough. Likewise, Morawska and coworkers reported that counting aloud for 10 seconds followed by 10 seconds of breathing, repeated over two minutes, releases half as many particles as 30 seconds of continual coughing, which in turn releases half as many particles as saying “aah” for 30 seconds.

[M T]

A couple of numbers:

The infectious dose of SARS-CoV (the original SARS virus) is estimated at 300 or so viral particles (this is typically represented as an ID50, or the dose that causes infection in 50% of people) ... The infectious dose of SARS-CoV-2 which causes COVID-19 is probably similar.

Normal talking can generate up to 3000 1-micron particles per minute in exhaled breath,4 and each particle could contain more than 250 virions, which means that a single minute of speaking potentially generates more than 750,000 virions. (link, it is in the second letter).  ...

Figuring out how many of those viruses end up being inhaled by the exposed individual is complicated and is affected by a whole host of issues, including mask-wearing, distance, confinement, ventilation, etc.

With these numbers, inhaling one of the 1 micron particles, puts you close to the ID50 dose.   Inhale two particles and you're well over that dose.   (And, of course, the exhaled particles have a wide spectrum of sizes, and I presume a wide spread of viral particles.)   1 micron particles are well within the aerosol range (typically described as < 5 microns) and in the range to float around in air for a long time.

I believe these numbers are misrepresentative at best.  Specifically, I don't see the "3000 1-micron particles per minute in exhaled breath" as stated in the Brosseau(et al) letter to NEJM.  The reference in the letter is to a study by Asadi. As I understand it, he was looking for aerosolizable particles emitted during vocalizations.  I presume the letter authors' "3000 1-micron particles per minute" would come from the paper's slightly less than 50 particles per second.  This number is the largest measured value for the loudest continuous "ahhhh" sound (NOT "normal talking").  (I will also note that Asadi indicates that the instrument intentionally did not measure every particle.)   If you look at Figure 5A, you will see that the average loud speech (reading) measured less than 6 particles/second (<360 per minute;).  There is a high outlier at 17 particles/second (~ 1000/minute) for someone termed a "speech superemitter".

The letter didn't specify a reference for the 250 virons per micron-sized droplet.

The letter's numbers aside, the Asadi study shows that loud talking (~5.3 particles/sec) results in 75% more particles that intermediate speech (~3.0p/s), and 4x quiet speech(~1.2).  Even quiet speech dwarfs mouth breathing (which appears to be ~0.1particles/sec) and nose breathing (too low for me to estimate).  Unfortunately, he didn't measure yelling (as in an argument, or at a sporting event).

If anyone is interested, I put together a HTML file (with Javascript) that simulates someone emitting particles at a specified rate in a specified room, with another person inhaling the particles for a specified period, to get a concept of how all these numbers work together to give a dose to the 2nd person.
For example, if you are in a 10'x10'x8' room with an infected person who talks quietly for 15 minutes (1.2 aerosol particles/sec @ 250 virons/particle), with air exchanges at the low end recommended for a household (0.35/hour), and neither has a mask, you will inhale about 580 virons.  If the infected person is quiet, you'll get maybe 50 virons.


I don't know about others, but when I've been around people that have drunk too much, they've typically been overly loud.  Here's one more reason to avoid anyone in that state.   (I would also say that a trip to the library might be one of the safest places to go.;)

From the Asadi article:

Chao et al. used an interferometric imaging technique to obtain the size distribution of particles larger than 2 μm and found that counting aloud from 1 to 100 releases at least 6 times as many particles as an individual cough. Likewise, Morawska and coworkers reported that counting aloud for 10 seconds followed by 10 seconds of breathing, repeated over two minutes, releases half as many particles as 30 seconds of continual coughing, which in turn releases half as many particles as saying “aah” for 30 seconds.

[M T]

And remember - October in Germany is rather cold. 
There are a lot of places in the USA that won't see coronavirus friendly conditions like that until November with numbers emerging in December.
Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.
Thanksgiving traditions probably will appear in the USA numbers in December.
Therefore, better to measure fall/winter success relative to another place by March 31 2021 rather than day to day.

As part of my trying to get some estimate of the number of virus particles one might inhale, I looked at what happens to the cloud of particles you exhale.  I was struck by how fast the condensation rises (see the first 2 seconds of Vimeo video 69391280) when you're in near-freezing weather.  In freezing weather, unless someone is right in your face, they won't inhale any particles you breath out, at least not until after that cloud goes well above your head, cool and then falls down again (giving plenty of time for dissipation).

I also looked at what happens when someone exhales smoke.  That cloud, when indoor, doesn't rise very fast. It seems smokers rarely exhale normally, but there is an almost normal breath of a smoker (emphasized, but not blown) at 3:15 of the Youtube video tOOkWk9tsDI (I don't want to give it a link!) where you can see the smoke doesn't rise anywhere near as fast.

I wouldn't be worried about inhaling someone else's exhalation in the cold of winter outdoors.  I would be more worried during the summer.  I don't know if the difference between indoor temperatures of 68 in the winter vs 78 in the summer will significantly affect convection of a 98-degree exhalation.  Any difference may be offset by what I suspect is increased air movement indoors in the summer.

The tidal volume of a normal breath is about 0.5 liters, which is about the size of a 4" diameter sphere.  If a mouth breather is emitting 0.1 micron-sized particles per second (Asadi study), that's about 0.7 particles per exhalation (each having 250 virons), you'd need to be around them quite a while OR be unlucky without a mask to inhale two such particles to get to an infectious dose.   As that breath diffuses into the surrounding air (to, say, a 1-foot or 2-foot spherical volume) the likelihood drops, especially if convection takes it above your head.

OTOH, if you go into a closed environment where an infected person has been breathing (talking, exercising, etc.), without enough air replacement, then you may be receive an infectious dose almost immediately.

And, by the way, when someone is walking at a normal pace, they exhale about every 18 feet.  Likewise, you'll inhale that often.  If you're walking perpendicular or toward them, you're highly unlikely to breath any of that 4" sphere.   If you're following them, you would sometime inhale near where they exhaled, but like two watches running at slightly different speeds, you are unlikely to do that more than once in a long while.

[M T]

And remember - October in Germany is rather cold. 
There are a lot of places in the USA that won't see coronavirus friendly conditions like that until November with numbers emerging in December.
Octoberfest traditions in Germany, even if dramatically cut down, could show big numbers in November.
Thanksgiving traditions probably will appear in the USA numbers in December.
Therefore, better to measure fall/winter success relative to another place by March 31 2021 rather than day to day.

As part of my trying to get some estimate of the number of virus particles one might inhale, I looked at what happens to the cloud of particles you exhale.  I was struck by how fast the condensation rises (see the first 2 seconds of Vimeo video 69391280) when you're in near-freezing weather.  In freezing weather, unless someone is right in your face, they won't inhale any particles you breath out, at least not until after that cloud goes well above your head, cool and then falls down again (giving plenty of time for dissipation).

I also looked at what happens when someone exhales smoke.  That cloud, when indoor, doesn't rise very fast. It seems smokers rarely exhale normally, but there is an almost normal breath of a smoker (emphasized, but not blown) at 3:15 of the Youtube video tOOkWk9tsDI (I don't want to give it a link!) where you can see the smoke doesn't rise anywhere near as fast.

I wouldn't be worried about inhaling someone else's exhalation in the cold of winter outdoors.  I would be more worried during the summer.  I don't know if the difference between indoor temperatures of 68 in the winter vs 78 in the summer will significantly affect convection of a 98-degree exhalation.  Any difference may be offset by what I suspect is increased air movement indoors in the summer.

The tidal volume of a normal breath is about 0.5 liters, which is about the size of a 4" diameter sphere.  If a mouth breather is emitting 0.1 micron-sized particles per second (Asadi study), that's about 0.7 particles per exhalation (each having 250 virons), you'd need to be around them quite a while OR be unlucky without a mask to inhale two such particles to get to an infectious dose.   As that breath diffuses into the surrounding air (to, say, a 1-foot or 2-foot spherical volume) the likelihood drops, especially if convection takes it above your head.

OTOH, if you go into a closed environment where an infected person has been breathing (talking, exercising, etc.), without enough air replacement, then you may be receive an infectious dose almost immediately.

And, by the way, when someone is walking at a normal pace, they exhale about every 18 feet.  Likewise, you'll inhale that often.  If you're walking perpendicular or toward them, you're highly unlikely to breath any of that 4" sphere.   If you're following them, you would sometime inhale near where they exhaled, but like two watches running at slightly different speeds, you are unlikely to do that more than once in a long while.