Why don't viruses reach broad concentration outdoors in a city like allergens?

Question

Why don't airborne viruses reach concentrations that infect most people vulnerable outdoors in a city the way an allergen can cause inflammation to everyone sensitive to it. Both are (often) microscopic airborne biological particles produced by a bunch of scattered organisms.

Obviously my assumptions are wrong. Even at the height of Delta or Omicron Covid variants it was generally considered safe to be without a mask if you were alone and outside. The question is which assumptions are wrong?:

1. Does particle size mean that allergens stay near the ground and dilute less?
2. Are there just nowhere near enough organisms emitting at once to matter?
3. Does the life cycle of a virus just prevent enough people from being infectious at once the way a whole species of tree will start producing pollen?
4. Since producing virons isn't a normal function of a body are they just produced massively less than something like pollen from a tree?
5. Is the immune system so powerful that it's easier to trigger an allergic response than an infection to establish a foothold?

Why don't people catch colds like allergies.

Answer 1

2, 4, 5, and 6.

6 being that the UV light (from the sun), fluctuations in temperature, humidity, wind etc mean that the virions are decayed relatively rapidly for most virus species.

To address (1): In general a virus, such as SARS-CoV-2 or influenza is about 100 nanometres (0.1 micrometres (μm)) in diameter, whereas a pollen is about 10 μm (sizes in links for the respective viruses) - the virus is 100x smaller, but is largely spread through droplet transmission. Droplets are comprised of typical nasal secretions (i.e. snot) and/or saliva and those of about 5-100 μm in size are relatively dense and fall to the floor rapidly. Less than 5 μm can float for quite some time¹ but rapidly dehydrate and lose virion integrity, so can't cause infection.

(2) and (4) Infections produce a lot less virus than a tree does pollen. A typical allergenic tree like a Silver Birch (Betula papyrifera) can produce about 2 million pollen grains from a single catkin - up to about 2 billion grains per tree. Multiply that by the number of trees, and you'll get some idea of the number of pollen grains for that species alone. On the other hand an infected person has about 7 million virions per millilitre (ml) of saliva. However, not all of each ml is turned into droplets when speaking and it turns out that only about 37% of 50 micrometre droplets will contain a single virion, and that this drops to 0.37% for 10 μm droplets (see ref 1). This means that each infected person at their infectious peak is only producing putting a tiny proportion of the virus they contain.

(3) and (4) Sort of - plenty of people can be infected at once, as you will have seen during the waves of infection, but once sick they aren't out there walking around constantly emitting virus into the environment, they are in bed, at home or in a hospital (assuming they are following good public health advice). This also ties into the answer for (1) - the droplets just don't last like a pollen grain can. Pollen's purpose is to travel to find a new flower to fertilize, so trees that are wind pollinated have a selective pressure to produce pollen that can last in the environment and still fertilize another tree. Viruses don't have the same selective pressure because transmission relies on "close" contact of a mobile organism - not a tree that sits in the same place and the closest one might be miles away.

(5) How an infection takes place is multifactorial - you need the conditions to be just right for transmission to take place, and a big enough dose of virions to cause an infection (often for things like Influenza or Adenovirus this is in the 3-10 virions range), you then need it to hit the right tissue in the body and evade the immune system. All pollen has to do is hit a mucosal membrane (nose, mouth, eyes etc) for there to be an immune cell there to activate the immune response.

Long story short infection is much harder to do, produces less virions and has lower likelihood of happening.

1: Stadnytskyi V, Bax CE, Bax A, Anfinrud P. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proc Natl Acad Sci U S A. 2020 Jun 2;117(22):11875-11877. doi: 10.1073/pnas.2006874117. Epub 2020 May 13. PMID: 32404416; PMCID: PMC7275719.

Answer 2

In another answer elsewhere on StackExchange, a poster estimated that there might be something like 100 g to 1 kg of SARS-CoV-2 virus worldwide, and that's an estimate of all the virus, including what remains in the bodies of infected individuals, not just what makes it to the outside world.

I'm not certain of the accuracy of that estimate, but consider that a single pine tree can make a couple kilograms of pollen. Even if it's off by a couple orders of magnitude, the simple answer is there isn't nearly as much virus out there as there are environmental allergens.

Anecdotally, there's enough pollen during the right season that if I leave my windows open, there is a light, greenish-yellow coating on surfaces near windows. The comparison just isn't anywhere close.