Like probably many of you, I’m thinking about traveling for the holidays. Really, the number I’d like to know is: if I travel, what’s my chance of getting Covid?

Microcovid.org has been helpful in this. 1 microcovid = 1 in a million chance of catching covid. 10k microcovids = 1% chance. They’re roughly additive, so doing one thing worth 10k microcovids is the same as doing 50 things worth 200 microcovids*.

Normal life, I try to stick to about 200 microcovids a week. I don’t really calculate it, but here are some things I regularly do:

go to the grocery store for 1/2 hour with a cotton mask: 30 microcovids

see a few friends outside for 2-3 hours, with cotton masks, not really worrying about spacing: 50 microcovids

go biking with 2 friends all day: 10 microcovids

and I probably do about 3-4 of these a week; add in a x2 fudge factor because I’m probably forgetting all kinds of stuff and I’m at about 200/week.

Now, what are travel risks like? Before counting, I’m going to say that 0.1% chance would be ok. That’s 1000 microcovids**. Let’s imagine some scenarios:

- spending 1 week with parents from Cleveland, who have been quarantining long enough that we can model them as “they only grocery shop”: 300 microcovids.****

- same but with sister’s family in Charlotte exurbs: 100 microcovids.

- 1 hour in SFO before a flight where I’m wearing a well-fitted n95 mask: 100 microcovids***.

- 6 hour flight from San Francisco, n95 : 50 microcovids.

- 1 hour in Charlotte airport before a flight, n95: 300 microcovids.

- 6 hour flight from Charlotte, n95: 100 microcovids.

So imagining I do all these things; that’s only about 950 microcovids, or a 0.095% chance. Maybe a little less if I spend 2-3 days with them, not a week. I’m really not trying to cook the numbers here, but it looks like I might be just under my risk budget. Some things I learned:

- the well-fitted n95 (really, kn95, but same thing AFAICT) is a game changer. Without it all these airport numbers go up by about a factor of 5.

- adding a layover makes this much worse. 1 hour in Denver: 600 microcovids. Chicago: 500. And who knows with layovers; especially in the Christmas season, you might be there much longer!

- but uh when you’re in the airport, if you stand 10ft+ apart from people, your risks halve again - so I’ll be hiding in a faraway corner as much as possible.

* this breaks down when probabilities get big. Imagine doing something worth 500k microcovids, then doing it again - you’d have a “100% chance of covid”, but clearly that’s not the case. But for our purposes, when we’re in the hundreds or thousands of microcovids, it’s pretty close. For example, doing 2 things worth 100 microcovids each should really be counted as 199.99 microcovids, not 200… but this is not going to be the biggest source of error in your calculations!

** is this number reckless? did I just inflate my okayness with risk from 1% to 1.1% over the course of the year? I’ll try to compensate by doing less stuff most weeks - like this week I’m under 100 for sure - but I’m open to the possibility that I’m fooling myself. (in the same way, though to a lesser degree, than most people who are fooling themselves.)

*** tracking the risks for the airports/planes is a little weird because who knows where these people come from? but let’s just model them as all coming from the city that the airport is based in. that’s probably true for half of them at least.

**** upon re-checking this page, these numbers have shifted. that’s a good sign: it’s being updated! as of Dec 18, we’re at even lower than 950 microcovids.