WINDOWS! Hands, Face, Space

We're in a respiratory pandemic, with an airborne pathogen, mostly spread by breathing in someone-else's exhaled air. Shops, Hospitality, Transport (and others), open your damn windows!

By keeping all the windows and doors open, we can replace the inside air with fresh air, multiple times an hour. Eliminating second-hand "shared air" makes us all safer: if you could smell someone's cigarette smoke, you could be inhaling their viruses.

Maximise ventilation as well as wearing masks indoors, 2m+ distancing, and other measures.

We can think about coronavirus-transmission if it were passive smoking (but far more deadly). We already know how to visualise this (we have a good mental picture of the way smoke moves) and understand how to mitigate it (distance, fresh air). But too many people misunderstand how the virus spreads, and so they do what they think is most obvious. Poor public-health information, which omits this factor, makes most people focus primarily on frequent hand and surface sanitisation. Sanitisation is a good idea, but it is not as important as ventilation.

Opening windows and doors is also simple, and low-cost - we can just keep our coats on inside. This virus is highly transmissible (even more so in the new variants), and severe enough to crush the economy: so no excuses!

Ventilation Requirements

Maximise ventilation. All shops, supermarkets, restaurants, cafes, workplaces, buses, trains, taxis, care-homes (especially with visitors), pubs, theatres, gyms, hairdressers, (and houses with indoor visitors): KEEP ALL DOORS AND WINDOWS WIDE OPEN.

It should be windy indoors. Open side/back/loading-bay doors to promote a through-breeze.

Open windows wide on public transport (especially when crowded). If there are no windows... make some!

Wherever possible, move service outside. Reduce density by extending operating-hours.

Explanation: see for yourself

We know how much safer it is when air is not recycled - this is precisely why Flu (and Covid-19) peak in the cold season, when everyone crowds indoors and tries to save heat by avoiding draughts. This is not the moment for energy-efficiency. Outdoor events are generally safe (e.g. there was no spike in cases caused by the black-lives-matter protests).

Droplets: Take a perfume or aftershave mister, hold it in front of your face (pointing away), and spray it. You'll see tiny droplets.

Aerosols: Now, fry some onions or garlic. With the doors and windows closed, where else in your house can you smell it? Or observe how you can smell aftershave/perfume on another person from a distance.

Outdoors: Repeat the experiments outdoors. Can you smell the perfume or the onions?

Exposure is based on proximity × time. For example, two people spending a morning at opposite ends of an unventilated 10m long room are far more exposed to each other than passing briefly in the hallway.

The transmission numbers are scary right now: ventilation is vital, in addition to masks indoors and vaccination as soon as possible.


We can't see viruses, so how should we imagine them? Think of them like passive smoking, just far more toxic. The virus particles are tiny: they float around with the air, like smoke; they do not move like projectiles, such as raindrops or tennis balls. more

This video shows how the virus moves.
Covid-19 itself is about 0.1 μm in size. In comparison, dust is about 5 μm; cigarette smoke is about 0.1 μm; droplets are around 100 μm.
1 μm = 1 micron = 0.001 mm.
Droplets do fall out of the air quite fast; aerosols do not.

We already know how to protect against passive smoking: for Covid-19, exactly the same approach works, and you should imagine it in the same way. Imagine that 1 person in 50 is smoking, but you can't actually see the smoke, and that just "one breath" of smoke covid can get you infected.
(whereas second hand smoke takes more exposure, over longer-term to harm you.)

You are most likely to breathe in that smoke if you are close to them for several minutes, but smoke moves around inside, and persists long after the smoker has left the room.

In summary:

If you could smell someone's cigarettes/perfume/aftershave, you are exposed to them!

You can't socially-distance from an "infected ghost" (i.e. the trail where somone was, minutes earlier)

But you can replace their air to reduce your exposure.

Links and References

Here are some links to more information:

Misunderstandings and Errors

Here are some other things that people are doing wrong....

Tier 5, then what?

Our current strategy is not working. The newest strain is more transmissible and harder to contain, even by the strictest lockdown measures we had in the springtime. We have to do everything possible, so why isn't maximising ventilation part of that strategy?

The above numbers illustrate the problem we face; the same logic applies everywhere, worldwide. more

Here are some statistics and references:
Lockdown value of R for the UK in May 2020 was: 0.5 - 0.7.
The new variant is: 70% - 93% more transmissible.
In London (as at 23rd Dec), in Tier 4, R is measured at 1.2 - 1.5, with infections growing at 4%-8% per day.
To explore the figures further, see Our World in Data.

The unfortunate truth is this: we applied selective pressure on the virus to make it evolve to be more transmissible. The maths means that it's going to grow and keep growing, and even the strictest lockdown measures cannot control the virus sufficiently. While we race for the vaccine (and hope for springtime), we need to do better than we are. Maximum fresh-airflow is the only additional public health policy, which we are not yet doing, which could make a significant impact.

Q: What if we'd be a bit chilly?
A: Dress warmly. (And reassure your customers that the cold is evidence of reduced exposure).

Q: What if your building or train is designed with non-opening windows?
A: Screwdriver/Sledgehammer. (Seriously, we've had 9 months to get this fixed.)

Q: What if your supermarket has auto-closing doors to save energy?
A: Turn off the power, and wedge open. Turn down the heating to avoid wasting it.

Q: What if you can't guard all the doors to your shop?
A: Open them anyway: the risk of theft is small, compared to the risk of bankruptcy from remaining closed if you're stuck in Tier 5 till June.

Open your damn windows!

Come on everyone... Don't make me say it again! If you run a: bus, train, taxi, restaurant, shop, supermarket, cafe, pub, hairdresser, school, care-home, pharmacy, gym, ... then you have to help with the fight against Covid-19. There are many things you need to do, but at the very least:


This costs you nothing in time or effort, makes people safer, and helps reduce the need for other restrictions.

About OYDW

The authors are a Cambridge University Physicist and Biologist, who have been finding the lack of ventilation extremely frustrating, as it is the single most effective thing we could do to reduce risk, without imposing cost, harming the economy, or needing to add even further social restrictions. And yet most people don't know, don't understand, and the public health messaging says very little - though this is finally beginning to change.

I have also had extensive discussions with multiple engineers and biologists. I am a scientist and engineer (and so I can visualise fluid dynamics and airflow), but I'm not an epidemiologist. If there is something here which is factually wrong, please let me know and I'll correct it - outdated messages, or misinformation are harmful. If there's a better resource on this, we'll link to it, or redirect.

Copyright: please feel free to share this, link it, or copy it wholesale. Anything here is placed into the public domain.

Last updated: 30th Dec 2020.

Contact: send email to: contact AT openyourdamnwindows DOT com.

Post Script

As of December 2020, the vaccine is coming on-stream, but expected to take many months to reach full coverage. We also have increased transmissibility, and the newer variants have evolved to be more infectious, increasing "R". So, we really have to go all-out.

However, the economic, social, and mental-health costs of indefinite lockdown are mounting; while we must do everything to speed the vaccinations, we still need to find more ways of "living in a world which has the virus prevalent", rather than just "waiting it out". We have to leverage all our creativity and effort to find options. Doing more of the same is not sufficient. more

Here is an example, of the kind of change we could do: minimise density - spread out in time, by extending operating hours. This makes the venue-capacity limits far more effective, without reducing service.

Unless we close everything down (and if so, that also has some very severe consequences), we need to optimise the trade-off to maximise economic (similarly: manufacturing, teaching, retail, exercise, social...) activities, while minimising the covid-harm. The benefit is proportional to the total number of customers per day; while the covid-cost is proportional to the square of the number of customers present within each other's "shared airspace" at the same time. Many shops and venues have adopted restricted hours, to get more of their customers in within a shorter workday: this increases social contact and mixing - indeed, we already recognised the failure of the 10pm curfews on restaurants in October.

What we should do is encourage greatly extended opening hours (possibly even to 24/7), including allowing licenses to run late, and waiving Sunday trading restrictions. It wouldn't be perfect, but many of us could adapt to the inconvenience. Here are some examples to illustrate:

Situation Normal Lockdown Proposed
Supermarket 8am-10pm (less on Sundays). 1000 customers/day. Typical occupancy: 35 customers. 8am-8am. 1000 customers/day. Typical occupancy: 42. 8am-8am. 1000 customers/day. Typical occupancy: 20.
Restaurant 7pm-11pm. 100 customers/day. Typical occupancy: 50 customers. Closed. Business eventually goes bust. 6pm-2am. 100 customers/day. Typical occupancy: 12.
Shop (non-essential) 9am-5pm. 100 customers/day. Typical occupancy: 3 customers. Closed. Business eventually goes bust. 7am - 3am. 100 customers/day. Occupancy: 1.

And, obviously, keep the doors and windows open too, or it totally defeats the purpose!