Our institute has always recommended 30 m³/Person/h fresh air supply. The air exchange rates given by this will be different in a school (with 30 persons sitting in a small room) / a daycare center / an office (with e.g. only 1 person sitting there). So, air exchange rates can only be discussed keeping the situation of the room considered in mind. Fresh air rate per person is the adequate measure.
Do the math! In a typical school room that will result in 3…6 ach. But, keep in mind, what you can implicitly read in papers recommending ventilation: Even the highest ventilation-rate cannot protect you 100%; you’ll have to wear a mask, too. Because the droplets are in the airstream; and still can hit you before they leave the room.
So, that’s the academic discussion. Now let’s be clear, what could be a best possible path to protect us:
1) If indoors and together with other people – wear a mask, and a good one*. The mask can’t be substituted by ventilation.
2) Ventilate the room as good as possible – but don’t “over-ventilate” in winter, because dry air will make you sick. ERV and HRV will make this easier to achieve, but humidification might be needed.
3) Keep indoor humidity between 50-60% during such a virus pandemic. Lower rel.hum. lets the virus stay longer in the air and also makes humans more susceptible for infection. (How? If not installed in the system, use an evaporative humidifier. These are not expensive.)
4) Clean the surfaces people are in contact with in regular intervals. That’s another infection path. The virus from the big droplets can sit on a surface for quite a time.
5) Using air cleaners might help additionally. If equipped with an adequate filter (better than MERV16, F9 (epm1(85%))), these will reduce aerosols significantly. This is especially a method, if you have a potentially infected person at home.
6) If we are at this home setting: infected persons should stay in a different room. Have the window open in that room (even in a #passivehouse, you see, why we have always recommended openable windows) and close the door to that room. Wear a ffp3 mask (that’s for 14 days at maximum).
Just to give us a relation: In Korea, they checked for infection rates in the households of infected persons and found these to be less than 30%. So, yes, you can protect yourself. But yes, there will always be a remaining risk.
And now, let us be honest: The danger comes from these 1 to 2 days a person might be infectious but without symptoms. The problem is: You just don’t know. It’s crazy, but if we are dealing with high incidence**, there is no other way than to assume, that any person you meet, is infectious.
That was the reason why all responsibly thinking humans try to keep the infection rate low. That is easy (yes, it’s easy!): Keep the distance, wear a mask (whatever mask), ventilate properly (the 30 m³/Pers/h are ok), avoid large indoor gatherings, wash your hands, don’t shake hands and stay at home if feeling ill.
This alone will not reduce your personal risk to zero – but it will reduce the Reproduction***-factor R to smaller than 1. And that would lead to reduced infection numbers day after day after day … until these become so small, that the risk becomes low.
Example: R is in the range of 1.2 to 1.3 in Germany right now. If we improve proper mask wearing (including the nose, stupid!), reduce the number of huge indoor gatherings, add a bit of ventilation,.. in sum by just 40%, R will come down to 0.9 and we result in an exponentially falling incidence rate. Still, it reduces your personal risk at the start only by 40%.
If you want to reduce your personal risk in an “infectious environment” with high incidence, you’d have to improve by (90%? 95%?); what would mean wearing ffp3-masks all the time, ventilate like hell (is 10 ach enough? No, it still isn’t), clean all incoming stuff and you still might get it from.., well from piece of deep frozen meat coming from an infected meat factory (e.g., there are potential other paths, too. We just don’t know and can’t know all of them).
Conclusion: If (most of us) act responsibly, we could drive R down below 1 & start a declining exponential; the small incidence remaining can be kept under control by test/trace/quarantine (you see that in Korea, NZ, …). That’s the recommendation until the vaccine arrives, what will be January 2021 (the latest).
That’s 3 Months to go … Yes, we can!
* A “good mask” which will also protect you and not only the others from you will be ffp2 or even ffp3.
** “Incidence” is the number of new infections divided by the number of persons in a population.
*** The reproduction number R is the number of persons who get infected in average by 1 already infected person.