kleinbl00 is right, this article & tool is too good not to share.
If you dig deeper, you can find out why it does that: this Zeit article is based on this Max Planck Insitute model which points to this scientific article which has the ventilation assumptions in table 3. 'No ventilation' is replacing air 0.3 times per hour, 'Active ventilation' (windows open) is twice per hour, and 'Ventilation system' assumes nine times per hour. kleinbl00's bitchin' medical-grade system does 11 per hour, so no, regular people don't have that system. I'm pretty sure that, since home ventilation systems are very rare in Germany, this is a lost-in-translation thing where ' ventilation system' means something else to you than to bratwursteaters.
11 per hour if we leave the fart fans on. 8 if we don't. It wouldn't surprise me if european standards, due to cubic meters per minute, work out to 9 airchanges for hospitals while American standards, due to cubic feet per minute, work out to 8. If this was a hospital-based study they may have just assumed "hospital ventilation" which, for basic-bitch exam rooms, emergency rooms and recovery rooms, is at 8 air changes per hour in the US.
basing it around .3 replacements per hour is hilariously misleading https://www.engineeringtoolbox.com/air-change-rate-room-d_867.html https://www.contractingbusiness.com/service/article/20868246/use-the-air-changes-calculation-to-determine-room-cfm etc no comment, of course, on the differences between germany and the usa. but over here we do have extremely aggressive air conditioning systems just as a cultural thing
ASHRAE 62.2 specifies a minimum of 0.35 airchanges per hour for residences built in 2008 or later. Your certainty about the situation, however, is noted.ASHRAE Standard 62-1973 required ventilation in most buildings of 20 cfm per person. In 1981, ASHRAE 62 reduced the rate to 5cfm per person in an effort to address the energy impact of ventilation. This was quickly found to be far too low a ventilation rate. ASHRAE 62 was updated in 1989 and set a residential ventilation rate of 15 cfm per person or 0.35 ACH, whichever was higher. However, this version of the Standard contained only a half-page on residential ventilation. In the early 1990s, ASHRAE began the process of updating Standard 62 and in 1997 separated the overall standard into two documents with two committees, SPC 62.1 that dealt with all the occupancies other than low-rise residential and SPC 62.2 that dealt with low-rise residential only.
So, in my house we have radiators. And it is cold outside so all the windows are closed up nice and tight. The only ventilation is when we turn on the exhaust fans during morning showers. So, no, we don't have a whole lot in the way of ventilation. And most homes in the north east section of the US are like mine. Perhaps you live in the south or a much newer home (built in the last 30 years) where you have central HVAC.
I'm glad that it exists. If I could just have the Max Planck Institute take over the wheel, that'd be grrreaat. Important sidenote: these percentages do assume social distancing is kept. It only models the aerosol part of COVID transmissions, not the droplet part, so real percentages are a bit to a lot higher.