The experiment that proved airborne disease transmission

It's pretty much settled also for sars-cov2. A study in China found the virus in air samples taken inside the central air-condition system, far away from any patients (see bottom). Another found correlation between infected and where they were sitting vs AC duct outlets.

Also the fact that mandatory masks have had zero effect on the R value in any country, same as for influenza and other "flu" viruses. If it was spread by droplets/surfaces then masks should at least have some effect to slow or contain the spread, like make a dent in the curves.

Yes there are masks that prevent TB transmission but this bacteria is more than 10x larger than typical respiratory viruses. They look like duck beaks, are very uncomfortable and require training to use properly. I haven't seen these anywhere during this pandemic and they wouldn't help either as they are designed to prevent aerosols down to about 2um size. There is plenty of space for ~0.1um viruses even in 0.5um droplets. Some researchers even claim the virus can stay potent a while after the droplet evaporates and float around like dust.

CDC meta study showing medical masks had no effect neither as PPE or source control: https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article Study comparing medical masks and N95 masks found no difference in transmission of influenza: https://pubmed.ncbi.nlm.nih.gov/31479137/ More links and summaries collected here: https://swprs.org/face-masks-evidence/ This Time article has lots of links and info (but the argument for masks is complete fail): https://time.com/5883081/covid-19-transmitted-aerosols/

It's funny how some claim we didn't need masks before, but now we need them since it's spread by aerosols?? The truth is in fact the opposite and don't get me started on cloth masks...it's absurd. This seems more like covering their asses than following the science.

Most of these studies involve health personell, with high-end masks, the general public will of course do much much worse. This survey of studies and history of mask use in dentistry is also informative (why was it taken down?): https://archive.is/My2jr

The WHO reversal on mask use is also revealing: On July 12, Deborah Cohen, the medical correspondent of BBC2’s Newsnight, revealed on Twitter that: ‘We had been told by various sources [that the] WHO committee reviewing the evidence had not backed masks but they recommended them due to political lobbying.’ She said the BBC had then put this to the WHO, which did not deny it. In March, the WHO had said: ‘There is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can protect them from infection with respiratory viruses, including Covid-19.’

-Van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. 2020;382:1564-7. -Fears AC, Klimstra WB, Duprex P, Weaver SC, Plante JA, Aguilar PV, et al. Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions. Emerg Infect Dis 2020;26(9). -Chia PY, for the Singapore Novel Coronavirus Outbreak Research T, Coleman KK, Tan YK, Ong SWX, Gum M, et al. Detection of air and surface contamination by SARS-CoV-2 in hospital rooms of infected patients. Nat Comm. 2020;11(1). -Guo Z-D, Wang Z-Y, Zhang S-F, Li X, Li L, Li C, et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020. Emerg Infect Dis. 2020;26(7). -Santarpia JL, Rivera DN, Herrera V, Morwitzer MJ, Creager H, Santarpia GW, et al. Transmission potential of SARS-CoV-2 in viral shedding observed at the University of Nebraska Medical Center (pre-print). MedRxiv. 2020 doi: 10.1101/2020.03.23.20039446. -Zhou J, Otter J, Price JR, Cimpeanu C, Garcia DM, Kinross J, et al. Investigating SARS-CoV-2 surface and air contamination in an acute healthcare setting during the peak of the COVID-19 pandemic in London (pre-print). MedRxiv. 2020 doi: 10.1101/2020.05.24.20110346. -Liu Y, Ning Z, Chen Y, Guo M, Liu Y, Gali NK, et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature. 2020;582:557-60. -Ma J, Qi X, Chen H, Li X, Zhan Z, Wang H, et al. Exhaled breath is a significant source of SARS-CoV-2 emission (pre-print). MedRxiv. 2020 doi: 10.1101/2020.05.31.20115154. -https://pubmed.ncbi.nlm.nih.gov/19797474/ -https://pubmed.ncbi.nlm.nih.gov/19216002/

I'm actually curious: how many 'virus particles' does a person need to be exposed to on average to contract the virus?

That's what an ERV or HRV is for: https://en.wikipedia.org/wiki/Energy_recovery_ventilation https://en.wikipedia.org/wiki/Heat_recovery_ventilation

Et voilà, now it's no longer terrible for efficiency since you're recovering 70-90% of the energy. For fixed-plate exchangers there's no cross-contamination, either.

The meta study does not "[show] medical masks had no effect", it shows that we have limited evidence for it:

"Most studies were underpowered because of limited sample size, and some studies also reported suboptimal adherence in the face mask group"

https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article

And indeed you can sympathise with them that it would be hard to perform such an experiment.

But the whole point of science is that it allows us to make predictions based on theories. And the current scientific consensus is that wearing masks is beneficial.

Challenge trials in animal models suggest somewhere in the 100-1000 range. Lower initial does also seem to lead to less severe courses of disease.