Mask wearing for airborne diseases
A lot of what happened during Spanish flu was repeated during the COVID 19 pandemic. The Great Influenza talks about how doctors quickly identified mask-wearing as one of few interventions that work.
It has been shown that face masks are effective tools for preventing SARS-CoV-2 transmission (e.g., Rebeiro2021impact, Howard2021evidence, Cheng2021face). This Nature news article or this meta-analysis (Talic2021effectiveness) may be a good summary of evidence.
Cheng2021face argues that there exist two regimes—virus-limited and virus-rich—and the effectiveness of the mask can vary depending on the regime. There is also at least one randomized trial.
Simulations & estimations
Eikenberry2020tomask estimates how mask-wearing can curtail the spreading. Ku2021safe argues that mandatory mask-wearing and social distancing with masks during peak hours reduced infection rates by 93-98%.
BMJ2021should showcase two arguments for and against mask wearing outdoors.