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., 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 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 showcases two arguments for and against mask wearing outdoors.