Mask wearing for airborne diseases
A lot of what happened during the Spanish flu was repeated during the COVID 19 pandemic. The Great Influenza talks about how doctors quickly identified mask-wearing as one of the few interventions that work.
It has been shown that face masks are effective tools for preventing SARS-CoV-2 transmission (e.g., Rebeiro2021impact, Howard2021evidence (update in 2022 July), Cheng2021face). This Nature news article or this meta-analysis (Talic2021effectiveness) may be a good summary of evidence. Cowger2022lifting used the Difference in differences method to estimate the impact of masking in Schools, concluding that lifting of the masking mandate is associated with an additional 44.9 cases per 1000 students and staff.
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.