Display omitted •Predictive estimation approach improves airborne infection risk assessment methods.•SARS-CoV-2 estimated to be nearly as contagious as Mycobacterium tuberculosis.•SARS-CoV-2 much more transmissible through indoor air than seasonal influenza.•Current ventilation standards in public buildings are unlikely to control SARS-CoV-2. The infectious emission rate is a fundamental input parameter for airborne transmission risk assessment, but data are limited due to reliance on estimates from chance superspreading events. This study assesses the strength of a predictive estimation approach developed by the authors for SARS-CoV-2 and uses novel estimates to compare the contagiousness of respiratory pathogens. We applied the approach to SARS-CoV-1, SARS-CoV-2, MERS, measles virus, adenovirus, rhinovirus, coxsackievirus, seasonal influenza virus and Mycobacterium tuberculosis (TB) and compared quanta emission rate (ERq) estimates to literature values. We calculated infection risk in a prototypical classroom and barracks to assess the relative ability of ventilation to mitigate airborne transmission. Our median standing and speaking ERq estimate for SARS-CoV-2 (2.7 quanta h−1) is similar to active, untreated TB (3.1 quanta h−1), higher than seasonal influenza (0.17 quanta h−1), and lower than measles virus (15 quanta h−1). We calculated event reproduction numbers above 1 for SARS-CoV-2, measles virus, and untreated TB in both the classroom and barracks for an activity level of standing and speaking at low, medium and high ventilation rates of 2.3, 6.6 and 14 L per second per person (L s–1 p–1), respectively. Our predictive ERq estimates are consistent with the range of values reported over decades of research. In congregate settings, current ventilation standards are unlikely to control the spread of viruses with upper quartile ERq values above 10 quanta h−1, such as SARS-CoV-2, indicating the need for additional control measures.