Air exchange between people has emerged in the COVID-19 pandemic as the important vector for transmission of the SARS-CoV-2 virus. We study the airflow and exchange between two unmasked individuals conversing face-to-face at short range, which can potentially transfer a high dose of a pathogen, because the dilution is small when compared to long-range airborne transmission. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking the initial phase of a conversation. The evolution and dynamics of the jets are affected by the vertical offset between the mouths of the speakers. At low offsets the head-on collision of jets results in a `blocking effect', temporarily shielding the susceptible speaker from the pathogen carrying jet, although, the lateral spread of the jets is enhanced. Sufficiently large offsets prevent the interaction of the jets. At intermediate offsets (8-10 cm for 1 m separation), jet entrainment and the inhaled breath assist the transport of the pathogen-loaded saliva droplets towards the susceptible speaker's mouth. Air exchange is expected, in spite of the blocking effect arising from the interaction of the respiratory jets from the two speakers.

As the restrictions imposed due to the COVID-19 pandemic are being gradually relaxed and people are resuming their usual activities, the configuration of two unmasked individuals at proximity having a face-to-face conversation poses a risk of transferring a high dose of a pathogen from an infected person to a susceptible one. To study this airflow and exchange between two individuals, we conduct fog flow visualization experiments and direct numerical simulations of colliding respiratory jets mimicking a short conversation. It is found that the vertical offset between the mouths of the speakers is an important parameter governing the propagation and evolution of the respiratory jets. A 'blocking effect' is observed at low offsets, which temporarily protects the susceptible speaker from the pathogen-loaded saliva droplets in the jet from the infected speaker. Whereas at large offsets, the interaction between the jets is minimum. At certain intermediate offsets, jet entrainment and inhaled breath, assist the pathogen-containing jets to propagate towards the susceptible speaker's mouth. Thus, the interaction of the respiratory jets permits air exchange to a varying degree depending upon the effectiveness of the blocking effect and jet entrainment.