Mixing airflow through ducts

Hi all – we are renovating a brownstone that has an old forced-air heating system with supply ducts running vertically through the house. We are separating off the ground floor into its own apartment, and I’m wondering if air will mix through the ducts between the apartments when the system isn’t running. We’ve always lived in apartments with steam heating, so I’m generally curious about this, but also specifically wondering in our COVID-era if its possible for aerosols generated on the ground floor to enter the ducts, and then be blown upwards when the system kicks in. Is this a concern, or nothing to worry about? Thanks in advance for any insights!