In the context of the regulatory debates about the appropriateness to ban or not e-vapour use in public places, the need to understand and identify the factors affecting bystanders’ exposure is particularly useful.

The propagation, dilution and extraction of indoor intermittent, localised emission sources are described by a simplified macro scale modelling based on basic physical principles and reproducing the following steps: puff inhalation, retention in air pathways, exhalation in the environment, aerosol propagation, dilution in air, air renewal, deposition on surfaces, exposure of bystanders and dose inhaled by breathing during a certain period of time. The modelling input parameters were the quantity of constituents inhaled per puff; the retention rate; the puffing session frequency; the number of puffs in each puffing session; the speed of aerosol propagation in the environment; the volume of the room; the air exchange and recycling rates; the speed of deposition on the surfaces; the distance from e-vapour user to bystanders; bystanders’ breathing pattern and the time spent in the room. Outputs were then the exhaled constituent concentrations in space and time, and the quantity of constituent breathed.

The model was applied to describe a scenario considering an e-vapour user and a bystander colleague working in the same office. It was possible to simulate the effect of intermittent aerosol constituents’ exhalation in environmental air over time and to compare calculations with experimental data.

Even if further experimental studies in real-life conditions need to be conducted, modelling is a good way to understand how indoor air quality can be changed by e-vapour users and for identifying the key factors related to exposure. This also provides elements for discussion on the need or not of a ban in public places.