Objectives While COVID-19 is known to be spread by respiratory droplets (which travel <2m horizontally), much less is known about its transmission via aerosols, which can become airborne and widely distributed throughout room spaces. In order to quantify the risk posed by COVID-19 infectors exhaling respiratory aerosols in enclosed spaces, we undertook a computer modelling study to simulate transmission in an office building. Methods Respiratory droplet data from four published datasets were analysed to quantify the number and volume of droplets <100μm diameter produced by a typical cough and speaking event (i.e. counting from 1 to 100). This was used in a stochastic model to simulate (10000 simulations) the number of respiratory particles, originating from a COVID-19 infector, that would be inhaled in one hour by a susceptible individual practicing socially distancing in a 4 x 4 x 2.5m office space. Several scenarios were simulated that mimicked the presence of both symptomatic and asymptomatic COVID-19 infectors. Results On average, each cough and speaking event produced similar numbers of droplets <100μm diameter (median range = 971.9-1013.4). Computer simulations (ventilation rate=2AC/h) revealed that sharing the office space with a symptomatic COVID-19 infector (4 coughs and 10 speaking events per hour) for one hour resulted in the inhalation of 16.9 (25-75th range = 8.1-33.9) aerosolised respiratory droplets, equating to about 280-1190 particles inhaled over a 35-hour working week. Sharing with an asymptomatic infector (10 speaking events per hour) resulted in the about 196-875 particles inhaled over 35 hours. Conclusions Given that live SARS-CoV-2 virions are known to be shed in high concentrations from the nasal cavity of both symptomatic and asymptomatic COVID-19 patients, the results suggest that those sharing enclosed spaces with infectors for long periods may be at risk of contracting COVID-19 by the aerosol route, even when practicing social distancing.