The risk of airborne infection for COVID-19 needs to be better understood and is especially urgent during the current pandemic. We present a method to determine the relative risk that can be readily deployed on either modelled or monitored CO$_2$ data and occupancy levels within an indoor space. Moreover, for spaces regularly, or consistently, occupied by the same group of people, e.g. an open-plan office or a school classroom, we establish protocols to assess the absolute risk of airborne infection of this regular attendance at work or school. In so doing, we are able to calculate the expected number of infections arising from a single regular attendee becoming infectious and remaining pre/asymptomatic, i.e. we present a robust methodology to calculate the absolute reproductive number of these spaces. We demonstrate our model by calculating risks for both a modelled open-plan office and by using monitored data recorded within a small naturally ventilated office. Results suggest that attendance at work is unlikely to significantly contribute to the pandemic if relatively quiet desk-based work is carried out in the presence of adequate ventilation. However, these spaces are likely to contribute significantly to the pandemic if ventilation is inadequate and/or activity levels increase.