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The year 2020 has seen the emergence of a global pandemic as a result of the disease COVID-19. This report reviews knowledge of the transmission of COVID-19 indoors, examines the evidence for mitigating measures, and considers the implications for wintertime with a focus on ventilation.

We develop an analytical model to describe the generation of vapour as water moves through a hot porous rock, as occurs in hot, geothermal reservoirs. Typically the isotherms in the liquid lag behind the water-vapour interface and so water is supplied to the interface at the interface temperature. This temperature is lower than that in the rock far ahead of the interface. Therefore, as the hot porous rock is invaded with water, it cools and the heat released is used to vaporize some of the water. At low injection rates, vapour formed from the injected liquid may readily move ahead of the advancing liquid-vapour interface and so the interfacial pressure remains close to that in the far field ahead of the interface. The mass fraction that vaporizes is then limited by the superheat of the rock. For larger injection rates, the interfacial vapour pressure becomes considerably greater than that in the far field in order to drive the vapour ahead of the moving interface. As a result, the interfacial temperature increases. The associated reduction in the thermal energy available for vaporization results in a decrease in the mass fraction of vapour produced.Since the vapour is compressible, the motion of the vapour ahead of the interface is governed by a nonlinear diffusion equation. Therefore, the geometry of injection has an important effect upon the mass fraction of water that vaporizes. We show that with a constant supply of water from (i) a point source, the mass fraction of water which vaporizes increases towards the maximum permitted by the superheat of the rock; (ii) a line source, a similarity solution exists in which the mass fraction vaporizing is constant; and (iii) a planar source, the liquid-vapour interface steadily translates through the rock with a very small fraction of the injected water vaporizing.

The design of a natural ventilation strategy requires the establishment of the location and size of a series of purpose provided ventilation openings (PPOs). The success of the design is dependent on knowledge of the aerodynamic performance of the PPOs often described by their geometry (normally an area) and resistance to airflow. The incorrect interpretation of this information can lead inappropriate ventilation strategies and buildings that overheat and have an excessive energy demand.Many definitions of PPO area are used by standards, guidelines, text books, and software tools. Each can be assigned multiple terms and a single term can be assigned to different definitions. There is evidence that this leads to errors in practice. An effective area of a PPO, defined as the product of its discharge coefficient and its free area, is proposed as a standard description because it is unambiguous and its measurement is governed by recognised standards. It is hoped that PPO manufacturers will provide an effective area as standard and that its use will be recognised as best practice. It is intended that these steps will reduce design errors and lead to successful natural ventilation strategies and better buildings. HIGHLIGHTS Definitions of free, effective, and equivalent ventilation opening areas are given  A review of current definitions highlight contradictions in national standards and guidelines  The contradictions are shown to lead to unintended design errors  An unambiguous term that describes ventilation opening performance is proposed  This will help to mitigate against design errors in ventilation strategies KEYWORDS Natural Ventilation; Design; Standards; Effective area; Equivalent area; Free area; Policy. 3 INTRODUCTIONOpenings located in the thermal envelope of a building comprise those that are intentional, known as purposeprovided openings (PPOs), and those that are unintentional, known as adventitious openings (Etheridge, 2012).It is desirable to minimize adventitious openings to minimize a building's energy demand and to ensure the satisfactory operation of a system of PPOs (Jones et al., 2015). When designing a ventilation strategy that comprises a system of PPOs, a fundamental objective is to establish their location and size. Both factors depend on the airflow rates required through each PPO for a given pressure drop in order to maintain adequate indoor air quality (IAQ) and to dissipate heat gains under limiting conditions (CIBSE, 2005). Accordingly, a description of the geometry of each PPO and its resistance to airflow are required in order to enable a designer to establish the performance of a system using envelope flow models (CIBSE, 2005;Etheridge, 2012). The same information can also be used when working with more complex simulation tools to ensure that a building meets relevant energy and indoor environment quality (IEQ) criteria, such as indoor air quality (IAQ), thermal comfort, overheating, and noise levels. The geometrical information and resistance to airflow of a specific P...