A critical evaluation of the dynamic nature of indoor-outdoor air quality ratios

Abstract

Long term, continuous indoor and outdoor pollutant monitoring was evaluated from a case study hospital, school, office and 18 apartments in the UK. Data was examined in order to explore the dynamic behaviour of indoor-outdoor ratios (I/O) for both particulate matter and nitrogen dioxide. Traditionally I/O ratios have been determined as single aggregate values or static parameters, from passive sampling or short periods of continuous monitoring. Whilst widely reported, I/O ratios are seen as too variable to be of wider use. However, this work reveals the dynamic nature of I/O ratios, with strong diurnal and seasonal variation observed for both particulate matter and nitrogen dioxide. Higher I/O ratios tended to be seen during core or occupied hours, associated with increased human activity and higher ventilation rates. This means that static I/O ratios determined by passive sampling techniques, rather than continuous measurements filtered to core hours, may underestimate I/O ratios associated with occupant exposure. Further, the I/O ratio is shown to be strongly influenced by occupant activity and window opening behaviour. As such, it may represent a personal variable as much as one associated with a building. It is argued that traditionally reported static I/O ratios simplify these dynamic behaviour and modes of operation into a single aggregate value, losing key information in the process. Further, without contextual information on the operation and use of a building during measurements a reported I/O ratio may be hard to interpret or compare to wider studies. Finally, it is argued that the I/O ratio, whilst a limited metric, when evaluated dynamically provides a useful building operation parameter, describing the relationship the building has with the outdoor environment. This can help better define ventilation strategies, schedules, the influence of occupant behaviour and significance of indoor sources.