Air pollution measured as particulate matter (PM) has been shown to be detrimental to human health and can lead to increased mortality rates. There are four main indoor sources of episodic PM emissions: smoking, cooking, cleaning and resuspension. This study has eliminated all human activity and provides data on the variability of the contribution from external sources via mechanical ventilation. The transportation of PM from an external to internal environment by mechanical ventilation is investigated in the same room in two, similar timber-framed houses, constructed identically apart from details affecting their airtightness. There was significant variation in the transportation of PM from an external to internal environment in two similar houses ([PM] Control > [PM] Test (rho = 0.001)) despite both houses operating the same mechanical ventilation system. Mean internal PM10 concentrations = 2.4 mu g m(-3) (Control) and 1.3 mu g m(-3) (Test) with corresponding mean external PM10 concentrations of 5.4 mu g m(-3) and 5.2 mu g m(-3) respectively. Particle removal efficiency between the two houses varied by approximately 20%. These findings indicate that there is considerable variation in filtration efficiencies even when the same mechanical ventilation system is in use in similar homes in the same location. The infiltration factor (F-inf) was calculated and demonstrated considerable variability both between houses (Control-Test PM10 0.40-0.23) and over time (Control PM10 0.40-0.18) which indicates that relationship between external and internal concentrations of PM is not linear and should therefore be used with caution. This questions simplifying F-inf into one factor as there are likely to be multiple contributing factors. For example, the effect of air flow on particle adsorption to internal surfaces, natural variations in filter efficiency and variations in particle loss. Over the duration of this study, PM concentrations decreased in both bedrooms by 52% (Control) and 37% (Test), which may be due to a number of factors including changes in internal environmental conditions, filter age and the cumulative effect of the use of mechanical ventilation over time reducing the transportation of PM into the houses.
