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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 (ρ = 0.001)) despite both houses operating the same mechanical ventilation system. Mean internal PM10 concentrations = 2.4 μg m−3 (Control) and 1.3 μg m−3 (Test) with corresponding mean external PM10 concentrations of 5.4 μg m−3 and 5.2 μg 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 (Finf) 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 Finf 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. Keywords: Particulate matter, Indoor air quality, Mechanical ventilation, Infiltration factor, I/O ratios, New Zealand

It can be difficult to tease apart the sources of the many chemical substances that can adversely affect indoor air quality (IAQ). These can be generated from primary sources, e.g. cleaning products, fragrances and wall decorations, and created in-situ (secondary emissions). Inputs due to human activities (heating, smoking, etc.) add to the catalogue of chemical possibilities. This study examines the effect of airtightness and ventilation on volatile organic compounds (VOC) concentration in newly-furnished unoccupied residential houses. Further objectives included investigating the effects of airtightness and mechanical ventilation on internal VOC concentrations in two rooms with controlled simulated occupancy. Chemicals that appear to have been generated directly from or supplemented by furnishings were acetone, 2-butanone, ethyl acetate and tert-butyl alcohol. In the absence of mechanical ventilation, total VOC (TVOC) concentrations were over three times greater in the more airtight room. The appearance of concentration maxima after furnishing and/or decoration was delayed in the airtight room and the subsequent decay rate was slower. With the exception of benzene, none of the individual compounds exceeded national guidelines for outdoor air (in the absence of IAQ guidelines). Mechanical ventilation reduced TVOC concentrations by 300 μg m−3 (more airtight room) and > 340 μg m−3 (less airtight room) to below European threshold recommendations; although the reduction took approximately five days. The time lag indicates that simply providing adequate ventilation during chemical use indoors may not be sufficient to protect inhabitants from prolonged exposure following use.