Outdoor PM_2.5 air filtration: optimising indoor air quality and energy

Human inhalation exposure to fine particulate matter (PM_2.5), including the PM_2.5 of outdoor origin, predominantly occurs indoors. To limit outdoor PM_2.5 penetration into buildings, ventilation standards often require the filtration of outdoor air with a minimum efficiency. Nevertheless, the PM_2.5 filter selection recommended by the standards is based on the annual average outdoor concentrations without considering seasonal or diurnal fluctuations. This could result in a waste of energy or elevated indoor PM_2.5 exposures. Representative outdoor PM_2.5 data from 37 cities worldwide in conjunction with a simulated office building are used to examine the impact of filtration strategies on indoor PM_2.5 levels and the fan’s energy consumption. Two energy-saving methods are tested: (1) the optimum filter selection that maintains the indoor PM_2.5 below the World Health Organization’s (WHO) air quality guidelines; and (2) the baseline filter recommended by standards in combination with a filter bypass. Relative to a standard recommended baseline case, the two applied methods could reduce energy demand by between 4% and 17%. This indicates that the outdoor air is over-filtered in the majority of the investigated cities. In cities with low-to-moderate outdoor PM_2.5 levels, using a filter bypass can be an effective energy conservation method without compromising PM_2.5 exposures indoors.

Practice relevance

Protecting building occupants from outdoor originating PM_2.5 without a high energy penalty is not a simple task. The majority of recommendations for ventilation system design typically do not consider temporal variation in outdoor PM_2.5. This study’s data suggest that outdoor air filtration efficiencies required by building standards are not sufficient to protect the building occupants living in severely polluted areas. In these areas, outdoor air filtration should be supplemented with other measures to limit outdoor PM_2.5 penetration. In areas with low or intermittent outdoor PM_2.5 levels, bypassing the filter can significantly reduce the energy consumption from fans without compromising indoor air quality. The energy-saving potential increases with the increase of outdoor air quality. This study suggests that improved outdoor air filtration can be achieved by a stronger reliance on continuous outdoor air quality data recorded on site.