Extensive research indicates that adverse health effects, and markers of adverse health effects such as asthma exacerbations, hospital admissions, and death rates, increase with increased concentrations of particles in outdoor air [8-10]. Much of people's exposures to particles from outdoor air occurs when people are indoors and these exposures can be reduced by particle air cleaning. Consequently, it is reasonable to expect health benefits from particle air cleaning systems that substantially reduce indoor concentrations of particles with an outdoor-air origin. These particle air cleaning systems can be either standalone air cleaners or particle filtration systems installed in heating, ventilating, and air conditioning systems. Filtering the incoming outdoor air as it enters the building is a particularly effective strategy for reducing indoor air concentrations of particles from outdoors. Published relationships of outdoor air particle concentrations with adverse health effects have been used in models to predict the related health benefits of particle air cleaning. The resulting papers, reviewed by , indicate substantial health benefits with particle air cleaning, with benefits generally proportional to the reduction in total exposure to particles less than 2.5 microns in diameter. The reductions in adverse health effects of outdoor air particles reported by these studies, or inferable from study results, range from 7% to 21%. One paper  compared particle filtration costs in an office building with the economic benefits of avoided health effects. The predicted annual filtration operating cost was $2.6 per person, which included filter material, maintenance, and energy costs. Predicted annual mortality-related economic benefits were $37 to $144 per person and predicted annual morbidity-related economic benefits were $8 to $30 per person, thus, predicted health-related economic benefits far exceeded costs. A second paper  projected that particle air filtration upgrades in Europe would annually prevent 27,000 to 100,000 premature deaths. Another paper evaluated use of higher efficiency electronic particle air cleaners in forced-air heating and cooling systems of homes, together with continuous operation of the fans of the forced-air systems when windows were closed . For a population of 2.7 million, the authors projected health benefits including 700 avoided premature deaths and 130,000 avoided asthma exacerbations per year. A more recent analysis for three regions of the U.S.  estimated the potential reductions in premature mortality from various improvements in filter efficiencies in both central forced air heating and cooling systems of homes and heating, ventilating, and air conditioning systems of commercial buildings, as well as from use of portable air cleaners in homes. The predicted annual reductions in mortality ranged from approximately 0.25 to 2.4 per 10,000 population. For Los Angeles County, the analyses indicated that some of the filtration system improvement scenarios would prevent more than 1000 premature particle-related deaths per year. Improvements in filtration in homes reduced premature deaths more than improvements in filtration in commercial buildings. The most effective filtration scenarios incorporated continuous operation of portable air cleaners in homes or use of higher efficiency filters in the central forced air systems of homes coupled with increases in fan operation time. In some cases, the predicted annual mortality related economic benefits exceeded $1000 per person. Economic benefits always exceeded costs with benefit-to-cost ratios ranging from approximately 3.9 to 133. Restricting filtration improvements to the homes of the elderly further increased the mortality reductions per unit population and the benefit-to-cost ratios. This analysis is available as one of the subsequent downloadable papers.
Because the health outcomes considered in the models, such as premature death, annually affect a small proportion of the total population, large empirical studies would be needed to confirm these predictions, and such studies have not been performed. Two studies [15, 16], but not a third , found statistically significant improvements, with particle filtration, in biomarkers that predict future adverse coronary events , providing some empirical support for the model predictions of health benefits.