Figure 6. Enhanced filtration improved performance-related mental states as well as many environmental dissatisfaction ratings. (LBNL-48217)
 

Many of the indoor-generated pollutants important for asthma are particles with diameters ranging from a fraction of a micrometer ot approximately 20 micrometers. Many bioaerosols associated with asthma, particulary dust mite allergens, whole pollens, cockroach allergen, and many fungal spores are large particles greater than a few micrometers in diameter. See the following link for more details.