That is a very good question. It has everything to do with Application. Semiconductor and Biotechnology facilities are very controlled environments, to the extent there are few if any particles being generated by activity within these environments. Personnel wear Clean or Containment Suits to prevent the emission of particles from human hygiene or activity and entry and exit is through Air-showers and Airlocks. Moreover the ceiling tiles in these facilities are replaced with HEPA Filters so the air flow in these facilities is measured in Room Changes per Minute. Bottom line is that these facilities can be considered “closed or contained environments”. In the home and office environment, doors are opening as people come and go allowing allergens and other contaminants to enter, pets and people are generating particles and pathogens through all of their activities and microbes are growing despite our best efforts to maintain environmental cleanliness. Therefore, we are more concerned with removing as many particles as we can (Net Particle Removal) instead of measuring particle filter retention efficiency. IE retention efficiency matters little if you are only processing 10% of your home or Office air once every hour. Bottom line is that there is no practical way to create a closed or contained environment in a home or office application. Therefore, we focus on removing as many particles as we can as quickly as we can.