Humans in the developed world spend more than 90 percent of their lives indoors, where they breathe in and come into contact with trillions of life forms invisible to the naked eye: microorganisms.

The quote reframes “indoors” as an ecological habitat rather than a sterile refuge. By stressing that people in industrialized societies spend the vast majority of life inside, it highlights how built environments shape daily exposure to microbial communities through air, surfaces, dust, and human contact. The emphasis on “trillions” and “invisible” organisms underscores both scale and perceptual blindness: what we cannot see still profoundly affects health, comfort, and disease risk. Implicitly, it supports the idea that architecture, ventilation, cleaning practices, and occupancy patterns are biological as well as engineering choices—suggesting that managing indoor microbiomes is a public-health and design challenge, not merely a matter of hygiene.