Over time, buildings have been built progressively tighter for improved durability, along with increased insulation required by building codes to gradually increase the energy efficiency of new buildings.
A tighter building is better for multiple reasons. Air leaking through the building enclosure carries water vapor into the building cavities, which can condense and lead to mold growth, degrade materials, and reduce the life-span of the building enclosure. Air moving through insulation in those building enclosure cavities also reduces the effectiveness of the insulation and can potentially make it useless.
Buildings do not need to breathe, but we do, and we do not want to be breathing the air that leaks in through the dusty building cavities around windows, electrical outlets, and other cracks.
Air leaking into the building also carries outdoor air pollutants. The outdoor air pollutants that can leak into the building include ozone and particulate matter pollution. Particulate matter pollution is tiny particles released into the air from wildfires, disc brakes, gasoline or diesel vehicles, and fuel combustion in buildings and factories. It should be noted that particulate matter pollution is also created when cooking or burning a candle at home. Particulate matter pollution is typically categorized by size, measured by the diameter of the particle. PM10 (roughly 1/6 the diameter of a human hair) is the size of dust and pollen and PM2.5, which is the size of some combustion particles, is 1/4 the size of PM10. Ultrafine particulate matter, PM0.1, is 1/100 the size of PM10 and includes the dust produced by disc brakes.
A recent Lawrence Berkeley National Lab (LBNL) study estimated that the chronic health impact from PM2.5 is on par with or greater than secondhand smoke and radon.
Both LBNL and the Built Environment Research Group at the Illinois Institute of Technology have found that a tighter building enclosure helps reduce the amount of outdoor air pollution within the building.
Lastly, it is important to remember that all buildings require proper ventilation, ventilation filtration, and ventilation system testing to confirm systems are operating correctly to ensure adequate indoor air quality.