As studies increased in sophistication,
other measurable factors came into
play. Building construction materials
and techniques changed radically. A
reduction in natural ventilation, or
"fresh" air, in the interest of saving
energy became a concern and, finally,
people realized that pollutants could
actually originate within a building. The
World Health Organization (WHO)
estimated that more than 30 percent of
all commercial buildings have
significant IAQ problems.