Energy efficiency is a good thing, and one component of an energy efficient home is preventing air leaks that lead to increased heat or air conditioning use. However, good ventilation is also needed to prevent health problems, like sick building syndrome.
The National Institutes of Health (NIH) gives us the formally accepted definition of sick building syndrome:
“The term sick building syndrome (SBS) is used to describe a situation in which the occupants of a building experience acute health- or comfort-related effects that seem to be linked directly to the time spent in the building. No specific illness or cause can be identified. The complainants may be localized in a particular room or zone or may be widespread throughout the building.”