The EPA estimates that indoor air contains 2 TO 5 TIMES MORE contaminants than outside air.  Considering that we now spend (on average) 90% of our time indoors, this might be a factor to take into consideration.  The World Heath Organization is now estimating that indoor air quality is responsible for 3% of  the global burden of disease.  They've also concluded that up to 80% of all cancers are attributed to environmental factors, not genetic factors as previously thought.  Whether you're planning on remodeling or building from scratch, it is encouraged to set aside a portion of the budget toward addressing IAQ (indoor air quality).