Nitrogen Dioxide - is a reddish-brown, irritating odor gas that can be a byproduct of indoor fuel-burning appliances, such as gas stoves, gas or oil furnaces, fireplaces, wood stoves and unvented kerosene or gas space heaters.
Dust Mites - are too small to be seen, but can be found in almost every home in mattresses and bedding materials, carpets, upholstered furniture, stuffed toys and curtains.
Mold - can grow indoors when mold spores land on wet or damp surfaces. In the home, mold is most commonly found in the bathroom, kitchen and basement
Secondhand Smoke - is a mixture of smoke from the burning end of a cigarette, pipe or cigar and the smoke exhaled by the smoker that is often found in homes and cars where smoking is allowed. 
       Surgeon General Issues Call to Action to Promote Healthy Homes

Asthma
The Mayo Clinic has documented a 72% increase in respiratory asthma in children and a 61% increase in adullts over a 12 year period.  Also, that 1 in 3 people have allergies severe enough to seek medical attention on a routine basis. source American Lung Association
 
 
The Surgeon General has called Asthma a national epidemic. 20 million Americans suffer from asthma which is 3 times more than just 25 years ago, including 1 in every 8 children and 12 people die each day from asthma. source msn
The Association between Asthma and Allergic Symptoms in Children and Phthalates in House Dust: A Nested CaseControl Study
Maine has the highest rate of Asthma in the country!!
Humidity - moisture can accumulate and provide the right conditions for mold and mildew growth.

Indoor Air Quality Issues

Radon - 2nd Leading cause of lung cancer

Do you have a musty basement smell?

Do you have a crawl space?

Did the Doctor tell you to use a humidifier?
It may be causing an IAQ problem!!

Measuring Humidity & CO2 differences at floor and ceiling

Ultrafine Particle counts & HEPA filter