If you have a heating or cooling systems that uses air ducts, you could have a problem.
There are many duct systems that are poorly designed and installed that have the ability to bring pollutants into your home.
Some are ducts are located in wet or damp crawl spaces and basements.
I've seen some of these areas and some look like an open sewer.
Having ducts that are not properly sealed or insulated in these locations can draw moisture, mold, bacteria and even carbon monoxide into the building.
Unsealed ducts have the same effect as opening a hatch to these ares, the only difference is that you don't see it and the fan from the heating or cooling appliance will force more of the pollutants into the living space at a much higher rate.
Ducts located in attics can bring in cold dry air in winter, and hot, humid air in summer, dust, dirt and fibers from the fiberglass insulation that is there to keep your house warm in the winter and cool in the summer.
It doesn't have to be this way.
Hiring the right professional who uses duct diagnostic tools and good industry practices will put you on the road to good indoor air quality and a more comfortable environment.