What are the most common concerns your clients have? Mold and mildew? Radon? Dust and particulates? Other pollutants?

For the most part, in smaller projects, it's usually a catastrophic episode that causes the issue. Most are molds, odors or other microbials. The larger projects are usually for a regular maintainance program. MOst ductwork and AHU's contain dust and dirt paritcles. I guess I should thank the people who invented and use fiberglass lined ductwork. If you want a disaster in your system, install this wonderful preduct. I a few years when it breaks down, and it will break down, you'll be calling me to stop the "black stuff" from blowing out on you desks. It is also a wonderful home for molds like cladosporium and Penicillium. Of course when the studies are finished on fiberglass I'm sure we'll all be removing tons of it from ductwork. Then on course there os still the asbestos threat. It is still around in abundant supply. There are many commercial desicant units out there that use chrysotile as part of the filtration unit.
Most residential customers are concerned about odors, mold and pet danders. Too many worry about cigarette smoke but the nicotine is almost impossible to remove from standard galvanized ductwork. They fail to understand the big picture. It's not what you can see that will hurt you, it's the contamiants too small to see.

Radon, a known carcinigen. #2 cause of lumg cancer behind smoking. Every home in the country has some radon gas. Outdoor air has radon gas. The EPA set recomendations for how much radon is too much. Nebraska is #3 in the percentage of homes failing radon tests (54% of the homes tested in Neb. fail, 63% in my county fail). It's soil type, construction methods and climate. The good news is radon is a very fixable problem.

Mold is #2 in the last couple of years.