Order one of these. They send you the machine and you send it back and they analyze the sample.
https://homeaircheck.com/products/If you haven't done any remodeling lately, get the version without formaldehyde. I had the same problem 2 years ago. Turns out my HVAC system balance was off and was creating negative pressure and pulling air and dust from the garage. I could tell because the test showed elevated levels of gasoline and paint in the VOC analysis. I had the system balanced and coughing quit. I'm not saying that is your problem, but the test is almost the same quality a professional will do for a fraction of the price. Most times you can figure out the source of the problem based on what is elevated. If it comes back with elevated mold, then have a professional look at it.
