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.
I keep thinking about this, I have never heard of this:
"Turns out my HVAC system balance was off and was creating negative pressure and pulling air and dust from the garage"
How do you balance a AC system?
What do you mean by "Negative pressure"
And you are saying storing gas cans in the garage, caused your AC to suck fumes into the house? with the house door closed 98% of the time?
Reason I am asking, is I have been dealing with a stuffed up nose that happens at about 3am each night.
-Jason
