It turns out my air handler in my garage isn’t properly sealed and needs to be fixed. I was getting ready for the new spring weather last month by cleaning out my air conditioning components with a light vinegar solution before I crank the system on again for the first time since before winter. Everything in my process was fine and went according to plan, I was able to get my coils and filter tray clean, and I also opened the air return vent and the output vents to clean those as well. But after I had the system on for about a day or two, I was starting to get the sort of allergic reactions that I get to heavy dust and mold, but hadn’t had any issues with either since easily last autumn or longer. I scoured the internet for various possible sources and probably spent more time worrying over unlikely worst-case-scenario situations instead of learning anything constructive about my problem. I finally called a reputable local heating and cooling supplier to drop by and take a look at my HVAC system to see if anything was wrong under the sheer chance that somewhere something was getting introduced to my air stream. Well, it looks like my hunch was correct, the HVAC technician told me that my air handler itself was not properly sealed and was drawing in air from my dusty garage and pushing it into my house. He had to schedule a separate service call to get that fixed but once he did I noticed a massive improvement. My lungs were clearing up and I was finding less dust on surfaces when I cleaned. I would have never thought to check the seals on the air handler if I hadn’t called the HVAC contractor over.