I had the same issue with Midnite when we moved into the house and the heat went on. I had him tested and one thing that was definite was Petrochemical.
"Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane."
Obviously I need heat in the winter so I went the route of strengthing his immune system. No issues this year at all.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD