(Bloomberg) Migraine headaches, fatigue and dizziness were sidelining Bert Henriksen several times a week. Evenings were the worst, after his 30-mile commute home in his 2017 Ford Explorer.

His behavior grew erratic. He’d get angry over minor things. “We were getting scared that he had some kind of a brain problem,” said his wife, Megan.

An answer came last March in a phone call from his doctor: A blood test revealed that Henriksen had been exposed to toxic levels of carbon monoxide gas. But how? The result was consistent with someone who’d been in a house fire, his doctor said, but Henriksen hadn’t been through anything like that.

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