Workup shows a decrease in oxygen carrying capacity with no change in PO2 of arterial blood

A 64-year-old man complains of severe headache, dizziness, nausea and deterioration of vision. Workup shows a decrease in oxygen carrying capacity with no change in PO2 of arterial blood.
Which of the following is most likely responsible for this patient’s symptoms?

A- Ozone
B- Carbon monoxide (CO)
C- Sulfur dioxide
D- Nitrogen