A 75-year-old man who lives alone in a poorly ventilated house without central heating uses a portable unvented kerosene heater to warm the house during the winter months. One morning, a neighbor finds him in an obtunded state. On physical examination, he appears cyanotic. Results of blood gas measurement on room air are PO2, 90 mm Hg; PCO2, 35 mm Hg; and pH, 7.3. Pulse oximetry shows low oxygen saturation. Exposure to which of the following is most likely to have produced this man's illness?