Drs. Chaitman and coworkers at the St. Louis University School of Medicine examined whether there is a link between carbon monoxide exposure and arrhythmias in subjects with coronary artery disease. Carbon monoxide is a ubiquitous air pollutant. It is found in cigarette smoke and emissions from motor vehicles, industrial processes, and poorly ventilated combustion sources. The investigators studied 25 men and 5 women, aged 45 to 77 years, all of whom were nonsmokers with stable coronary artery disease and who had moderate levels of ventricular arrhythmias. Most of the subjects were taking medications for heart disease. The subjects were exposed for one hour to air only or to air containing one of two levels of carbon monoxide sufficient to produce 3% or 5% carboxyhemoglobin; these carboxyhemoglobin levels were maintained for an additional 90 minutes. Blood carboxyhemoglobin was measured using highly accurate gas chromatographic techniques. Subjects were monitored continuously for arrhythmias by Holter monitors, which record the electrical activity of the heart. They were monitored for a total or 20 hours while at rest, during a brief exercise period, during recovery from exercise, and during their usual activities.