Ten patients with angina had cardiopulmonary tests done in the control state, after being driven for 90 minutes during heavy morning freeway traffic, and 2 hours after return. The cardiac tests were repeated on a subsequent morning, with the patients breathing compressed, purified air during freeway travel. The expired-air carbon monoxide and arterial carboxyhemoglobin levels significantly increased after breathing freeway air. There was a significant decrease in exercise performance until angina, in systolic blood pressure at angina, in heart rate at angina, in systolic blood pressure times heart rate at angina, and in the FEV1/FVC (forced expiratory volume in 1 second/forced vital capacity) after breathing freeway air. Ischemic ST-segment depression occurred in 3 of 10 patients while breathing freeway air. No significant change from control values occurred in the above variables after breathing compressed, purified air. Exposure to heavy freeway traffic increased carboxyhemoglobin levels, causing angina to develop sooner after less cardiac work.