Even for healthy people, swimming can lead to an accumulation of fluid in the lungs, called pulmonary edema.

New research suggests that difficulty breathing and a transient drop in lung function are common features of swimming-induced pulmonary edema. However, just why it happens - and whether it has any long-term effects - remain unanswered.

The current study, in the medical journal Chest, looked at 70 cases of pulmonary edema related to swimming that occurred in healthy young men participating in a fitness program.

All the subjects had a physical examination and a chest X-ray, and had the level of oxygen in their blood measured.

Breathing difficulties were noted in all cases, lead author Dr. Yochai Adir, from the Israel Naval Medical Institute in Haifa, and colleagues report. Nearly all of the young men had a cough with significant sputum production, and about half actually coughed up blood.

Swimming-induced pulmonary edema caused a significant fall in blood oxygenation. Also, lung function dropped considerably, and this persisted for a week.

In all cases, the chest X-rays, which were obtained 12 to 18 hours after symptoms began, were normal.

Pulmonary edema caused by swimming recurred in 16 subjects, with an interval between episodes of at least three months.

Adir’s team thinks the problem results from a combination of factors.

As they write: “We speculate that the physiologic effects of immersion, swimming in the semi-supine position with the head and shoulders above water, thus increasing the pressure difference between the lower extremities and the thorax, and the relatively cold water temperature, all serve to increase the risk of exercise-induced pulmonary edema.”

SOURCE: Chest, August 2004.