Researchers have spent hundreds of hours trying to understand what is causing deaths in triathlon swims.

Though deaths during triathlon races don’t occur often, they do occur – at a rate of about one in every 76,000 participants. Of those deaths, more than 72 percent happen in the swim.

It’s understandable, then, that people would start to question the safety of the triathlon swim – are races doing enough to keep their swimmers safe? Many races have responded to these concerns with new initiatives, like rolling or wave swim starts instead of mass swim starts, increased lifeguard presence, and an abundance of caution – even going so far to cancel swims in weather conditions that would have been considered acceptable a decade ago.

And yet, swim deaths still occur. Researchers are now beginning to better understand one possible reason why: SIPE, or Swimming Induced Pulmonary Edema. Researchers like Dr. Richard Moon of Duke University have dedicated hundreds of hours trying to understand this phenomenon – and hopefully, to reduce the likelihood of it happening during triathlon races.

What is SIPE?

In layman’s terms, SIPE is a condition in which the lungs fill with fluid and blood. “This occurs when there is greater to normal pressure within the pulmonary capillaries, causing a breakdown of the membrane separating the blood from the gas spaces, or alveoli,” explains Moon. “Some have referred to SIPE as ‘drowning from the inside.’ This condition occurs in swimmers and divers, particularly – but not always – during heavy exertion, such as in the swim portion of the triathlon.”

SIPE presents itself as shortness of breath and cough, often with blood-tinged sputum (or, in some cases, thick blood). This cough leads to a severe hypoxemia, or low blood oxygen level, and subsides with a cessation of exercise. However, if a person persists in the race, it can be serious – and even deadly.

It’s important to note that SIPE is different from cardiac arrest, or a heart attack. In fact, Moon’s research into swim-death autopsies has found that in the majority of triathlon deaths attributed to simple cardiac arrest, the cause was more likely to be SIPE.

How do I know if I’m at risk?

SIPE occurs in all ages of adults, and all fitness levels – even Navy SEAL trainees experience SIPE. However, the frequency increases in middle-aged or older adults who subject themselves to heavy exertion during a swim. Some medical conditions, such as hypertension, cardiac muscle disorders, heart valve disease, and coronary artery disease, are risk factors; however, Moon warns that SIPE can occur even in the absence of such conditions: “We have identified some individuals who have normal blood pressure under ‘doctor’s office’ conditions, but whose blood pressure rises excessively when immersed in cold water.”

Though there’s currently no standard blood test or chest scan to screen for SIPE at the doctor’s office, Moon and his team has found one way to screen for the condition: “We have a test that requires an oral fluid load exertion and 40 minutes of exercise immersed in cold water. In this test the majority of SIPE-susceptible individuals will experience mild symptoms and signs of SIPE.” It’s possible that this test may one day lead to simpler testing methods to screen for SIPE in a traditional healthcare setting.

In the interim, it’s a good idea to see a physician regularly for health checks that include a medical examination for the above-mentioned risk factors. Athletes should report any heart-related symptoms, including shortness of breath, chest tightness, or chest discomfort, to their doctor.

Can I cure it?

At the moment, the answer is no. But Moon and his team are working hard to change that. In fact, his team is currently testing a theory that the medication sildenafil might reduce the occurrence of SIPE. You may not have heard of this drug by its generic moniker, but you’ve definitely heard it called by its brand name: Viagra. Though it’s now known as a medication for erectile dysfunction, sildenafil was originally developed in 1989 as a way to treat high blood pressure (what – er, popped up – was a surprising discovery). The Viagra theory is still in the testing stages (Moon is recruiting volunteers ages 18-45 who have previously experienced SIPE; those interested in serving as test subjects can register here), but if successful, a dose of the medication prior to racing may one day serve as a prophylactic against SIPE.

What should I do if I suspect SIPE on race day?

If you experience shortness of breath and blood-tinged cough while swimming, it’s important to stop exercise immediately. Persisting – even for a brief period of time – can be deadly. Hearing a cough in a fellow athlete may not be a reason to worry (many conditions have a cough, including asthma) but it is a reason to check in (simply asking, “Hey, are you okay?” can be enough to get a hyper-focused athlete to notice something might be off). If necessary, inform that athlete of the SIPE warning signs and help him or her to get to the nearest lifeguard.

Should I give up swimming altogether?

If you’re a healthy individual, no. Though swim deaths do happen, they shouldn’t overshadow the many health benefits of swim, bike, and run. The majority of people will experience more benefit than harm from regular exercise – so swim on.