If you’ve ever strived to become an elite Navy SEAL, you likely will have tried military-grade breath-holding techniques used by the most elite special forces in the field.

The average SEAL can hold their breath for around three minutes during high-pressure underwater exercises meant to simulate life-threatening scenarios, but the average American does not have to undergo the same rigorous testing to see how well their lungs work.

Lung capacity tests have cropped up all over TikTok, where users post videos showing a countdown of about 40 seconds. The first few should be dedicated to a profound inhale, followed by an extended period of holding that breath in before exhaling.

The online tests, though far less strenuous than the versions aspiring SEALs must complete, came easily for some people, while many others said they ‘almost fainted’, ‘turned blue’, and ‘saw my grandma halfway through’.

The user is meant to inhale profoundly for around four seconds before holding their breath for the amount of time it takes for the dot to travel clockwise to the other side for the ‘exhale’

TikTok is saturated with videos made by average civilians who assert that if you can hold your breath for around 40 seconds without lights popping in your periphery, you may want to consider a career in the military.

Navy SEALs, named for the environments in which they operate (sea, air, and land), are an elite military force, famous for having raided the Pakistani compound of terrorist Osama bin Laden, that trains day in and day out to prepare for any possible wartime scenario.

Their grueling training exercises include one in which a candidate lays face down in a pool of water after instructors have tied knots in their breathing regulator, forcing them to remain calm with little oxygen while also having to untangle the knots before resurfacing.

The tests on TikTok are far simpler and don’t require viewers to even leave their desk chairs. Simply inhale deeply for about four seconds, which many online say is the hardest part, then hold that breath for 40 to 50 seconds. Then, release a measured exhale.

The test making its rounds on social media is strikingly similar to one used by the UK’s Royal Air Force during World War I through 1939 to measure a military candidate’s physical fitness. 

Men were instructed to make a full exhalation followed by a deep inhalation, and then hold it for as long as possible, which was around 45 seconds on average.

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Even though the test was used through the start of World War II, scientists began to realize years prior it was a poor measure of physical fitness, with one saying it only measured a person’s ability to withstand discomfort, not their bodies’ abilities to adapt to more efficiently use oxygen.

A fierce cardio lover may have an easier time holding their breath because their lungs are stronger, while a person who vapes or smokes regularly will likely struggle more.

A 2017 study conducted by doctors in India reported smokers were able to hold their breath for 34.85 seconds while non-smokers could hold it for 46.61 seconds.

When anyone holds their breath, whether training to be in the military or swimming with friends at the pool, the body makes compensations for the sudden cut off from oxygen.

Holding one’s breath stops oxygen from coming in, and it also stops carbon dioxide from being able to leave the body because exhalation has stopped. 

That CO2, rather than being released, is able to cross the blood-brain barrier, a network of blood vessels and tissue that serves as a protective layer lining the inside of the brain.

This can cause a drop in the delicate pH balance of the blood. The brain then issues an urgent signal to the lungs to breathe out that harmful CO2.

A smattering of studies have reported some health benefits of practicing breath-holding techniques, including one from 2015 that reported breath-holding could help regenerate brain tissue, though the study was conducted on salamanders.

Another study 2014, argued the exercises could activate the body’s fight-or-flight response while producing anti-inflammation agents in the body that could help mediate an overzealous immune system response to foreign invaders or its own cells.

Doctors will sometimes perform their own versions of lung function tests to measure how well the lungs are working, though none that ask a patient to inhale, hold their breathe and exhale for a predetermined amount of time.