In an op-ed for The New York Times, emergency room doctor Richard Levitan sheds light on “silent hypoxia,” which is an infection that he claims is killing coronavirus patients.

His claim stems from a 10-day stint of treating coronavirus pneumonia at a Bellevue Hospital in New York City. He described the many patients he treated that suffered from COVID-19 pneumonia but did not have any respiratory complaints. For example, patients who experienced fall injuries were sometimes revealed to have the affliction by their CT scans.

“We are just beginning to recognize that Covid pneumonia initially causes a form of oxygen deprivation we call ‘silent hypoxia’ — ‘silent’ because of its insidious, hard-to-detect nature.”

Although various breathing problems — including pain with breathing and chest discomfort — are common with standard pneumonia, it’s coronavirus counterpart doesn’t appear to be the same. Even as patients’ oxygen levels dip, Levitan writes, many do not feel shortness of breath. When the infection is acute enough, Levitan said these symptoms begin to manifest, but by that time, the patients already have “alarmingly low oxygen levels.”

“Normal oxygen saturation for most persons at sea level is 94 percent to 100 percent” he wrote. “Covid pneumonia patients I saw had oxygen saturations as low as 50 percent.”

Another concerning part of coronavirus pneumonia patients is that they exhibit “relatively minimal apparent distress,” with many continuing to use their cell phones as they are examined at the emergency department. Levitan contrasts this with the average patient that requires intubation due to acute hypoxia, who are often unconscious or barely functioning.

According to the physician, the reason for silent hypoxia is just beginning to become apparent. As of now, he said researchers understand coronavirus attacks the lung cells that create the surfactant necessary to keep the air sacs in the lung open between breaths. But as coronavirus pneumonia begins to take over, he claimed the air sacs remain “compliant,” which allows them to continue releasing carbon dioxide. Without a buildup of the gas in their lungs, patients don’t feel short of breath, even as the infection progresses.

Pro-Tip: take care of yourselves by checking your temperature and blood oxidation rate daily! Go get a pulse oximeter.

If you drop below 94-95% blood oxygen, seek help if you have symptoms. #COVID2019 pic.twitter.com/PFZi11aVb8 — Collin Myers ABC13 (@collinabc13) March 25, 2020

Levitan noted that avoiding a ventilator is beneficial for both the patients as well as the medical system and instead, he pushed for the use of pulse oximeters. According to him, the widespread use of such devices — with doctor guidance to ensure they are used and interpreted correctly — could help detect early coronavirus pneumonia signs.

In a piece for Business Insider, Aria Bendix credits the device as her “saving grace” as she battled COVID-19 and noted its relatively inexpensive price.

According to Bendix, the emergency room doctor who treated her felt the same.

“I wish everyone had one of these so they could monitor themselves,” they told her. “Then maybe we’d have fewer people come in.”