Doctors and public officials remain confused over a new effect from the coronavirus and COVID-19 — “happy hypoxia.”

“Happy hypoxia” is a mysterious condition where people who have severely low oxygen levels don’t appear worried or in distress, even though their oxygen levels could make them unconsciousness or on the brink of death.

Healthy people have oxygen levels around 95%. But patients who have this symptom might have oxygen levels around 80% or 70%, if not lower than 50%, according to The Guardian.

Dr. Jonathan Bannard-Smith, a consultant in critical care and anaesthesia at Manchester Royal Infirmary, told The Guardian that the condition is a little confusing.

“It’s intriguing to see so many people coming in, quite how hypoxic they are,” he said. “We’re seeing oxygen saturations that are very low and they’re unaware of that. We wouldn’t usually see this phenomenon in influenza or community-acquired pneumonia. It’s very much more profound and an example of very abnormal physiology going on before our eyes.”

Normally, patients with this level of oxygen would appear sick or worried. It’s not the case with COVID-19.

In fact, patients have been scrolling their phones or talking with health service workers, according to Science magazine. Some even considered their condition as comfortable.

Dr. Mike Charlesworth, an anaesthetist at Wythenshawe Hospital in Manchester, told The Guardian: “With pneumonia or a pulmonary embolism they wouldn’t be sat up in bed talking to you. “We just don’t understand it. We don’t know if it’s causing organ damage that we’re not able to detect. We don’t understand if the body’s compensating.”

Normally, people will feel breathlessness because the body senses the rise of carbon dioxide — not the low oxygen levels, New York Post reports.

But this doesn’t seem to be the case with COVID-19 patients. People who have low oxygen levels don’t show signs of it.

Dr. Reuben Strayer, an emergency doctor at Maimonides Medical Center in New York, told Science magazine that clotting might be to blame. But it’s unknown.