Each week we answer some of your pressing questions about the coronavirus and how to stay safe. Email us your questions at[email protected]with the subject line: "Weekly Coronavirus Questions." This week, we're considering questions about pulse oximeters.

As the coronavirus continues to spread, a small medical device called a pulse oximeter has started to fly off the shelves. In fact, demand has spiked to such an extraordinary degree that you may not be able to buy one right now in your local pharmacy or online.

What is the value of the device during this pandemic? Do you need to have one at home? The first question is relatively easy to answer. The second is a little more complicated.

What is it?

A pulse oximeter is a small electronic device that estimates the saturation of oxygen in your blood. You want a number in the 95% to 100% range. If the number drops to 92% or lower, that's a cause for concern. That's generally the level where a doctor might put you on supplementary oxygen and keep you in the hospital for observation.

To come up with that percentage, the device clamps onto your finger or earlobe and shoots different wavelengths of light through the small capillaries, explains Dr. Richard Levitan, a New Hampshire-based emergency room physician who specializes in airway management. (For a more thorough scientific explanation, read this.)

The term for low blood oxygen levels is hypoxemia. In this condition, your body's organs may not work right. Severe cases can interfere with heart or brain function.

You've likely had your blood oxygen monitored by a pulse oximeter already. It's used during some physical exams and is considered essential during operations and intensive care. The devices also give your heart rate.

Why are these devices suddenly getting so much attention?

COVID-19 can bring on what's called COVID pneumonia — an infection in which the lung's air sacs fill with fluid or pus. And it's possible that someone infected with the novel coronavirus might be in the early stages of COVID pneumonia – including a drop in blood oxygen level — without experiencing any difficulty breathing.

In such cases, a pulse oximeter might signal that you're in trouble before you realize it. That's what Levitan saw when he spent 10 days working in the ER at Bellevue Hospital in New York City earlier this month: Many COVID patients were already very sick with COVID pneumonia by the time they arrived. They were breathing rapidly, their blood oxygen levels dangerously low. Like mountain climbers, the patients had grown accustomed to gradually decreasing levels of oxygen and didn't realize they were in distress.

Many of them said they only recently started feeling short of breath though they had experienced symptoms of COVID-19 for days. By the time the patients went to the hospital, says Levitan, the virus had already damaged their lungs, and many were in critical condition. He saw COVID pneumonia patients with oxygen saturation levels as low as 50 percent.

"These COVID patients have adapted to this slow, insidious drop in their oxygen and they don't know it," he told NPR. "Then when they come in with shortness of breath, it is late in the process." In his opinion, a pulse oximeter could have caught the warning sign of low blood oxygen level earlier on.

The disease "kills by silently causing [a decrease in] oxygen. When you come in late in the disease, which is basically everybody who we now have coming to the E.R., there is a high mortality," he says.

Levitan says that in medicine, earlier treatment leads to better results — "and that's true of COVID, as well."

Should you get one?

Levitan thinks that in the midst of a respiratory pandemic, it makes sense to have a pulse oximeter at home – just as you might have a thermometer to track fevers. If you have symptoms of COVID-19, he says, like weakness, muscle aches or fever, you could use the device to measure blood oxygen levels.

That 92% figure (or lower) is a sign that "you should get evaluated because this disease kills silently and you don't have to have significant shortness of breath" to be at risk. (Although people at high altitudes might have levels "in the low 90s and be fine," he adds.)

Dr. Elissa Perkins, a professor of emergency medicine at Boston Medical Center, doesn't necessarily think everyone needs to buy a pulse oximeter. She worries that people will depend on the device's reading rather than calling a doctor if they're feeling sick.

"I think it's a bit of a stretch to say that people should all have these at home," says Perkins, though she understands why people would want them.

"In general, I think honestly, if people are starting to feel symptoms or feel even a little bit short of breath, they should probably contact their physician, if possible," she says. "Most places or many places at this point are offering telemedicine visits, telehealth visits and that can give a more thorough picture of what somebody is experiencing than just one number on a pulse oximeter."

But she's not completely opposed to home pulse oximeters. She says if you have symptoms consistent with the novel coronavirus, using a pulse oximeter and consulting with a doctor can be a good strategy.

She is concerned about one risk: People could be falsely reassured by good numbers into thinking they're not very sick. People with COVID "can get incredibly dehydrated, people can get very weak," says Perkins. "We've seen people fainting or passing out, who don't have particular respiratory symptoms and don't have shortness of breath. So it's another piece of data, but it's certainly not the only piece of data."

There are other concerns.The reading can be less accurate if a person is wearing nail polish or artificial nails, has cold hands or poor circulation.

It's possible to accidentally read the numbers upside down and panic over a seemingly eye-popping number. Or the device itself could be inaccurate.

The American Lung Association chief medical officer Albert Rizzo issued a statement on Thursday advising against "buying pulse oximeters unnecessarily."

"[U]nless you have a chronic lung or heart condition that affects your oxygen saturation level on a regular basis, most individuals do not need to have a pulse oximeter in their home," Rizzo said. "Most importantly, if you're interested in purchasing a pulse oximeter for home use, please discuss the necessity and use of the device with your health-care provider. A medical professional can determine if it would be helpful and also provide direction for interpreting the readings in the context of how you are feeling. Your pulse oximeter reading should be used along with your other symptoms such as shortness of breath, fever, fatigue, as parameters that would prompt a call to your doctor."

Abraar Karan, internal medicine doctor at the Brigham and Women's Hospital and Harvard Medical School, is a proponent of caution as well when it comes to pulse oximeters for home use. Hesays it remains to be seen how predictive low oxygen levels are at correctly identifying patients who will have a severe course with the disease. He points to a recent study looking at 5,700 patients hospitalized with COVID-19 in New York City, which found only 27.8% needed supplemental oxygen at triage.

And he shares Perkins' concern about people being either falsely comforted or unnecessarily alarmed.

"What we don't want to happen is that people are in real distress but think that because their home pulse oximeter reads normal, that they are fine," Karan writes via email. "We also need to make sure that patients are using the pulse oximeter properly and that it is reading correctly — otherwise, you could have more people coming to the emergency room that may not need to be there."

But Levitan says that kind of strain on ERs already happens when people interpret home blood pressure monitors or thermometers incorrectly. A much bigger concern, Levitan says, is people dying suddenly from COVID. And he doesn't worry that the device will just make people more anxious: "You know what? We've got fear. We've got anxiety. We need reassurance."

Where do I buy one — and how worried should I be if there's a delay in getting it?

If you do think a pulse oximeter would be valuable, Levitan suggests buying one from a medical source like a pharmacy rather than a random site on the internet – and it's a good idea to look for one that's been FDA-approved, if possible. You can go to the FDA's 510(k) Premarket Notification page here and search for "oximeter."

It's possible to buy devices that haven't been approved for medical use, and such devices may be less accurate.

Just a few months ago, it would have been easy to find consumer models costing about $30 typically on pharmacy shelves. They are often used by people with chronic lung diseasewho are taking supplemental oxygen to make sure they were getting the right amount of oxygen.

With many pharmacies sold out, people are turning to sites like eBay and Amazon, where devices can be found in the $60 range.

But Levitan says you shouldn't fret if you have to wait a few weeks to get one: "You know what? We have 12 to 24 months of this [coronavirus]," he predicts.

He says in an age of respiratory pandemics, there is very useful health monitoring that people can do form home, in concert with a phone call to a medical professional.

"We need to change the public perception of what health monitoring means in this age of respiratory pandemics," he says. "When you combine heart rate measurement, oxygen saturation and temperature, which are all three things you can do at home — and especially if you combine it with a phone call to your doctor or to your local emergency department ... that's incredibly informative."

Copyright 2021 NPR. To see more, visit https://www.npr.org.