One of the mysteries of Covid-19 is why oxygen levels in the blood can drop to dangerously low levels without the patient noticing.
It is known as "silent hypoxia".
As a result, patients have been arriving in hospital in far worse health than they realised and, in some cases, too late to treat effectively.
But a potentially life-saving solution, in the form of a pulse oximeter, allows patients to monitor their oxygen levels at home, and costs about £20.
They are being rolled out for high-risk Covid patients in the UK, and the doctor leading the scheme thinks everyone should consider buying one.
A normal oxygen level in the blood is between 95% and 100%.
"With Covid, we were admitting patients with oxygen levels in the 70s or low-or-middle 80s," said Dr Matt Inada-Kim, a consultant in acute medicine at Hampshire Hospitals.
He told BBC Radio 4's Inside Health: "It was a really curious and scary presentation and really made us rethink what we were doing."
Dr Inada-Kim became the national clinical lead of the Covid Oximetry@home project.
A pulse oximeter slips over your middle finger and shines a light into the body. It measures how much of the light is absorbed in order to calculate oxygen levels in the blood.
In England, they are being given to people with Covid who are over 65, younger but have a health problem, or anyone doctors are concerned about. Similar schemes are being rolled out across the UK.
People measure and record their oxygen levels three times a day.
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If oxygen levels drop to 93% or 94%, then people speak to their GP or call 111. If they go below 92%, people should go to A&E or call 999 for an ambulance.
Studies, which have not been reviewed by other scientists, have shown even small drops below 95% are linked to an increased risk of dying.
Dr Inada-Kim said: "The point of this whole strategy is to try to get in early to prevent people getting that sick, by admitting patients at a more salvageable point in their illness."
Chris Harris, who is 70, was one of the first patients to benefit from the scheme.
He was being treated for a urinary infection in November last year, but then when he developed unexpected flu-like symptoms his GP sent him for a Covid test. It was positive.
"I don't mind admitting I was in tears, it was a very stressful, frightening time," he told Inside Health.
His oxygen levels dropped a couple of percentage points below the normal zone, so after a call with his GP, he went to hospital.
At this point he was still feeling fine, but things changed the day after he was admitted.
"My breathing started to get a little bit laboured, I had a high temperature as the days went on, [my oxygen levels] were progressively getting lower, they were in their 80s," he told me.
Chris was treated, did not need intensive care and has made a full recovery.
He said: "I may have gone [to hospital] as the very last resort and that's the frightening thing. It was the oxygen meter that forced me to go, I would have just sat it out thinking I would recover.
"I am extremely lucky and very, very grateful."
His GP, Dr Caroline O'Keefe, says she has seen a massive increase in the number of people being monitored.
She said: "On Christmas Day we were monitoring 44 patients, today I have 160 patients who I am monitoring daily. So we are certainly busy."
"We've had to quadruple the size of our team in the last two weeks."
Overall, NHS England has supplied around 300,000 pulse oximeters for the home-monitoring scheme.
Dr Inada-Kim says there isn't definitive proof that the gadget saves lives and it could take until April to know for sure. However, the early signs are all positive.
"What we think we can see are the early seeds of a reduction in the length of stay after a hospital admission, an improvement in survival and a reduction in the pressures on the emergency services," he said.
He is so convinced of their role in tackling silent hypoxia that he said everyone should consider buying one.
"Personally I would, and I know a number of colleagues who have bought pulse oximeters to distribute to their loved ones," he said.
He advised checking they had a CE Kitemark and to avoid apps on smartphones, which he said were not as reliable.
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