If you've seen those heart-wrenching photos from India of long lines of people waiting to get oxygen cylinders refilled, you'll appreciate the benefit of personal, portable, oxygen concentrators. Especially during the COVID crisis, oxygen is at a premium. SARS-CoV-2, the virus that causes Covid-19, reduces the ability of cells lining the lungs from functioning as "oxygen in/carbon dioxide out" filters, resulting in the blood being starved for oxygen. In extreme cases, oxygen-enriched air has to be forced into a patient's lungs from a ventilator inserted into the windpipe, essentially doing their breathing for them until they recover. Of course, you don't have to have COVID to need extra oxygen — anyone with COPD (chronic obstructive pulmonary disease), pulmonary fibrosis, asthma or a host of other breathing problems needs more of it than that supplied by regular breathing.
Before the 1970s, oxygen therapy required those heavy, high-pressure, steel cylinders, which involved inconvenient and costly schlepping from factory to hospital or home, along with the inherent fire risk. The technology for portable oxygen concentrators came out of the invention of "molecular sieves" by Union Carbide in the 1950s. The idea is simple: Air is mostly nitrogen (78 percent) and oxygen (21 percent). Remove the nitrogen and you end up with nearly pure oxygen. Nitrogen molecules (N2) are larger than oxygen molecules (O2), so an appropriately sized "sieve" will allow O2 to pass through while trapping the N2. The three-minute video How Oxygen Concentrator Works neatly explains it.
Turns out, the plentiful and cheap sieve material zeolite is readily available. Zeolite is the collective term for a group of porous minerals that consist of hydrated aluminosilicates, having a large ratio of surface area to volume (think of a sponge). When air at high pressure (about 20 pounds per square inch in portable units) is forced through zeolite, it adsorbs the nitrogen while allowing the smaller oxygen molecules to pass through. The output for portable concentrators is around 94 percent oxygen.
What happens when the zeolite gets clogged up with nitrogen? The clever trick employed by portable oxygen concentrators is to have two zeolite-filled cylinders. Call them A and B. First, A is the active cylinder then, a few seconds later, the unit switches to B. At that point — this is the genius of the system — part of the oxygen output from cylinder B is diverted into the output of cylinder A, where it "backflushes," that is, the oxygen desorbs, or frees, the nitrogen from the zeolite which is then vented into the atmosphere. The resulting enriched oxygen output is made available to the patient through a nasal cannula after passing through a pressure-reducing valve.
At rest, we humans breathe about 8 liters of air per minute, of which a fifth is oxygen. Some people need enriched oxygen; for them, these portable units, slung over the shoulder, can give them back their regular lives. Score one for science.