Kidney cancers can be cystic, and differentiating between a single benign cyst of the kidney and a kidney cancer can occasionally be a problem. When many cysts are present, as in polycystic renal disease, the distinction is easier. Cysts may also be caused by some parasitic infections.
It will actually collect the oxygen in the ambient air (~21%) and convert it into a concentration of around 85-94%. One of the main reasons you would want a concentrator over a tank and regulator would be when you are using a lot of oxygen for taking ozone saunas or ozonating oils.
Hydrogen has a RMM of 2; nitrogen has a RMM of 28 and oxygen has a RMM of 32. Air is a mix of nitrogen and oxygen, so will have an average RMM of about 29. That is 14.5 times as much as hydrogen, so air is 14.5 times as dense as hydrogen. And it's simple to do the same for other gases. Carbon dioxide has a RMM of 44 (12+16+16), so it is 44/29 = 1.5 times as dense as air.