The average physically non-active person breathes in 7 litre of air/min:
https://health.howstuffworks.com/human-body/systems/respiratory/question98.htm
Oxygen concentrators,medical ones, only cost $250/ps:
https://contecmedical.en.made-in-china.com/product/FKQEmMHTOirq/China-Contec-Oc3b-3L-Cheap-Mini-China-OEM-Medical-Neonatal-Pediatric-Oxygen-Concentrator.html?from=ButtenAD
More: https://www.made-in-china.com/cs/hot-china-products/Concentrator.html?gclid=EAIaIQobChMIudGZkMWs8AIVBbqWCh0oDQFvEAAYASAAEgIlmfD_BwE
Household non-medical versions even cheaper at 888 Yuan
