For guinea pigs, we place them in a 4'x6' plexiglass chamber and bleed in 100%N2 to lower oxygen level of room air (21%O2) down to whatever we want (16%, 12%, 10%O2). We have an oxygen sensor in the chamber to monitor oxygen levels and a servo controller on the box to regulate the % we want. We place dessicant inside to lower humidity and air flow through a silica gel canistor to remove ammonia.

For smaller animals (mice and rats) many groups are using systems like the Baker Ruskinn VelO2x system. This system ramps up and down the oxygen levels quickly and is ideal for intermittent hypoxia studies.

For more information visit this website: https://www.scintica.com/products/baker-ruskinn-velo2x-animal-hypoxia-chamber/