This chapter of the Breathedge guide contains information on health recovery of the main character.
Health in Breathedge can decrease due to various factors. These can be environmental factors (radiation, temperature), or lack of oxygen (most often).
The first available method is simply sleeping. Lie in bed in your capsule to regenerate your health to the maximum. Some time will pass in the game (which does not matter to the hero) and you will lose some of the hunger and thirst bars.
The second method available at any time and anywhere is the use of first aid kit. You need to get the scanner first and get the blueprint. You will find the blueprint in the toilet of your capsule by scanning the first aid kit located there.
To make a first aid kit, you need a common fabric and slightly less common (but crafted) alcohol.