The dots are the air — nitrogen, oxygen, a little carbon dioxide, and some germs. Skin, hair and clothing bounce everything away, so the mask is the only way in or out. Make it finer or looser and watch what it stops and what slips past.
Inhaling ▸
Air is drawn toward the mask. Small things pass; germs are caught.
The mask
Loose (big holes)Tight (fine)
Clean roomCrowded room
Nitrogen (N₂) · 78%
Oxygen (O₂) · 21%
Carbon dioxide (CO₂)
Virus
Bacteria
Air, in vs out
Air you breathe inO₂ 21% · CO₂ 0.04%
O₂N₂
Air you breathe outO₂ 16% · CO₂ 5%
O₂N₂CO₂
Your body takes in oxygen and gives out carbon dioxide. That is why the breath you exhale has less O₂ and much more CO₂. The mask lets all of these gases through — it only stops the big germs.
What the mask did
Germs caught by the mask0
Germs that reached the lungs0
A simplified model. Sizes are not to scale, CO₂ is exaggerated so you can see it, and the inward pull just stands for breathing airflow. A real mask also traps germs smaller than its holes by clinging to the fibres — which is how masks stop viruses a simple sieve would miss.
Noescio