Gas mask and sewage treatment plant: what they have in common

1. Step 1. Gas mask — evolved from — Respirator. Respiratory protection.
2. Step 2. Respirator — evolved from — Surgical mask. A filtering mask.
3. Step 3. Surgical mask — uses — Germ theory of disease. Protection against droplets.
4. Step 4. Germ theory of disease — uses — Culture medium (agar). Koch’s postulates.
5. Step 5. Culture medium (agar) — is used in — Sewage treatment plant. Waste water is cleaned by activated sludge — bacteria.

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