[photochemsyn]

Clean air and water issues are not limited to biological contamination with pathogenic microbes and viruses, and some of the suggestions (ensuring good ventilation) run into problems when external air quality is dangerously bad (when health agencies tell people to keep their windows closed).

It's not entirely unlike water issues, for example the Thames was used to dispose of wastes from animal slaughtering, leather tanning, production of dyes from coal tar, alcohol distillery wastes as well as for human excrement. Cleaning up air quality requires addressing these issues as well (coal power plants, diesel truck emissions, agricultural dust, etc.).