- Arsenic: many areas of Bangladesh have it in tubewells. Test at your local public-health office; tubewells are marked red/green.
- Sand–charcoal–gravel filter: reduces cloudiness but does not reliably remove arsenic or germs.
- Boiling: kills germs, not arsenic.
- Wells and tubewells: keep them away from latrines and waste; raise and seal the platform.
Never guess about drinking water — test it.