How to choose a water filter for emergencies

What to look for

Tick each point the product you are considering meets.

• For parasites such as Cryptosporidium and Giardia, choose a filter labelled with an absolute pore size of 1 micron or smaller, or certified to NSF/ANSI Standard 53 or 58 for cyst reduction.^[1]
• Prefer 'absolute' over 'nominal' pore size: absolute means every pore is that size or smaller, nominal is only an average.^[1]
• To remove bacteria too, choose an absolute pore size of 0.3 micron or smaller, or ultrafiltration, nanofiltration or reverse osmosis.^[1]
• Only nanofiltration and reverse osmosis are listed as removing viruses.^[1]
• Look for NSF certification on the label and check the specific product in NSF's online database.^[1]
• For chemicals such as lead, check the label for the exact chemicals the filter is certified to reduce: filters that remove germs often do not remove chemicals.^[1]

Avoid

• Do not rely on a jug (pitcher) or fridge filter to make unsafe water safe: most are not designed to remove germs.^[1]
• Do not treat a portable filter as enough on its own: it will not remove viruses, so add a disinfectant such as chlorine, iodine or chlorine dioxide after filtering.^[2]
• Do not assume a disinfectant alone is enough for Cryptosporidium or Giardia: disinfectants do not work as well as boiling against them.^[2]

After you buy

• Replace cartridges as often as the manufacturer recommends.^[1]
• Wear gloves when changing a filter and wash your hands afterwards; people with weakened immune systems should not change filters themselves.^[1]
• When in doubt and you can heat water, boiling is the best way to kill germs.^[2]