The choice between water purification tablets is a choice between chemicals, and the CDC settles it in one sentence: iodine, tablets with iodine, or chlorine tablets kill most germs, but will not kill Cryptosporidium. Chlorine dioxide tablets will kill Cryptosporidium as long as you follow the instructions correctly. For an emergency kit that may have to treat flood water or a lake, that puts chlorine dioxide first; for treating water you have stored yourself, plain chlorine is enough and far cheaper per gallon.
Six products below, sorted by chemical, with what the listing says each treats and the catch for each. The CDC’s order of preference is boiling first, then chemical disinfection, then filtering — tablets are the backup, not the plan, and they do nothing against fuel or chemical contamination. Our water purification guide covers the whole ladder; this page is the tablet rung.
The three chemicals, in the CDC’s words
- Chlorine dioxide (tablets or two-part drops): kills bacteria, viruses and, given the full contact time on the label, Cryptosporidium and Giardia. The slowest — typically hours for the parasites — and the least aftertaste.
- Chlorine (NaDCC tablets, or household bleach at 8 drops per gallon): kills bacteria and viruses; the CDC says it will not kill Cryptosporidium. Cheapest per gallon, the right choice for water you stored clean.
- Iodine (tablets or tincture): kills bacteria and viruses, not Cryptosporidium, and the CDC adds two limits — avoid it if pregnant, with thyroid problems or iodine sensitivity, and nobody should drink iodine-treated water for more than a few weeks.
Whatever the chemical: if the water is cloudy, filter it through a cloth or coffee filter first, double the dose if it is cloudy or very cold, and wait the full time on the label.
At a glance
| Product | Best for | Chemical / quantity (listing) |
|---|---|---|
| Potable Aqua chlorine dioxide tablets, 30 count | The go-bag and the car kit | ClO₂ / 30 tablets |
| Chlorine dioxide two-part drops kit | Treating many litres at home or in a shelter | ClO₂ drops / 2 fl oz |
| Aquatabs 397 mg, 100 pack | Treating a barrel, a bathtub, a rain tank | Chlorine / 100 × 397 mg |
| Water Preserver concentrate, treats 55 gallons | Keeping stored tap water drinkable for years | Chlorine additive / 55 gal |
| Potable Aqua iodine tablets with PA Plus, two bottles | A hiking pack, a short-term backup | Iodine / 2 × 50 |
| Coghlan’s germicidal iodine tablets, 50 count | The lightest, cheapest backup in a bag | Iodine / 50 |
Chlorine dioxide: the kit choice
The only tablet chemistry the CDC credits against Cryptosporidium.
Potable Aqua chlorine dioxide tablets, 30 count
For: The go-bag and the car kit
What the listing says: 30 chlorine dioxide tablets, dosed per quart or litre, effective against bacteria, viruses, Giardia and Cryptosporidium with the stated contact time.
Before you buy: Thirty tablets is thirty litres: about a week for one adult, two days for a family of four. Buy two boxes for a household kit, and read the contact time on the label before you need it — the parasite figure is hours, not the 30 minutes of chlorine.
Chlorine dioxide two-part drops kit
For: Treating many litres at home or in a shelter
What the listing says: Two-part liquid kit (mix to generate chlorine dioxide), 2 fl oz components, for stored reserves, well water and travel.
Before you buy: Two bottles to mix, a wait before use, and dosing that needs a level surface and clean hands: fine in a kitchen, fiddly in a car at night. Best as the home stock, with tablets in the bag.
Chlorine: for stored water and large containers
Aquatabs 397 mg, 100 pack
For: Treating a barrel, a bathtub, a rain tank
What the listing says: 100 individually sealed sodium dichloroisocyanurate (NaDCC) tablets, 397 mg, for larger containers.
Before you buy: The 397 mg tablet is the large-volume size: one tablet treats a big container, not a bottle. Check the litres per tablet on the packet before buying, and remember chlorine does not touch Cryptosporidium — it is for water that was clean when you stored it.
Water Preserver concentrate, treats 55 gallons
For: Keeping stored tap water drinkable for years
What the listing says: Liquid additive for 55 gallons of clean stored water, to inhibit microbial growth in sealed containers.
Before you buy: This is a preservative for clean water going into a sealed drum, not a treatment for dirty water. Used as directed it saves you rotating the drum every six months; it will not make flood water safe.
Iodine: light, cheap, with limits
Potable Aqua iodine tablets with PA Plus, two bottles
For: A hiking pack, a short-term backup
What the listing says: Two 50-count bottles: iodine (tetraglycine hydroperiodide) tablets plus PA Plus taste-neutraliser tablets.
Before you buy: The CDC’s limits apply: not for pregnancy or thyroid conditions, not for more than a few weeks, and no effect on Cryptosporidium. Once the bottle is opened the tablets have a short life; write the date on it.
Coghlan’s germicidal iodine tablets, 50 count
For: The lightest, cheapest backup in a bag
What the listing says: 50 iodine tablets in a small bottle, dosed per quart.
Before you buy: Same chemistry and the same limits as the Potable Aqua, without the taste-neutraliser. Keep it as the last-resort layer behind a filter, not as the family’s main treatment.
The bleach alternative, from the CDC and EPA
If the tablets run out, unscented household bleach does the chlorine job: the EPA and CDC figure is 8 drops of 6% bleach per gallon (6 drops of 8.25%), stirred, left 30 minutes, doubled if the water is cloudy or very cold. It has the same limit as chlorine tablets — no effect on Cryptosporidium — and bleach itself loses strength after about a year on the shelf. A dropper in the kit is worth more than a fourth box of tablets.
Tablets and filters together
The CDC’s sequence for untreated surface water is filter first, then disinfect: a filter with an absolute pore size of 1 micron or smaller removes the parasites that chlorine and iodine miss, and the chemical then kills the viruses the filter lets through. Our portable filter comparison and gravity filter review cover the first half of that pair. For the quantity to store in the first place — a gallon per person per day — use the supply calculator.
FAQ
Which tablets kill Cryptosporidium?
Per the CDC, chlorine dioxide tablets do, when the label instructions and contact time are followed. Iodine and chlorine tablets do not; boiling for one minute kills everything.
How many tablets does a family need?
One tablet per quart or litre for most products. A family of four drinking two litres each a day uses eight tablets a day, so a 30-count box is under four days; two boxes plus a bottle of bleach and a dropper is a realistic two-week reserve.
Do purification tablets expire?
Yes. Sealed tablets are typically rated for several years; opened iodine bottles degrade quickly. Write the purchase date on each pack and check the label’s shelf life when you rotate the kit.
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Official sources & further reading
The guidance in this article comes from official sources. Consult them directly for the authoritative version:
Dan Lockland is a preparedness instructor and survival skills educator with over 15 years of hands-on experience. He shares practical, no-nonsense guidance on emergency preparedness, self-reliance, and sustainable living at PreparingWithDan.com.