Giardia is a microscopic parasite that may or may not be present in any water you drink from natural sources.

Giardia Cyst Giardia parasites are protected by an outer layer called a 'cyst.'  This shell allows giardia to exist outside of the body for relatively long periods of time, and also makes the parasite resistant to chemical water treatment (such as chlorine).  Research has shown that cysts can live up to three months in water.

Ingestion of giardia cysts may result in a disease called giardiasis, commonly known as 'beaver fever.'  The disease may not manifest itself for 2 - 10 days, but when it does, it's not a pleasant experience.  Symptoms include nausea, weight loss, fever and diarrhea.

For the most part, symptoms last a few days, and within a month or two the body can usually rid itself of the parasite using its own natural immune systems.  For individuals with weakened immune systems, the disease can last much longer and medical treatment may be required.

Purifying Water

The oldest, simplest and cheapest method for purification is by boiling your drinking water.  By the time water reaches a boil, nearly all pathogens will be killed, but the generally accepted guideline is a rolling boil for five minutes.  

Chemical treatments such as chlorine or iodine can also be used to purify water, but caution must be taken with the process.  For a start, water should be at a temperature of 25°C (75°F), which is a much higher temperature than is found in most lake and river water in Canada. To be safe, water should sit until it reaches that temperature (possibly overnight).  Chlorine is an 'iffy' choice - iodine is a much better option, although it has the disadvantage of making the water taste unpleasant.  The addition of ascorbic acid (vitamin C) will neutralize this taste. 

Prolonged use of iodine can cause thyroid problems, so it should only be used occasionally or as a short-term solution.  Pregnant women or people with thyroid problems should not use iodine treatments.

It should be recognized that boiling is a far better option than either chlorine or iodine for water treatment.  If boiling is not practical or possible, the following guidelines should be used for chemical disinfection:


Regular bleach (for laundry use) usually has about 4% to 6% available chlorine. Check the chlorine content before using, and add according to the following guidelines:

1% chlorine (10-20 drops / litre)
4% to 6% chlorine (2-4 drops / litre)
7% to 10% chlorine (1-2 drops / litre)

Water should stand after treatment for 30 - 60 minutes.  If water is murky or very cold, the treatment time should be lengthened to several hours over even overnight.


Regular tincture of iodine (first aid) can be used. A 2% iodine solution should be added to water at a rate of 5-10 drops per litre.  If iodine tablets are used, follow the directions on the bottle.  Remember that iodine tablets are not stable, and the effectiveness will begin to deteriorate after the bottle has been unsealed.

As with chlorine, the water should stand after treatment for 30-60 minutes, or longer if water is turbid or cold.

Water Filters

Water Filters will efficiently remove giardia and suspended solids from water.  They will not remove viruses unless they are also classed as purifiers, which generally use an iodine-impregnated resin to kill the viruses.

Detailed information on water filters can be found on the water filter page in the equipment section of this site.  

Water Treatment - What Works, What Doesn't

  Boiling Water Filter Water Purifier Chlorine Iodine
Giardia good good good fair fair
Crypto good good good fair fair
Viruses good poor good good good

The Bottom Line

Giardia is a reasonably common contaminant, and cryptosporidum less so.  The odds of encountering a virus in North American waters are very low.
Although there are never guarantees, most people use a water filter only and never experience a problem.  If you're fanatical (or worried) about treatment, then boil your water.  It's the simplest and most effective treatment method available.