Water is an extremely complex element. There was a time when I had a full file drawer of test data from water purification device manufacturers, all supposedly written by scientists, and all full of contradictory data. Unfortunately, each manufacturer of a water purification device had its own paid scientists to prove that its product was the best.  
But I was constantly seeking “the bottom line.” What product is, objectively, “the best?”  It wasn’t an easy answer to find. In part, this is because of the enormous complexity of water and the various components that contaminate water under different circumstances. 
Plus, to the best of my knowledge, no company, agency or government had ever taken all the water purification devices and tested them all with control water which contained measurable amounts of known pathogens. Such a test would be time-consuming, expensive and perhaps still leave many questions unanswered. 
Even though there are countless variables, here is my 25-cent version of water purification that I have taught my students for nearly 30 years.
Distillation is the only absolute method of purification. Boiling kills everything that can make us sick, so it’s the best field method of purification.
If you can’t boil, all the commercial filters that meet the federal guidelines of blocking everything larger that .02 microns is fine.
Two of the halogen class of chemicals (chlorine and iodine) can be used, but each has its pros and cons. Tincture of iodine, for example, has a shelf life of about two years. But if you have iodine crystals in a glass jar with a bit of water, the solution will be viable as long as there are crystals in the water.
Household bleach (2 percent chlorine) is ineffective with seriously polluted water. It is useful for retarding the growth of algae in water you intend to store, however.
That’s the outline of “Water Purification 101,” how to purify water in a nutshell. But there is so much more to the subject. Even some of what I thought was true in my 25-cent synopsis is not true!
Most biologists and hydrologists agree that there is no way you can determine whether water is safe to drink simply by looking at it.  
How can the average person, backpacking in a wilderness area or in need of water after a major earthquake or tsunami, decide which water to drink and how to purify water if necessary?
Most health and wilderness experts worldwide say you should always assume that open water sources are unsafe to drink, unless you find out otherwise. This very conservative viewpoint does not mean that all open sources of water are polluted; it’s simply solid advice to avoid getting extremely sick from bad water. In most areas, water is tested by hydrologists or biologists, and if you do your homework before entering a new wilderness area, you’ll have good information about the water’s purity.
Even when the water in an area is believed to be pure, it’s important to use common sense. Make sure there’s no dead animal upstream, and always be observant for sloppy campers. In some heavily used campgrounds, lazy or ignorant campers toss garbage, baby diapers and old food into streams.  
OK, so the water’s polluted. How do we purify it?
Boiling is generally considered the best way to purify water from biological contaminants. Nearly every water expert I spoke with agreed on this point. 
Anton van Leeuwenhoek (1632-1723) discovered that some organisms do survive boiling. Fortunately, the majority of organisms that cause sickness in humans are killed by boiling. It turns out, however, that typhoid spores survive this process. Normally, you’ll only encounter typhoid in the water after a hurricane, or a major earthquake, when sewage water mixes with the drinking water. Though somewhat rare today, it’s still possible to get typhoid spores in open sources of water that would survive boiling. 
So how long would it take to actually kill those spores? This has to do with such factors as temperature, exposure to sunlight, the amount of spores in the water and whether you did any of the primary or secondary levels of purification. In areas with heavily polluted water, up to 20 minutes may be required to break down those spore casings. Even if you consumed some, they do degrade with time. Whether you get sick depends on your state of health.
When water is suspected of being impure, experts suggests a three-step process of purification.  
First, filter open sources of water through something like cotton to remove all larger particulate matter.
Then, let the water settle, then siphon out the clear water.
Third, if you cannot boil water, chemicals can be used.
According to Talal Bala’a, whom I interviewed for my book, one should follow the two “Rules of Three.” There is the Rule of Three for determining relative purity, and there is the Rule of Three for the process of purification. 
“Always begin with an obvious common-sense observation of the water,” states Bala’a. “Begin with three simple observations. 1. Does it look good? Is the water cloudy? Are there things in the water? 2. Does it smell good? Do you detect the odor of chemicals? Do you detect the odor of rotten material? And lastly, does it taste alright? Taste some with your tongue. Is there any astringency? Do you detect anything unpleasant? 
Next, if you suspect the water is impure, follow the three fundamental steps of water purification already listed above.  
“If you seriously doubt the water’s purity,” adds Bala’a, “you should follow the three-step process of 1. Filtering the water through a cloth, 2. Allowing the sediments in the water to settle, and then 3. Boiling the water or using chemicals. If anything is left, your body’s immune system should protect you, if it can.”
There is much more to say about water purification, from chemicals to water filtration devices to rain collection. Get my book, “How to Survive Anywhere,” from Amazon, your local bookstore or my Web site. 
Christopher Nyerges is the author of books on the outdoors, including “How to Survive Anywhere.” He does a weekly podcast for Preparedness Radio Network, and he blogs at ChristopherNyerges.com.  A schedule of his classes at the School of Self-Reliance is available by writing to PO Box 41834, Eagle Rock, Calif., 90041.