Iodine was the standard for several decades, and is still widely used today. NOLS, until recently, used iodine for appox. 40 years with their students. Many of the old timers who have used iodine for many years have built up an immunity and I constantly hear their arguments that they've never gotten sick from using it. However, if your system isn't used to iodine, it could potentially cause problems, especially if used over the course of 2 weeks or more. Much of the backcountry water that has little human influence can be treated successfully using iodine. It does work against low concentrations of giardia, but isn't as effective as chorine dioxide.

It is true that iodine will degrade over time. Potable Aqua, considered the standard for iodine tablets, recommends that you not use a year-old open bottle. If you have an unopened bottle, it should be good for 3-4 years.

Chlorine Dioxide does have a shelf life of about 5 years, although it should be effective even after this time period. The makers of the tablets recommend that you replace after 5 years on the shelf.

I feel I need to plug the MIOX here because it doesn't have a shelf life (other than the lithium battery life which is approx. 10 years). Every time you operate it, you make a fresh chemical. It is also a "mixed oxidant solution" meaning it has more than just chlorine dioxide in the resultant chemical (hypochloris acid, hydrogen peroxide, chorine dioxide, ozone, sodium hypochoride, chlorine, etc.). These are known as "radical" oxidants because they have a very short half-life. After the chemical is made it can be used only for about 9 days until it is ineffective. The best time to use the solution is immediately after it is made. The fresher the chemical, the more potent it is.

Chlorine Dioxide tablets are very potent because they need to be in order so that they will be effective after several years. Because of this, they are potentially harmful if swallowed. The EPA mandates that the tablets have a child-proof container due to the danger they pose to children (the wrapper must be cut with scissors or a knife). However, the MIOX solution is completely safe and is non-toxic. In fact you can use it as a topical solution if a person experiences a wound or a severe cut. If you ingest the raw solution, it will taste nasty, but will pose no risk. Both iodine and chlorine dioxide can be poisonous and even lethal to some people. Always treat them as poisonous meaning if someone swallows an iodine or chlorine dioxide pill that they immediately begin to consume fluids and they be taken to a doctor if possible. It is highly unlikely that someone will die from ingesting a pill (i.e. a Potable Aqua has 1/10 of the lethal dose of iodine per bottle for a normal healthy adult; Polar Pure Iodine Crystals does have a lethal dose per bottle), but it can make a person fill ill and be harmful. I found this on the chlorine dioxide package: CAUTION: Corrosive. irreversible eye damage and skin burns. Harmful if absorbed through skin. Harmful if swallowed. This, when comparing the different chemical disinfectants, and in the case of really bad water not knowing the amount of chemical added is sufficient to kill the bugs is chlorine dioxide's only major disadvantages (MIOX uses test strips in the case of very bad water to tell you if you've added enough chemical).