Safety is a relative term. This is true for most activities in our lives.
Personally, I never gave crawling around in uncharted caves a second thought, but I wouldn't ever go sky diving. Both high risk activities, but to my mind, one is safer than the other.
Safety with regard to the chemicals in our environment is also a relative term. Breathe enough oxygen and you will die. Long term exposure to oxygen does lead to death. Of course, the absence of oxygen is guaranteed to kill you much quicker!
This is an extreme and somewhat absurd example, but it does highlight the point that whether or not we deem a chemical compound as "safe" is a matter of perspective. Whether or not a chemical compound is "safe" depends upon such factors as dose, duration and personal genetics.
Large doses of any chemical compound can kill, even large doses of oxygen. The same can be said about water or food - which is just a conglomerate of chemical compounds - or pretty much everything we encounter in our daily lives. And the absence of chemical compounds can just as easily kill.
But to use a more familiar example of a poisonous chemical, consider arsenic. A large dose is almost always fatal. Remember the movie Arsenic and Old Lace with the little old ladies steadily poisoning their boarder? Certainly, arsenic is a poison that should be avoided at all costs in large doses. Fortunately, this is fairly easy to do.
However, research would suggest that even small doses of arsenic should be considered unsafe. A study published by the National Research Council in the U.S. points out even at very low concentrations, arsenic in drinking water appears to be associated with a higher occurrence of cancer, specifically bladder and lung cancer.
In this instance, duration is the important factor in determining toxicity. Accumulation of small doses of arsenic over a long period of time can result in a significant health risk, just as a massive dose over a short time can prove fatal.
How small is a small dose? The report estimates at three parts per billion (about a teaspoon in a million litres) long term daily exposure results in a 1-in-1,000 chance of developing bladder or lung cancer. That is, one person in a population of one thousand will develop cancer as a result of the arsenic in their drinking water.
At 10 parts per billion, the risk is more than 3-in-1,000 and at 20 parts per billion, it is approximately 7-in-1,000. These may not sound like very high numbers but they do represent a significant risk well above, say, the risk of being shot to death.
Arsenic in drinking water represents both an acute and a long term health hazard. It is a compound for which there is no safe dose. A large dose kills quickly. A small dose can kill in time.
This means arsenic is not a "threshold compound." There is no acceptable lower limit for arsenic concentrations in drinking water below which it is safe to consume.
This makes legislating exposure limits much more problematic. Particularly as arsenic is certainly in the drinking water in many, if not most, localities around the country and the world, as it occurs in many natural soils and minerals. The World Health Organization has set a limit of 10 parts per billion as acceptable given the fact arsenic is unavoidable.
It is important to remember that arsenic is one of the naturally occurring elements. It is impossible to eliminate it from our environment. It is difficult to ensure "zero" exposure. Indeed, because of the detection limits of our instrumentation, it is presently impossible to even determine if someone has "zero" exposure. All WHO and governments can do is apply upper limits.
Still, with our focus on drinking water safety, questions abound. Is the water system safe? Who should decide what are acceptable limits for the compounds present in water? Who should pay for the testing? Who should do it?
I would suggest, though, that a much more fundamental question is - can our water ever truly be safe? Can we ever be sure that every possible contaminant has been removed? The answer is likely "no."
Even getting contamination levels below detectable limits will not ensure long term exposure to a contaminant will not cause an increased incidence of death.
Thorough and comprehensive water testing is the only way to ensure some level of safety. And even then, it is only a relative measure.
