How often do we hear "I
think there's a problem -> Invoke the Precautionary Principle and
Stop it Now!!!" ?
It doesn't work that way.
The Precautionary Principle in fact is a well-grounded legalistic tool, to
the extent that in Europe at least one does not invoke it on a
case-by-case basis but rather must always use it. Using it, though,
doesn't mean you always have to take action, and that's an important
distinction.
The PP (let's call it that from now on because it's easier to type) really
is just common sense. It's rooted in what we mean by "proof"; commonly we
use a 95% confidence interval in science, which means we have to be pretty
sure of something before we consider it "proven". Now, that's fine if
we're looking for gravitational waves from coalescing black holes (though
for the Higgs boson they used 5-sigma) but it's not really appropriate for
environmental agents, where you'd want to be a bit more, well, cautious.
There's quite a good analogy in law, where for a criminal conviction we
look for "proven beyond reasonable doubt" but in a civil case it's
"balance-of-probabilities" or "more-likely-then-not".
So for the PP, you would consider taking action if it looks more likely
than not that inaction could lead to significant harm even if there's not
proof beyond reasonable doubt, or proof at 95% scientific confidence
levels or whatever. One issue with that is that "balance of probabilities"
is something that shifts with evidence; if you're pouring stuff in a lake
that you know is toxic to fish, you shouldn't wait until you start seeing
dead fish to decide it's more likely than not that you're killing them.
That would be a reasonable assumption based on what you know about the
stuff you're pouring in. It's actually pretty likely that if you pour
enough of it in, fish will die. You really hardly need the PP to tell you
not to do it. Where it really comes into play properly is when you either
don't know anything much (new agent on the block) or you know enough to
know that something is probably (but not definitely) a problem. The second
of those is the easier case to start to deal with: it's time to consider
action. The first is actually quite difficult, because if you don't know
anything at all, what's the balance of probabilities? It then usually
comes down to a pragmatic decision about outcomes. Which leads to the
second misunderstanding about the PP.
The PP is not a blank cheque to take action to ban an agent or prevent an
activity. If the balance of probabilities suggests that it's time to do
something, the something you do is take a look at what's an appropriate
action to take. That means balancing the costs and benefits of any action
against the costs (and potential benefits) of doing nothing. By "costs"
here we don't really mean financial costs (though that may also be a
factor) but more any societal or personal detriment accruing from action
or inaction. That's quite hard to appreciate unless you see a concrete
example, so here is one.
Across the years, and across many epidemiological studies, there has been
an association between magnetic field exposure above 0.4 uT and an
increased risk of childhood leukaemia. It's not established as causative,
it's an association, but it seems quite robust. Largely as a result of
this finding, IARC assigned a 2B category ("possible carcinogen") to power
frequency fields in 2002. Now "possible" doesn't meet the PP threshold of
more-likely-than-not, so one wouldn't expect a public health-driven change
to the way we distribute or use electricity. Nevertheless, it makes for an
interesting case study in "what if"; In the UK the SAGE process considered
what would be appropriate to do in terms of remedial action if one took as
axiomatic a doubling of risk of childhood leukaemia as indicated in the
epidemiological studies. The answer was "almost nothing". Some obvious
measures like not routing cables over schools where an equal-cost
alternative is available, and arranging the phases of conductor bundles to
minimise fields emerged as no-brainers, but actions such as moving
existing lines or relocating people who lived under them turned out to be
too hard to justify, not just in terms of financial cost but in terms of
lives: people are injured and die (statistically) when major engineering
works take place.