Book Review: Silent Spring Part 2

(See for the first part of this review)

Listen to one of the many stories detailed in Silent Spring.  It started in 1949 at a fishing reserve called  Clear Lake in America.  This was a project undertaken with great care.  The object was to get rid of the gnats that were troubling fishermen.  A close relative of DDT, DDD, was used.  This was thought to be less harmful to the fish.

The levels used were modest by any standards and should have been safe enough.  They started at 1 part per 70 million of water.  It worked well at first, but the gnats started to return  so in 1954 the dose was increased to a still very cautious 1 part per 50 million.

But soon the number of western grebes that lived on the lake started to fall.  The grebes, who dive below the surface of the water to catch their prey, were eating the contaminated fish that had eaten the contaminated insects, concentrating the insecticide in their bodies.  Some dead specimens were collected in 1957 and found to contain 1600 parts per million of DDD.  The concentration in the birds was around 8 million times higher than the dose that had been applied.

Even that was not enough to kill many of them, but it was enough to affect their ability to produce offspring.  The grebes soon became a rare sight.

What was going on? DDD is fat soluble, and so tends to accumulate in the fatty tissues of the insects to which it is applied.  The insects are eaten by birds or fish, which accumulate increasing quantities of the pesticide in their own fat, concentrating it as it moves up the food chain.  DDD is stable so it persists in the body for long periods.  Given its low toxicity, creatures can accumulate quite large amounts before it does them any harm.  In fact it is quite likely to do them no harm at all, enabling them to fly or swim around taking their dose of DDT around with them spreading it further afield.

But while they are getting on with living oblivious, the DDD might well still be at high enough level to harm their ability to reproduce.

This pattern was to be repeated time and again.  DDT  and other similar pesticides worked in exactly the same way.  Birds, at the top of the food chain but having a small body mass, would be hit worst.  The Silent Spring of the title was the spring that would come when there would be no bird song because all the birds had died. It is a harrowing image intended to shock.  But the diligent scientist Rachel Carson was able to back up what she was saying with a huge range of solid research.

Lets have a look at what was important.  It wasn’t the high toxicity of the chemical that was the problem. It was the combination of low toxicity, oil solubility and stability.  Also, look how difficult to predict the problem was.  The chemist we heard about earlier who ate the stuff had, we can be sure, done all the calculations to assess the risk of the dose to himself.   That was a simple enough problem and he knew what he was doing.   But he wouldn’t have been able to so easily predict how it would work in the field and would never have guessed the ability of biological systems to concentrate particular agents.  The conscientious project managers of Clear Lake had taken every reasonable precaution but had still created havoc for the wild life in their care.

At first sight this looks like a council of despair.  If things are so complicated shouldn’t we just conclude that there is no such thing as a safe level and ban everything?  But that is to give up too soon.  Mistakes can be learnt from.  For instance, why use such stable molecules?  We now know you have to understand the system as well.   But as our understanding has grown this has become possible.  Everyone who works in the development of chemical products is now very well aware that simply looking at a safety data sheet isn’t enough to be sure that something is safe.

And the converse is also true.   Just because a risk can be identified with a chemical, it is not necessarily going to actually be harmful for a particular use.  It is often stated as a fact that the Romans suffered from lead poisoning because they used lead water pipes.  I have seen that given as a reason for the collapse of the empire.  Lead is certainly toxic and does leach into water from lead pipes.  But in fact the water also contained calcium which coated the lead in the pipes making it harmless.  The Romans would probably have been highly offended at the way we talk about them.   They were well aware of lead poisoning – they mined and processed the stuff after all – and would certainly have worked out that lead pipes were a problem if they had actually been.  It makes a useful warning about assuming that we know best.  It’s always fun to think other people are ignorant and stupid, but it is rarely justified.

We only have the one planet so it is really important that we treat it well, and that is as good a reason as I can think of to take an interest in environmental issues.  But I am always finding that whenever you study an issue that comes up, you learn a great deal beyond the bare facts of the case.  I often learn some more about chemistry or biology, but also about human nature and myself.  In many ways, the way a person or a society or a species responds to its problems defines what it is.

Part 3 of my review of Silent Spring by Rachel Carson will be out on the 10th of February

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