182 Applications and Occurrences of Heterocycles in Everyday Life
Polybenzoxazoles (e.g. Zylon-PBO) form very strong lightweight fi bres, used, for example, in racing yacht rigging, but
a disadvantage is that this polymer degrades in light and therefore must have a protective coating. M5 fi bre has very
high strength and heat resistance, and is being developed for use in body armour.
Pesticides
Pesticides are designed to kill living organisms that are harmful to human interests. The ‘pests’ are mainly insects,
weeds, fungi and animals (particularly rats). Many of these pesticides are restricted to agricultural use under close
control, although a few are available for use by domestic consumers, and some others in professional public health
applications.
These substances are essential to modern agriculture and very useful in general life, but will always be controversial,
particularly the herbicides and insecticides, because of their ability to damage benefi cial organisms. There is also the
potential to leave trace residues in foods, which have been claimed to have adverse health effects in humans.
There is a certain amount of confusion, and constant change, in the usage of pesticides in various regions of the world,
with some widely used compounds being banned or severely restricted in certain countries, quite often in the EU and
the USA.
Insecticides
Most insecticides for domestic use are (non-heterocyclic) pyrethroids, which have very low toxicity to humans,
although some of those used by professionals for use in public hygiene situations are heterocycles. A much larger range,
including many heterocycles, is used in agriculture under rigorously controlled circumstances, to avoid adverse effects,
for example on pollinating insects such as bees.
The cholinergic system, which is similar to that in humans, is one of the main targets. Nicotine was widely used as
an insecticide, but is extremely toxic to humans (similar to hydrogen cyanide – lethal dose about 60 mg) and readily
absorbed through the skin. A number of compounds – ‘neonicotinoid insecticides’ – have been developed that have the
same mechanism of toxicity (acetylcholine agonists) but are much more selective for insects. Not surprisingly, many
are 3-substituted pyridines (e.g. imidacloprid), together with some thiazoles (e.g. thiamethoxam).
Other substances are choline esterase inhibitors, either reversible, such as the carbamates, for example pirimicarb, or irre-
versible (mainly organophosphates). A number of organophosphates have been modifi ed by the addition of heterocyclic