420 22 · ENVIRONMENTAL CHEMISTRY
DDT was discovered in 1939 and hailed as a miraculous insecticide. It was widely
used during the Second World War, and afterwards, to combat body lice that carry
typhoid, malaria-carrying mosquitoes and pests attacking cotton and food crops.
DDT was so effective that it began to be overused and, because it does not degrade
easily, its concentration in the environment rose quickly. Bird populations in the
United States began to decline because it had affected their ability to reproduce
efficiently and legal measures were introduced to curb the use of the insecticide. Small
doses of DDT are not immediately harmful to humans, but recent research suggests
that DDT may increase the incidence of breast cancer. DDT is still in use in developing
countries.- Herbicides
Herbicides are used to kill plants. Sodium chlorate, NaClO 3 and sodium arsenite,
Na 3 AsO 3 , were commonly used as weed killers in the first half of the twentieth cen-
tury, but inorganic arsenic compounds, in particular, are toxic to mammals. Organic
herbicides are now used. They are much more toxic to certain types of plants than to
others, so they can be used as selectiveweedkillers.Atrazine, which is a member of a
class of herbicides called the triazines, is widely used to kill weeds in cornfields. The
triazines contain six-membered rings with alternating carbon and nitrogen atoms.
The effects of atrazine on human health are not yet clear.BOX 22.5
Toxicity of mercury
The uses of the element mercury are
many, mainly because it is liquid at room
temperature and can be used as an
electrical conductor in a variety of
electrical switches. When given energy,
mercury atoms emit light in the visible
region and can be used for lighting. The
vapour of mercury, however, is highly
toxic and when breathed it damages the
central nervous system. Since the metal
is very volatile, it should be handled only
in a well ventilated area. Liquid mercury
is not particularly toxic and can be
ingested and excreted (for example if you
accidently break the bulb of a
thermometer in your mouth andswallow the mercury) without too much
danger.The Romans mined the mercury ore
mercury sulfide, HgS or cinnibar, and
roasted the ore in air to obtain the
metal, by condensing the mercury
vapour given off. Presumably because
of the high toxicity of the vapour, the life
expectancy of the slaves that worked on
the process was about six months!
Mercury forms solutions or alloys with
many different metals, called amalgams.
The dental amalgam used to fill cavities
in teeth contains mercury, silver and tin.
During chewing, it has been suggestedthat a tiny amount of mercury is vapor-
ized and may cause a long-term health
hazard. The matter has not been defini-
tively resolved, but alternative mercury-
free fillings are being developed.
Methylmercury derivatives are the most
toxic form of the element. Because they
are organometallics they will dissolve in
fatty tissue in animals and accumulate.
Most of the mercury present in humans
is in the form of methylmercury, formed
in rivers by microorganisms which
convert Hg^2 into CH 3 Hg+and
(CH 3 ) 2 Hg. Fish absorb methylmercury
through their gills and humans eat the
fish.