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Although this essay is devoted to solvents, much of the information
may be extrapolated to other organic halogenated substances, such as
refrigerants, fire control compounds, aerosol propellants, anaesthetics
etc.
There are a number of types and classes of halogenated solvents.
Practically all of them have some environmental and health and safety
issues. However, there are massive quantities of misinformation, even
disinformation, about them. It is hoped that this essay will help place the
church back into the centre of the village. It is not the intention to
discuss individual solvents in great detail, nor to indicate their
applications. They are most often used for cleaning mechanical parts and
garments and as carrier solvents for coatings and adhesives. In order to
simplify the page structure, I'll classify the solvents initially by their
environmental effects and secondly by their health and safety effects.
Ozone depletion
Many solvents are ozone depleting, in varying degrees, measured by their
Ozone Depleting Potential (ODP) which is an index related to the effect that
the simplest chlorofluorocarbon (CFC) gas, CFC-11, which is given as an ODP
of 1. The most popular CFC solvent, CFC-113 has an ODP of 0.8. This was
popular for much precision and electronics cleaning before it became a
controlled substance under the Montreal Protocol, along with all the other
CFC solvents.
Another class of controlled substance is the hydrochlorofluorocarbon
(HCFC) type. Generally these have ODPs between about 0.03 and 0.1. The
Montreal Protocol gives them a longer phase-out time, but some countries
have already eliminated or substantially reduced their use. The two main
HCFC solvents are HCFC-141b, with a high ODP of 0.1 and the isomers of
HCFC-225, with a relatively low ODP.
Chlorocarbon solvents are generally considered as non-ozone-depleting
with the exception of 1,1,1-trichloroethane, which has an ODP of 0.1 and
tetrachloromethane, more usually known as carbon tetrachloride (CTC), with a
massive ODP of 1.3.
The other non-ozone-depleting ones, exempt from control under the Montreal
Protocol, are trichloroethylene (TCE), popular for metal degreasing, perchloroethylene (PCE), frequently used for dry cleaning garments and
methylene chloride or dichloromethane (DCM), often used for adhesives or in
paint strippers. These are all toxic (see below) but are widely used.
Only one bromocarbon solvent has been considered useful in recent
years. This is 1-bromopropane, also known as n-propyl bromide (nPB).
This substance has become very controversial because, although it
decomposes in the atmosphere fairly rapidly, it has a relatively high
ODP and is apparently toxic (see below). It has very similar physical
characteristics to TCE and, because it was initially believed to be less
toxic than TCE, gained a certain popularity. The big stumbling block is
the fact that it has never been introduced into the Montreal Protocol as
a controlled substance. The reason for this may seem obscure but it is
because the Protocol's Scientific Assessment Panel has not been able to
reliably assess it's ozone depletion potential because of its short
atmospheric residency time. Unfortunately, the standard models for such
an assessment do not work because the ODP is related to the
meteorological conditions, and subsequently to the latitude of
emissions. As far as can be ascertained, the ODP is very low at higher
latitudes, rising to about 0.1 in the tropics. Nevertheless, this range
of ODP is within the range of controlled substances and it would seem
anomalous that nPB has not been included as a controlled substance into
the Protocol. It is possible that the lack of a decision by the Parties
to the Protocol may be at least partially political.
Bromochloromethane is a solvent that had a brief moment of glory in
the mid-1990s but its high ODP attracted the attention of the Parties to
the Montreal Protocol and it was quickly introduced as a controlled
substance.
Perfluorocarbons (PFCs), hydrofluorocarbons (HFCs) and
hydrofluoroethers (HFEs) are not ozone-depleting. They are therefore not
subject to any control by the Montreal Protocol.
Climate change
Nearly all halogenated solvents have a Global Warming Potential (GWP)
greater than that of carbon dioxide. In particular, CFCs -- controlled
by the Montreal Protocol -- have high values but PFCs, HFCs and HFEs are
particularly bad.
PFCs are rarely used by themselves as a solvent but they have
atmospheric residency times of the order of thousands of years and GWPs
often exceeding 10,000 times that of carbon dioxide; they are amongst
the worst substances for instigating climate change. On the other hand,
there are a number of HFCs which are used as solvents and these have
GWPs usually between 300 and 1500. As a general rule, HFE solvents have
slightly lower GWPs than the HFC solvents.
PFCs and HFCs are controlled substances under the Kyoto Protocol.
Health and Safety
Other than for pure PFCs and HFCs, most halogenated solvents are
quite toxic to very toxic, even suspected carcinogens in some cases. As
a rough guide, brominated solvents are more toxic than chlorinated
solvents which are more toxic than fluorinated solvents. However, many
commercial solvents are not pure substances but are blends, often with
other toxic solvents such as methanol. Even HFC and HFE blends may
contain chlorinated solvents which are much more toxic than the major
product.
The acute toxicity of a single dose of most solvent vapours is not high
under normal conditions. Nevertheless, heavy doses of some halogenated
solvents have been known to cause cardiac arrest. It should be
remembered that these substances are quite closely related to surgical
anaesthetics. Other than that, many deaths have been caused by
suffocation while cleaning equipment where the heavy vapours have not
been evacuated, displacing air. When cleaning equipment which has
contained any halogenated solvent, extreme precautions should be imposed
to ensure the safety of the personnel. In particular, large closed tanks
should be cleaned only with surveillance by a second person outside,
capable of pulling up an unconscious person by a safety rope.
Chlorinated solvents, such as TCE, PCE and DCM, are all toxic to some
degree to the liver. Operating personnel should never be exposed to
solvent vapours exceeding the Operator Exposure Level imposed by the
national authorities of the country concerned. Unfortunately, a
considerable amount of disinformation has been propagated about the
carcinogenicity of these solvents. Because they are known to cause
cancerous tumours in laboratory animals, scaremongering has been rife,
extrapolating these results to humans. It has been shown that this is
scientifically invalid because the manner in which the solvents are
metabolised is entirely different between rodents and humans. Very
important epidemiological tests, involving cohorts of thousands of persons exposed
to the solvents over many years, have never proved that these substances
are carcinogenic. The solvents have been used industrially for over 100
years, often under uncontrolled conditions with bad exposure levels. It
can be stated fairly categorically that if the operator exposure levels
are respected, the risk of contracting any cancer is negligible. The two
ozone-depleting chlorocarbon solvents have very different toxicity
ratings. Carbon tetrachloride is very toxic and has been banned from use
as a solvent since the 1950s in most western countries. On the other
hand, it is still used in some developing countries, despite the danger
of doing so. On the other hand, 1,1,1-trichloroethane has a lower
toxicity than most other chlorinated solvents.
Probably the most controversial of the halogenated solvents, from the
point of view of health and safety, is n-propyl bromide. This is because
it has not been used industrially for more than 10 years in any great
quantity and we do not have sufficient experience to provide valid
epidemiological studies. Animal tests, which are not necessarily able to
be extrapolated to humans, have shown that the solvent attacks both the
reproductive and central nervous systems. The little experience that we
do have would seem to indicate similar effects in humans subjected to
heavy exposures. Anecdotal evidence has shown that there is a great risk
of absorption through the skin, potentially causing neuropathic lesions.
Until we gain more knowledge about the effect of nPB on humans, it would
seem wise to reduce the exposure to a minimum, or not to use it at all.
This lack of knowledge is shown by different manufacturers recommending
time weighted average operator exposure levels as low as 5 ppm and as
high as 100 ppm. Some authorities have made recommendations of the 10
ppm and 25 ppm but none of these values are binding upon users, because
national permitted exposure levels have not been promulgated. The use of
the solvent is forbidden in a few places.
All halogenated solvents are classed as volatile organic compounds by
the EU and other authorities. It is therefore possible that they may
contribute to the formation of tropospheric ozone and smog, both of
which have negative impacts on the health of the population at large. It
is therefore necessary to ensure that their use is such that emissions
are reduced to an absolute minimum, no matter what other effects they
may have on the environment or on the people using them.
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