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Alcohol deserves a special section all to
itself,
and this is because of all the molecules,
all the
poisons, that have to be dealt with by
forensic scientists,
by far it's alcohol that generates the
most work.
Alcohol is an interesting and
controversial molecule.
For some societies, for some cultures,
alcohol is
an essential lubricant that keeps the
wheels turning.
And yet in other cultures, it's an evil
molecule which is reviled and must never
be consumed.
So, this is a famous picture.
This is a picture titled Gin Lane, which
was produced by the 18th century artist
Hogarth,
and it's intended to show the evils
of alcohol. Well actually, more
specifically, it
shows the evils of the very strong spirit,
gin, because at that time in England,
gin consumption was enormous. Well,
Hogarth
actually wasn't opposed to alcohol itself.
He just felt the consumption of gin was
very harmful.
And this picture is one of a pair, the
other picture is called Beer Street.
Beer Street, beer contains much less
alcohol
and therefore is much less damaging than
gin,
and Hogarth and his friends wanted to
encourage
people to switch from one to the other.
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Now, what happens when someone drinks
alcohol?
Well of course, the liquid goes down into
the stomach.
Some of it is absorbed by the stomach, and
the bulk of it is absorbed in the small
intestine.
It's absorbed very fast, it will appear in
the blood almost immediately,
and it will be almost completely absorbed
into the body within one hour.
There is some variation of the rate of
absorption, depending on the kind of
drink and what the stomach contents are,
but it's not a particularly big
difference.
Once absorbed into the blood, alcohol will
be
distributed throughout the body via the
cardiovascular system.
And it will, of course, penetrate into
the brain, the blood-brain barrier doesn't
stop it.
And the effect of alcohol is actually to
depress the Central Nervous System.
Now, this seems a bit illogical, because
normally when
people drink some alcohol, they don't seem
to be depressed.
This is because its not depressing the
person, its depressing the nervous system.
Part of that depression of the nervous
system is the depression of
inhibition, so that makes people feel
happier, but it doesn't just depress
inhibitions.
It depresses the ability to make
judgements,
it depresses the ability to react to
situations.
it depresses reflex times and this is why
alcohol, when combined with
machineries such as motor vehicles, can be
a very, very dangerous combination.
Because of this danger, pretty much all
countries around the world limit the
amount of
alcohol that can be present in a person's
body while they are driving a motor
vehicle.
Now, the question is how much is too much?
What concentration of alcohol is there
going to
be in the blood after someone takes a
drink?
Because
you don't know what's in your blood,
all you know is what is in the glass in front
of you.
Well, we can calculate the likely blood
alcohol
concentration from the amount of alcohol
taken in.
So, suppose someone drinks something that
contains a certain amount of alcohol.
So let's say the amount of alcohol in
grams is a.
This amount of alcohol is then going to
be distributed over the person's body,
so, divide by the person's weight in
kilograms, which is p.
However, as we shall see, alcohol is not
distributed uniformly.
So we have to allow for the fact that
it's only spread over a certain proportion
of the body,
and so we have to put in a fudge factor,
and this is called the Widmark factor.
And there are different Widmark factors
for men and women.
So r, the Widmark factor, is 0.68 for men
and 0.55 for women.
And this is because Widmark considered
that women's bodies
have a higher proportion of fatty tissue
than men's bodies,
and that is a scientific and not a
personal comment.
So when we use this formula, we can
calculate the maximum concentration
of blood alcohol, c, as being equal to a
divided by pr.
So, let's see how this works.
Suppose you have a 70 kilogram man who
drinks a double whisky.
How high is his blood alcohol
concentration going to be after this?
Well, if whisky is about 43% alcohol
and a single is 25 cubic centimetres,
taking
into account that the density of alcohol
is
0.79 grams per cubic centimetre, then we
can
calculate that he has ingested 17 grams
of
alcohol.
So, if we put 17 grams of alcohol in
a 70 kilogram man, so the Widmark factor
is 0.68,
then we can estimate that the blood
alcohol concentration
is going to be 0.36 grams per litre of
blood.
So, alcohol concentration is usually
expressed
as milligrams per 100 ml of blood.
So when we correct for the units, this
will be 36 milligrams per 100 ml of blood.
Now, what is the effect of alcohol? In
particular,
what is the effect of alcohol on the
brain?
Now, different parts of the brain are not
effected equally,
so the effects vary according to the
amount of alcohol.
So below about 50 milligrams per 100 mil,
for most people there would be no obvious
effect.
As the blood alcohol concentration goes
higher, as the person has more drinks,
then there are noticeable effects, loss of
coordination, and of course slurring of
speech.
Increase the concentration further and
these effects get worse.
Marked loss of coordination, poor sensory
perception and in a lot
of people, starting to feel ill and
wanting to be sick.
Say at 150 to 200 mg per 100 ml.,
now we have drunkenness, nausea, ability,
inability to stand up, and vomiting.
So, even after that, some people will keep
drinking, and this will lead to
coma, problems with the circulatory and
respiratory
system. And at high enough blood
alcohol concentrations,
this can lead to death.
Now, alcohol has always been there in
nature.
For instance, the fermentation of fruits
which can occur naturally
means that alcohol has always been there
as humanity has evolved.
And therefore, our body has a biochemical
mechanism for eliminating alcohol.
In particular, there's an enzyme
called alcohol
dehydrogenase present in the liver and the
small intestine.
And this enzyme, alcohol dehydrogenase,
will convert ethanol
into a higher oxidation state molecule
which is acetaldehyde.
Acetaldehyde is also toxic,
so we have a second enzyme called
aldehyde dehydrogenase,
which will further oxidize that
acetaldehyde to acetic acid.
And acetic acid, that is naturally
present as part of our metabolism,
our body can burn up acetic acid, and it
will
end up as carbon dioxide and be exhaled
from the lungs.
And almost all alcohol that is taken into
the body will end up as carbon dioxide.
A very small amount will be lost through
the kidneys
or breathed out through the lungs in
the form of alcohol, the unchanged ethanol
molecule.
So when you smell alcohol on someone's
breath, it's that
small percentage that doesn't get burned
up by the enzymes.
Now, when the alcohol is in the body, it
is pretty uniformly distributed.
At least it's pretty uniformly
distributed over all parts
of the body where there's a lot of water.
The parts of the body where there isn't
much water do not get alcohol,
that is the bones, the fat and the hair.
And this is the reason why we have the
Widmark factor.
So there's a little difference between
arterial
and veinous blood during the absorption
phase, and that's simply because the
alcohol hasn't been completely distributed
yet.
So in order to know how much alcohol is
in someone's
body, we can measure the amount of alcohol
in any body fluid.
So typically, blood alcohol is measured,
and we can assume that whatever level of
alcohol is in the blood, it will be the
same level inside the brain.
So to measure blood alcohol, a blood
sample has to be taken
and then this can be analyzed by a
technique such as Gas Chromatography
which we discussed in an earlier lecture.
Now, to measure someone's blood alcohol
level, this has to be done by
a medically trained person, it has to be
done in a proper place.
But if a motorist is pulled over
by the traffic police because he's
driving erratically,
how can the traffic police determine
whether this guy's got too much alcohol?
And the technique used is to measure the
level of alcohol in the breath.
Of course, if there's a traffic accident,
and
someone's dead, then you can choose, as I
said,
any of the body fluid you like.
Now,
if the traffic police can measure the
breath alcohol, how can we
relate this to the blood alcohol, which is
actually the important factor?
The two are closely linked and they're
closely linked because of Henry's Law.
And Henry's Law says that if you have a
solution of something, a solution of a
volatile substance in a liquid, then the
vapour
pressure, that is the amount of that
volatile substance
in the the vapour phase above the liquid,
will be proportional with concentration
in the liquid.
So if someone has a lot of alcohol in
their blood,
then there's going to be a lot of alcohol
in their breath.
If they only have a little alcohol in
their blood,
there will only be a little alcohol in
their breath.
And these are strictly proportional, and
that ratio
is typically taken as 2300 : 1. So you can
measure the breath alcohol, multiply it,
and then you have the blood alcohol level.
Now, breath alcohol can be very easily
measured, as we've
said, at the roadside using these kind of
breathalyzer devices.
And typically, these breathalyzer devices
rely
on a little bit of chromium chemistry.
Potassium dichromate is a very, very nice
bright orange crystalline substance.
Very nice colour indeed.
But if you take potassium dichromate and
you have some acid present, and then you
add alcohol, so the suspect motorist
breathes into the device,
the alcohol in his breath will react
with the potassium dichromate. And the
chromium is
reduced and ends up as chromium sulfate,
whereas
the ethanol is oxidized to acetic acid.
Now, chromium sulphate is green, it's
a quite different colour to potassium
dichromate.
And we can simply measure the amount of
potassium dichromate that is converted
to chromium sulphate by measuring the loss
of absorbance at 420 nanometres.
Well, it's not always as simple as
just measuring the breath or blood
alcohol levels.
Suppose there has been an accident,
and one of the cars involved in the
accident drives
off, and the driver is not apprehended
until some hours later.
Then you're not interested in the amount
of blood alcohol the guy has
at the time he's arrested, what you really
want
to know is how much alcohol was in his
blood
when the accident happened. And as we know,
the
body is working hard to eliminate alcohol
from the blood.
So what we need to do is to be able to
calculate
how much blood alcohol somebody had in the
past,
a certain number of hours, after they are
actually measured.
So suppose, someone takes a drink at time 0.
Okay, at time 0, all the alcohol is in
their stomach, none is in their blood.
Very rapidly, that alcohol starts to move
into their
blood, so their blood alcohol
concentration starts to go up.
At some point, it will reach a maximum
and
the biochemical machinery will be
eliminating it, so it will reach a
maximum and then it will drop off, and
after some hours, we'll reach 0.
The question is, how steep is the slope of
this graph?
How fast does the body get rid of alcohol?
Let's take an example.
Suppose there's an accident at 3: 00 a.m.
The man is arrested some hours later, and
eye witnesses
confirm that he had had a drink before
the accident.
So the question is, what is his blood
alcohol
level at 3:00 a.m at the time of the
accident?
Well, suppose it takes 2 hours to arrest
him and take him
to a police station and take the blood
sample and do the measurement.
So, we can only measure the blood alcohol
level at 5 a.m.
Now, at 5 a.m., let's suppose that the
measurement is 70 mg per 100 mil.
So in most countries, including
Singapore, 70 mg
per 100 mil is below the legal limit.
But what we need to do is to calculate
his blood alcohol level
two hours before, at 3 a.m.
We're going to do this two ways.
First way we're going to do this is using
algebra.
Now, what we need to know is how fast do
people's bodies eliminate ethanol?
Well, it's found that the slowest
elimination rate is something
like 12.5 milligrams of ethanol per 100 mil
blood per hour.
So let's take the equation Ct = C0 minus t beta,
where Ct is the concentration of blood
alcohol at the time of measurement,
C0 is the concentration of blood alcohol
at the time of interest i.e.
the time of the accident. t is the time
elapsed between the two,
and beta is the elimination rate.
If we rearrange this equation, then we get
C0 equals Ct plus t beta.
Now we can plug in the numbers.
Ct is the 70 mg per 100 mil that was
measured at the police station. t is two,
that's the two hours between 3 a.m. and 5 a.m.
And the low elimination rate of 12.5
gives
us a concentration of 95 mg per 100 mil.
Now, people eliminate alcohol at different
rates.
So, this is the calculation assuming a
slow elimination rate of 12 and a half.
A high elimination rate will be 25 mg of
ethanol per 100 mil of blood per hour.
So if we use the same algebra and put in
this new value for beta of 25 mg,
then we calculate that the concentration
at the time
of the accident is 120 mg per 100 mil.
Well, whichever value of beta we use in
this case, the man was above the legal
limit.
If you don't like algebra,
you can do it this
way.
If we assume the low elimination rate of
12.5 mg per 100
mil of blood per hour, 12.5 mg for 2 hours
is 25 mg.
Add that back on to the 70 that we
measured, and you get the 95 mg per 100
mil.
The same logic applies to the higher
elimination rate of
25 mg of ethanol per 100 mil of blood per
hour,
so 25 mg times 2 hours is 50 mg.
Add that onto the 70 mg measured at 5 a.m.
and you get 120 mg per 100 mil. So
however you do the math, you get the same
conclusion
that this man was above the legal limit.
So as I said in the beginning, alcohol
is
the molecule that generates the most work
for toxicologists.
So, to your good health.
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