The Gazette 1990

GAZETTE

JANUARY/FEBRUARY 1990

cause x-ray change but do not give

rise to any disability whatsoever.

The possibility of cancer of the

throat has been considered as

being associated with asbestos but

never proven. I intend only to con-

sider asbestosis itself, "ordinary"

lung cancer, and mesothelioma.

Asbestosis

Asbestosis is a disease character-

ised by scarring (fibrosis) of the

spongy tissue or parenchyma of

the lung. It is one of the

pneumoconioses. The difference

between asbestosis and other sorts

of pneumoconiosis is very obvious

to the respirologist, radiologist, or

pathologist dealing with an in-

dividual case, where the real skill in

diagnosis comes is at the very early

stages however. The very earliest x-

ray changes come at the base of

the lung and the very earliest

clinically detectable change is

crackles, or wet sounds, also at the

base of the lung. Diagnosis at this

early stage takes a great deal of

skill and experience. The severity of

any pneumoconiosis including

asbestosis is proportional to the

amount of exposure both in volume

and in time and no one knows why

some individuals become easily

affected and some individuals

never develop the disease. The

disease may not commence in fact

until after exposure has ceased. In

one study workers who developed

asbestosis developed it in ten to

twenty years after about two years

of exposure. The first symptom of

asbestosis is breathlesshess. Here

again other causes of breathless-

ness may lead the individual not to

consult his general practitioner, and

again there may be further diag-

nostic delays as an individual

moves through the system. Finally,

ten, twenty or thirty years after the

period of exposure, the individual is

seen by a specialist and told "you

have an occupational disease due

to exposure to asbestos". What is

the likely outcome? A survey,

finishing in 1963, showed that

50% of a population of asbestos

workers who had died during the

study period died of lung cancer.

" In one study workers who

developed asbestosis

developed it in ten to twenty

years after about two years of

exposure."

Lung Cancer

The term lung cancer is now used

by respirologists and other

clinicians to refer to what is

pathologically correctly termed

"carcinoma of the bronchus" (in

other words not all cancers of the

lung are what is colloquially known

as lung cancer). This cancer is in

fact a cancer of the lining of the

bronchial tubes. As it grows it

breaks through the walls of the

bronchial tubes and becomes the

ugly white mess so characteristic

of x-rays of lung cancer. A question

which I tried to deal with in an

earlier paper (Murphy, D. L.,

Journal

of the National Industrial Safety

Organisation,

July 1985) was

whether lung cancer could occur in

an asbestos worker with no

evidence of asbestosis. Professor

Sir Richard Doll and Professor R.

Peto in one of their reports on

asbestos related disease to the

British Health and Safety Executive,

felt that it was unlikely that lung

cancer in an asbestos worker who

had no evidence of asbestosis was

due to asbestos exposure. As I said

previously, 50% of a population of

asbestos workers finally died from

lung cancer, not asbestosis. It is

obvious from this that the lung

cancer comes at a later stage.

Occupational cancers in general

have a latent period of between

20-24 years. This latent period is

the period from first exposure to

the cancer causing agent (carcino-

gen) to diagnosis of the disease.

Another general characteristic of

cancer due to occupation is that it

tends to occur at an earlier age

than the same cancers normally

appear in the general population.

"Occupational cancars in

ganaral hava a latant pariod of

between 20-24 years."

Masothalioma

The lungs are surrounded by a

membrane known as the pleura.

This is often described, in popular

medical articles, as the "cellophane

wrapping" surrounding the lungs.

A certain form of asbestos known

as crocidolite or "blue asbestos"

was known for many years before

it was discovered, in about 1964,

that most cases of this disease

were due to exposure to crocidolite.

Sadly, some of the early cases were

in small black children who played

on the slag heaps outside the

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crocidolite mines in southern

Africa. As in all epidemiological

studies, estimates vary. On this

side of the Atlantic it is considered

that 75% of cases of mesotheli-

oma are due to exposure to

asbestos. In the United States at

least one expert, Professor Irving

Sellikof, feels that as many as 90%

of mesotheliomata are due to

exposure to asbestos and that this

need not always be just crocidolite.

Mesothelioma tends to kill more

quickly than ordinary lung cancer.

Multiple causes

Looking at these three diseases we

can now consider some of the

problems which may be general to

occupational diseases and occupa-

tional cancers. A smoker has a risk

of developing lung cancer nine

times the average. An asbestos

worker, exposed to the kind of dust

levels that would have been

experienced in the industry thirty to

forty years ago, who is not a

smoker, has five times the average

risk of developing lung cancer. An

asbestos worker who is also a

smoker has fifty times the risk of

developing lung cancer.. The well-

known association between as-

bestos and lung cancer and

smoking and lung cancer has been

measured. What cannot be

measured and must await future

cancer research is why one smoker

develops lung cancer and another