Asbestos is a term that refers to a group of naturally occurring fibrous
minerals. Because of their resistance to decay and their remarkable
insulating properties, asbestos fibers have been incorporated into thousands
of products and materials. Unfortunately, it is now clear that exposure to
airborne asbestos fibers can cause disease. The risk of developing
asbestos-related disease varies according to the intensity, duration and
nature of the exposure.
The Respiratory System
To be a significant health concern, asbestos fibers must be inhaled. An
understanding of the mechanics of the respiratory system will aid in
appreciating the potential for exposure and the resulting health effects.
Every cell in the body needs a constant supply of oxygen. The respiratory
system meets this need by bringing oxygen to the bloodstream, which delivers
it to each cell and carries away carbon dioxide. The lungs are the focal
point of the respiratory system, which also includes the respiratory tract,
the channel by which air flows into and out of the lungs. The drawing above
is an illustration of the respiratory system. Inhaled air passes through the
nose, where moisture and tiny hairs filter dust. It then passes down the
throat where air is also humidified. Air continues into the trachea. Just
above the heart, the trachea divides into two bronchi. Each bronchus
leads into a lung where it subdivides into bronchioles and smaller air tubes
- giving the appearance of an upside-down tree. The tiniest tubes end in
globular air sacs called alveoli.
The actual exchange of gases - respiration - takes place in the
alveoli. There, blood vessels only one cell thick allow oxygen and carbon
dioxide to trade places. The carbon dioxide is exhaled back up the
respiratory tract. The blood picks up fresh oxygen and transports it
throughout the body.
The lungs, cone-shaped, balloon-like, elasticized tissue, are
located on either side of the chest. Each lung is encased by a double layer
of membrane, or pleura. One layer is attached to the lung, the
other to the rib cage. Space and fluid between the two layers enable the
lungs to expand and contract in the chest cavity without friction. When we
breathe in, the diaphragm stretches out flat and muscles between the ribs
contract with it, pulling the ribs up and out. This expands the chest cavity
creating a vacuum between the linings that expands the lungs and sucks in
air. When breathing out, the diaphragm and rib cage muscles relax, the ribs
fall in and down, and the lungs contract and push out the carbon dioxide and
unused oxygen.
The respiratory system is sensitive to bacteria, viruses, and many
airborne particles that can be inhaled. Reactions to these irritants can
disrupt the functioning of the system, resulting in many ailments including
such things as: the common cold, hay fever, sinusitis, sore throat, acute or
chronic bronchitis, emphysema, and lung cancer.
Natural Filters. The body has several mechanisms by which it
filters the air it breathes. The tiny hairs in the nose filter out dust and
airborne particles. Like the nose, the trachea and the bronchi are lined
with small fine "hairs" called cilia. Together with mucous
secreted by cells lining the airways, cilia trap particles and help prevent
respiratory infections. the cilia beat in an upward direction sweeping
foreign particles up to the back of the mouth where they are expelled or
swallowed. Viruses and bacteria are also attacked by enzymes called
lysozymes in the mucous cells. Microbes that slip through are usually
handled by white blood cells called phagocytes that envolop and eat
these invaders in the lung.
Cigarette smoking temporarily paralyzes the cilia. If smoking
continues long enough, the cilia wither and die. They are never replaced.
The efficiency of the cilia is replaced by the smoker's inefficient cough
which attempts to rid the respiratory tract of foreign particles and excess
mucus.
Dirty, contaminated air presents the greatest challenge to the
respiratory system. Some of the particles entering the airways reach the
alveoli. When this occurs, white blood cells called macrophages
attempt to engulf and digest the particles. In the case of asbestos, we are
dealing with a mineral fiber, as a substance which macrophages can often not
successfully attack. As a means of secondary defense, the macrophages
deposit a coating on the fibers which are then deposited in the smaller
passages. Here they clog and actually scar the tissues. The walls of the
alveoli lose their elasticity and useful function in respiration.
Asbestos Related Diseases
Asbestos exposure can cause a number of disabling and fatal diseases.
The principal rout of exposure is by inhalation through the nose and
mouth. Asbestos, traditionally valued for it's indestructibility, is
especially resistant to the internal defenses of the human body. Once
lodged inside the lungs, most fibers will not break up or dissolve, and
they cannot be neutralized or removed.
Asbestosis
Asbestosis is a disease which is characterized by pulmonary fibrosis,
a progressive scarring of the lungs caused by the accumulation of
asbestos fibers. Asbestosis is associated exclusively with chronic,
occupational exposure. The build up of scar tissue interferes with
oxygen uptake through the lungs and can lead to respiratory and heart
failure. Often, asbestosis is a progressive disease, even in the absence
of continued exposure. Symptoms include shortness of breath, cough,
fatigue, and vague feelings of sickness. When the fibrosis worsens,
shortness of breath occurs even at rest.
Pleural Plaques
Pleural plaques and pleural calcification are markers of exposure and
may develop 10 to 20 years after initial exposure. Plaques are opaque
patches visible on chest x-rays that consist of dense strands of
connective tissue surrounded by cells. All commercial types of asbestos
induce plaques. Plaques can occur even when fibrosis is absent and do
not seem to reflect the severity of pulmonary disease.
Lung Cancer
Of all the diseases related to asbestos exposure, lung cancer has
been responsible for over half of the excess deaths resulting from
occupational exposure. Although tissues and cells react to the presence
of asbestos immediately, detectable symptoms take years, or more often
decades, to manifest themselves. Asbestos-induced lung cancer may not
show up on x-rays for twenty years or more after the exposure began.
This delay between exposure and onset is referred to as the "latency
period". Even in cases of prolonged heavy exposure, abnormalities
commonly appear on x-rays only after ten or more years following
exposure.
Asbestos as a Co-Factor: Other substances appear to cooperate with
asbestos to multiply the risk of lung cancer. Asbestos exposure in
combination with cigarette smoking can multiply the risk of developing
lung cancer as much as ninety times over the risk to a non-smoker with
no history of exposure to asbestos.
Mesothelioma
Mesothelioma, a malignant nodular type cancer of the membranes which
line the lung cavity, is another disease related to asbestos exposure.
Malignant mesotheliomas of these membranes (the pleura and the
peritoneum) are extremely rare in persons with no history of asbestos
exposure, but may account for 10% to 18% of excess deaths in workers
exposed to asbestos. Generally, a latency period of at least 25 to 30
years is required in order to observe mesotheliomas, and some victims
have had a latency period of forty years since their initial exposure to
asbestos. This form of cancer is incurable and is usually fatal within a
year after diagnosis. Mesothelioma has been associated with short term,
incidental exposure, but here is no evidence of a relationship between
cigarette smoking and mesothelioma risk.
