Nowadays, most people know that asbestos is extremely dangerous; it was once a very common form of building insulation, but has since been highly regulated in the United States and outright banned in several other countries. Last week we looked at a little bit of the early history and basic composition of asbestos; its resistance to fire and tensile strength made it a very popular building material from the 1850s onward. But all asbestos types are extremely hazardous to human health, and can cause a variety of chronic, difficult diseases.

Asbestos is quite durable in many respects, but the fibers are also incredibly thin - as small as 3 micrometers in width. By comparison, a human hair is 17-181 micrometers wide. The asbestos fibers naturally line up parallel with each other to form crystalline lattice structures. These crystals have three cleavage planes, two of which are much weaker than the third, and can be broken apart into fibrous fragments if sufficient force is applied. In fact, the structure is so soft that it can be broken with a simple finger crushing pressure, sending billions of tiny fragmented asbestos fibers into the environment. In buildings, asbestos can be disturbed and released via air erosion, vibration, fire damage, or abrasion. And if the settled asbestos dust is disturbed by human activities, it can fragment even further.

The tiny, needle-like fibers of the amphibole class are easily inhaled. In fact, asbestos occurs naturally in the air and in some drinkable water, and most people likely have several million asbestos fibers in their lungs. While naturally occurring asbestos may not have a significant effect on health, continuous exposure, through occupational handling or living in a house with asbestos insulation, is a different story.

When asbestos fibers are inhaled, they become stuck in the alveoli (air sacs) in the lung. The lung’s immune system releases macrophages, or white blood cells, which attempt to ingest the fibers and heal the lung using fibrous scar tissue. This is a normal process for the body. However, asbestos fibers are naturally resistant to digestion; the white blood cells eventually die off, releasing chemical signals to attract more white blood cells to clean up the mess and apply more fibrous tissue. The result is a fibrous buildup of asbestos in the lungs, reducing oxygen transfer to the blood; the scar tissue also causes the lung walls to thicken and lose elasticity. In 1924, a British pathologist named the disease ‘asbestosis’. Asbestosis is a chronic, slowly-progressing disease, and symptoms appear years or even decades after the initial exposure.

Asbestosis isn’t the only illness caused by asbestos fibers; they also cause abnormalities of the pleura (the protective outer lining of the lung), including hard plaques, calcification, and thickening. When the pleura is damaged this way, it impacts lung function. Asbestos also causes various types of lung cancer. Mesothelioma is a rare form of malignant cancer that infects the pleura; while it can occur spontaneously, asbestos exposure has been reported in about 70-80% of all mesothelioma cases. It’s theorized that asbestos fibers may be causing cancer not because of their chemical toxicity, but because they are small enough to tangle with cell chromosomes and disrupt natural cell division (mitosis), which can trigger tumor growth.

A single, high-dose exposure to asbestos is very unlikely to cause disease, but chronic or continuous exposure is absolutely detrimental to human health. Doctors knew about the link between asbestos and lung disease as early as 1906, but the United States didn’t start regulation until the 1970s and 1980s. Why did they wait so long? Next week we’ll examine the asbestos scandal that caused thousands of unnecessary deaths during the asbestos manufacturing heyday of the 20th century.