Asbestos is a naturally occurring mineral that has been used as a building material since the early 20th century. It was particularly popular during the post-World War II construction boom as it was an affordable, durable, and fire-resistant material. Asbestos was found in a variety of building materials, including insulation, roofing sheets, ceiling tiles, cement, and pipe insulation.

The problem with asbestos is that it is a hazardous substance that can cause severe health issues, particularly if it's inhaled. When asbestos fibers become airborne, they can easily be inhaled into the lungs, causing respiratory issues, lung cancer, and mesothelioma - a rare form of cancer that can take decades to manifest.

Due to the health risks associated with asbestos, it's no longer used in building construction. However, there is still a significant amount of asbestos present in older buildings that were constructed before 2000.