Asbestos is a naturally-occurring mineral that is both pliant and incredibly strong. It was a popular building material in the early 19th century because it is extremely flame-retardant and resistant to chemical or electrical damage. However, the use of asbestos has been banned in most countries because the inhalation of asbestos fibers is toxic in high concentrations. Asbestos air testing determines if asbestos is present in a building, and if harmful fibers are being released into the air. This test involves pumping air through a filter, mounting the filter on a slide, and then viewing this slide with Phase Contrast Microscopy. A trained analyst must study the slide and count the number of asbestos fibers present; from this number, the analyst calculates a concentration value which represents the levels of asbestos present in the air. Different countries have differing laws concerning acceptable asbestos levels, but if the test reveals an unsafe concentration of asbestos, any building materials containing it must be removed and replaced. Asbestos is generally found in surfacing materials (such as plaster), in thermal system insulation (found on boilers and heating or air conditioning systems), and other building materials such as fabric, siding, and floor or ceiling tiles.