Knob and tube wiring is an older type of electrical wiring that's no longer used and generally not considered safe. However, some homes still have it, or may have a knob and tube wiring system that was incorrectly modified or rigged to work with a modern electrical system. Knob and tube wiring may contain asbestos insulation, but the only way to know is to have the material professionally tested.

Asbestos was commonly used in cloth wiring, but not all types of cloth wiring contain it. The age, condition, and appearance of the wiring can provide clues as to whether asbestos is present or not, but the only way to know for sure if the wiring has asbestos is to have it tested.

According to the National Cancer Institute, asbestos-related diseases can take anywhere from 10 to 40 years for symptoms to appear. The health risks increase with prolonged and heavier exposure. Risk factors also include:

• Amount of exposure

• Chemical makeup, size, and shape of the asbestos fibers

• Length of time a person is exposed to asbestos

• Genetic factors 

• Source of exposure (direct exposure or secondhand exposure from someone directly exposed)

It can be safe to live in a house with asbestos as long as the asbestos-containing materials are intact, in good condition, and not disturbed. Asbestos becomes dangerous only when its fibers are released into the air, which happens if the material is damaged, crumbling, or disturbed during renovation work. If you suspect asbestos is present, do not disturb it. Instead of removal, materials in good condition can sometimes be encapsulated. It is recommended to have regular professional inspections to monitor the condition of the asbestos and prevent health risks from potential exposure.