Ductwork connectors join two courses of ductwork together with a tight seal, which prevents any leakage of cool or warm air from the air conditioning or heating systems. These connectors are used at the joints, which means that the material needs to be semi-flexible in order to allow for slight movement and changes in the temperature of the air both outside and inside the system. Without these ductwork connectors, the air conditioning or heating systems would be compromised because unwanted air could leak out, or the opposite, could infiltrate the system. Every HVAC system that is installed professionally and requires separate courses of ducting material will use ductwork connectors to seal and insulate the joints. Before 1977, thousands of construction and HVAC materials were manufactured in the United States using asbestos in the products. This included ductwork connectors. Manufacturers thought that adding asbestos fibers into the ductwork material would be beneficial because it would give it added flexibility that the connectors needed. In 1977 however, legislation restricted the use of asbestos in manufacturing construction materials, although it was permitted to continue using the pre-existing asbestos filled materials. Because of this, it is assumed that any building that was constructed before the mid 1980's may have asbestos materials inside it. It is impossible to tell by looking at the products, but asbestos may be present. If any remodeling or renovation needs to be done to a home that was built prior to this, it is very important that the proper safety precautions be taken, such as calling a licensed asbestos abatement contractor. It is also important to know that asbestos becomes brittle and breaks down over time, so the tiny asbestos fibers may go into the ductwork without any renovation work being performed. This is dangerous because it can cause mesothelioma and other asbestos related illnesses.