The U.S. Environmental Protection Agency (EPA) enacted a rule on June 1 making it easier for companies to use asbestos in products, Fast Company explained in a recent report. But what, exactly, is asbestos, and why is it dangerous?

Asbestos is a naturally occurring material made up of thin crystal fibers and is resistant to heat and corrosion, the National Cancer Institute (NCI) says. Humans used it for centuries, and by the early 1900s, it had become a staple material of the industrialized world, according to Scientific American. Companies packed it into walls for insulation, mixed it with plastics, sealed it into floors and built it into cars. An "Asbestos Man" greeted visitors to the 1939 New York World's Fair, celebrating the material's "service to humanity."

But what most of the public and most researchers hadn't realized at that time was that those tiny asbestos fibers can be inhaled and get trapped in a person's lungs, causing disease, the NCI says. [10 Scientific Quit-Smoking Tips]

By the 1960s, however, researchers had begun to suspect that a spike in a rare cancer of the lungs' lining called mesothelioma— especially common among World War II-era ship insulators working with asbestos — might be tied to the suddenly omnipresent substance, according to Scientific American. In 1973, as The Virginian-Pilot reported in 2001, a doctor testified before Congress that 1 million Americans would die of work-related asbestos diseases in the coming decades. In 1975, the brand-new EPA banned the use of asbestos in insulation, and by 1989, the EPA had taken steps to ban the use of asbestos entirely. In 1991, however, industry lawyers successfully blocked that rule from being fully implemented, according to The Mesothelioma Center, an advocacy group.

Technically, the EPA still allows the use of asbestos in certain products, including car and motorcycle brakes, fireproof clothing and certain construction materials. But until recently there were still strict limitations on how it could be used. Under the new rule, Fast Company reported, the EPA will make it much easier for companies to use asbestos, following a Trump administration "framework" for evaluating risks. Instead of considering whether an asbestos product poses a risk to air, ground or water contamination, the agency will restrict only the products that pose a direct risk to workers who come into contact with the material during the manufacturing process.

According to the World Health Organization (WHO), asbestos can cause mesothelioma, as well as lung, laryngeal and ovarian cancer. The material can also cause asbestosis, or fibrosis of the lungs — a scarring of the lungs and thickening of the lung lining, both of which cause shortness of breath, the WHO says.

People who get sick from asbestos were likely exposed to large amounts of the material, according to the NCI. Heavy exposure to asbestos is less common today, though people working in construction and ship repair, along with people who manufacture asbestos-containing products, are still at risk, the NCI says. Also at risk are all workers who were involved in the rescue, recovery and cleanup efforts after the World Trade Center attacks in New York City, according to the NCI.

Originally published on Live Science.