Some scientists feel that products made of nanoparticles are being marketed without sufficient testing of their effect on human health and the environment. Nanoparticles, which are 10,000 times thinner than a strand of hair, are being used in everything from fabric coating to teddy bears. The Toxic Substances Control Act states that if a substance has already been approved in a larger than nano-scale form, it is considered as existing in the Chemical Substance Inventory by the U.S. Environmental Protection Agency and requires no further testing. Some scientists, however, think nano-scale substances should be treated as entirely different, as studies have shown that some nanoparticles can cause health problems in animals. Very small nanoparticles, called quantum dots, have been shown to penetrate the skin of pigs. Inhaled nanoparticles also penetrate deeper into the lungs, causing concern about their use in spray products. The Project on Emerging Nanotechnologies works to ensure that both the benefits of nanoparticles are realized and the risks minimized. The EPA has started a voluntary reporting program, called the Nanoscale Materials Stewardship Program, which two companies have submitted information to. Ten more companies have pledged to hand over data. Nanowerk maintains a database of the nearly 2,000 existing nanoparticles to help manufacturers locate the nanoparticles they need. There are worries that silver nanoparticles might damage microbes essential for the health of the ecosystem, and sewage treatment cannot clear water of the particles. Silver is known to adversely affect water-living organisms. The particles have many uses, including killing drug-resistant bacteria in hospitals and coating socks to help eliminate odor.