Of the 84,000 chemicals in commercial use in the United States -- from flame retardants in furniture to household cleaners -- nearly 20 percent are secret, according to the Environmental Protection Agency, their names and physical properties guarded from consumers and virtually all public officials under a little-known federal provision. Under the 1976 Toxic Substances Control Act, manufacturers must report to the federal government new chemicals they intend to market. But the law exempts from public disclosure any information that could harm their bottom line. Government officials, scientists and environmental groups say that manufacturers have exploited weaknesses in the law to claim secrecy for an ever-increasing number of chemicals. In the past several years, 95 percent of the notices for new chemicals sent to the government requested some secrecy, according to the Government Accountability Office. About 700 chemicals are introduced annually. Some companies have successfully argued that the federal government should not only keep the names of their chemicals secret but also hide from public view the identities and addresses of the manufacturers.