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.

The policy was designed 33 years ago to protect trade secrets in a highly competitive industry. But critics -- including the Obama administration -- say the secrecy has grown out of control, making it impossible for regulators to control potential dangers or for consumers to know which toxic substances they might be exposed to.

At a time of increasing public demand for more information about chemical exposure, pressure is building on lawmakers to make it more difficult for manufacturers to cloak their products in secrecy. Congress is set to rewrite chemical regulations this year for the first time in a generation.

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.

"Even acknowledging what chemical is used or what is made at what facility could convey important information to competitors, and they can start to put the pieces together," said Mike Walls, vice president of the American Chemistry Council.

Although a number of the roughly 17,000 secret chemicals may be harmless, manufacturers have reported in mandatory notices to the government that many pose a "substantial risk" to public health or the environment. In March, for example, more than half of the 65 "substantial risk" reports filed with the Environmental Protection Agency involved secret chemicals.

"You have thousands of chemicals that potentially present risks to health and the environment," said Richard Wiles, senior vice president of the Environmental Working Group, an advocacy organization that documented the extent of the secret chemicals through public-records requests from the EPA. "It's impossible to run an effective regulatory program when so many of these chemicals are secret."

Of the secret chemicals, 151 are made in quantities of more than 1 million tons a year and 10 are used specifically in children's products, according to the EPA.

The identities of the chemicals are known to a handful of EPA employees who are legally barred from sharing that information with other federal officials, state health and environmental regulators, foreign governments, emergency responders and the public.

Last year, a Colorado nurse fell seriously ill after treating a worker involved at a chemical spill at a gas-drilling site. The man, who later recovered, appeared at a Durango hospital complaining of dizziness and nausea. His work boots were damp; he reeked of chemicals, the nurse said.