WASHINGTON — A decade after emergency trailers meant to shelter Hurricane Katrina victims instead caused burning eyes, sore throats and other more serious ailments, the Environmental Protection Agency is on the verge of regulating the culprit: formaldehyde, a chemical that can be found in commonplace things like clothes and furniture.

But an unusual assortment of players, including furniture makers, the Chinese government, Republicans from states with a large base of furniture manufacturing and even some Democrats who championed early regulatory efforts, have questioned the E.P.A. proposal. The sustained opposition has held sway, as the agency is now preparing to ease key testing requirements before it releases the landmark federal health standard.

The E.P.A.’s five-year effort to adopt this rule offers another example of how industry opposition can delay and hamper attempts by the federal government to issue regulations, even to control substances known to be harmful to human health.

Formaldehyde is a known carcinogen that can also cause respiratory ailments like asthma, but the potential of long-term exposure to cause cancers like myeloid leukemia is less well understood.