SEATTLE — The state’s largest industrial emitters would be required to reduce carbon emissions by 5 percent every three years, under a proposed rule released Wednesday by state regulators.

The Department of Ecology’s proposed Clean Air Rule would initially apply to about two dozen manufacturing plants, refineries, power plants, natural gas distributors and others that release at least 100,000 metric tons of carbon a year. Many more facilities would likely be covered by the rule as that threshold is lowered over time.

State officials say the rule is needed to protect human health and the environment from climate change.

“It’s important that we act now to protect our water supplies, infrastructure and economy for future generations,” Ecology Director Maia Bellon said in a prepared statement.

Environmental and other groups applauded the draft rule as a crucial step in addressing climate change. But business groups and others have worried the efforts could hurt the state’s ability to attract and retain industries.

The draft rule comes after Gov. Jay Inslee failed last year to get legislation passed on his ambitious cap-and-trade plan that would have charged industrial facilities a fee for carbon emissions. In July, Inslee directed Ecology to limit carbon pollution using its existing authority under the state’s Clean Air Act.

Meanwhile, two competing efforts are trying to limit carbon pollution through statewide initiatives.

In one effort, backers of Initiative 732 have turned over 350,000 signatures on a proposal to tax carbon pollution at $25 a metric ton while lowering other state taxes. If verified, I-732 would go before the Legislature. If lawmakers don’t act, the measure would go on the 2016 ballot.

A coalition of environmental, labor and social justice groups, the Alliance for Jobs and Clean Energy, is also planning a statewide ballot initiative. That measure hasn’t been finalized, but it could impose new fees on carbon pollution and direct the money for clean-energy projects, low-income communities and other projects.

Officials say its Clean Air Rule would capture about 60 percent of the state’s overall carbon emissions, but it would not get the state all the way toward its mandate to limit emissions of greenhouse gases to the 1990 level.

The rule would initially apply to facilities releasing at least 100,000 metric tons. As that threshold drops by 5,000 metric tons every three years, it would apply to more entities.

Ecology identified nearly four dozen facilities such as Joint Base Lewis-McChord and the University of Washington that could eventually be covered by the rule as the threshold is lowered over time. The University of Washington, for example, last reported over 90,000 metric tons of emissions, Ecology said.

“There are elements that are concerning to us,” said Brandon Houskeeper, government affairs director with the Association of Washington Business, which represents most of those who would be covered by the proposed rule.

“Lowering the threshold means more entities would be covered by this rule. What signal does that send to businesses hoping to relocate to Washington?” he asked.

Facilities would have different ways to comply with the rule, including buying credits from another carbon market system such as California’s or sponsoring projects that permanently reduce carbon pollution.

The agency has scheduled four public hearings. It expects to finalize a rule by summer.