Washington farmworkers must be provided with smoke-filtering masks when air quality reaches the threshold that federal regulators consider “unhealthy” for the public, the state Department of Labor and Industries says.

The emergency rule went into effect July 16, though L&I investigators won’t begin enforcing it until July 23, department spokeswoman Dina Lorraine said.

L&I pulled back from an earlier proposal to enforce a mask rule at an air-quality level the Environmental Protection Agency considers acceptable, except for people unusually sensitive to smoke.

Instead, L&I adopted the standard used by California, the only other state that regulates farmworkers laboring in wildfire smoke. The department will revisit the threshold as it writes a rule for next wildfire season, Lorraine said.

Gov. Jay Inslee ordered L&I to write a rule last fall as smoke clouded skies and air quality deteriorated from “unhealthy” to “very unhealthy” to “hazardous” on the EPA scale in places across the state.

L&I has yet to propose a permanent rule. The emergency rule will be in place for the rest of the fire season.

The rule’s main provisions kick in when the federal air-quality index reaches 151 for particulate matter, the equivalent of 173 on a separate state air-quality index.

At 151, the color-coded EPA index turns red. Some members of the public may have ill effects, while sensitive groups may have serious illnesses, according to the EPA.

At that point, farmworkers must move indoors or away from the smoke, or be given federally approved N95 masks, according to the rule.

For this wildfire season, KN95 masks are acceptable. Wearing the employer-provided mask is up to the worker.

Employers must train workers about the rule, though that requirement won’t be enforced until Aug. 2 because training materials must still be translated into Spanish, Lorraine said.

L&I last month floated triggering the mask rule when the federal air-quality index reaches 69, or 101 on the state scale. The rule would have been roughly twice as strict as California’s.

At 69, the air could be hazardous for the young, the elderly and people with respiratory or other health problems, but is acceptable for most people, according to the EPA.

Farms groups said L&I was overstepping its jurisdiction by proposing a standard for working adults based on public health warnings for sensitive groups.

Washington State Tree Fruit Association President Jon DeVaney said he was pleased L&I followed California’s rule. “It makes sense to follow that standard,” he said.

Growers already were preparing for wildfire smoke, but now have a new set of regulations to learn, DeVaney said. “Rolling out rules when people are at their busiest is a distraction,” he said.

United Farm Workers organizer Elizabeth Strater said L&I should have stuck with the lower threshold. Farmworkers inhale dust and chemicals, and smoke adds to the occupational risks, she said.

“These folks aren’t just going to and from their car,” Strater said. “I think the jump to 151 is probably arbitrary. We know there are risks at lower levels than that.”

L&I’s rule “encourages,” but does not require, workers to move indoors or have approved masks when the air-quality index tops 69. The department wanted to acknowledge that smoke poses a hazard at the lower level, Lorraine said.

Four large wildfires were burning in Washington on Friday, but no monitoring station in the state indicated an air-quality rating for particulate matter worse than the 66 at Winthrop in north-central Washington.