The heat is on for California businesses and workers as state officials write a safety rule protecting workers from high heat in indoor workplaces.

Under the California Division of Occupational Safety and Health (Cal/OSHA) proposal, state employers will have to implement a written heat illness prevention plan that could be part of a workplace’s already required illness and injury prevention program, which covers all types of risks.

The latest version of the proposed rule, released May 11, sets 85 degrees as the trigger temperature for employers to initiate precautions such as encouraging workers to take cool-down rest breaks...