The employer has the responsibility to protect employees from all identified hazards in the workplace.
“Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.”
Air quality listed as "extremely hazardous" certainly meets this requirement. If employers who require employees to work in this hazard must attempt to eliminate the hazard with engineering controls. This may include sealing up leaky doorways and windows, and using an air filtration system.
If the hazard cannot be eliminated with engineering controls, then the employer must provide respirators to employees. Face coverings do not protect from smoke. Particulate filtering respirators must be used.