Temperatures as high as 107 degrees Fahrenheit are hitting parts of California this week, and Cal/OSHA is reminding employers that heat illness prevention obligations apply even in October.

Extreme heat warnings and heat advisories cover parts of Southern and Northern California through Friday morning. Southern California faces daytime highs from the mid-90s along the coast up to 107 in the valleys. Bay Area advisories warn of temperatures up to 103 degrees, while the San Joaquin and Sacramento valleys expect highs of 94 to 100 degrees.

For HR and safety teams, the weather event puts two distinct regulatory regimes into effect at once.

Which rules apply to which workplaces?

California employers must legally protect workers from heat-related illness in both indoor and outdoor workplaces — a dual mandate that differs from federal OSHA's approach, which has no permanent heat standard.

Cal/OSHA's indoor heat illness regulation covers most indoor workplaces, including restaurants, warehouses and manufacturing facilities. The trigger temperature is 82 degrees: once the indoor workplace reaches that point, employers must take protective steps.

Employers with workers in both settings may fall under both the indoor and outdoor regulations simultaneously — a distinction that matters for multi-site operations such as food processors with loading docks, or retailers with warehouse space.

Heat illness remains a serious and potentially fatal workplace hazard, Cal/OSHA notes, which is why the agency treats prevention as a compliance issue rather than a comfort issue.

What must employers do right now?

Cal/OSHA lays out six core obligations:

• Plan — Develop and implement an effective written heat illness prevention plan that includes emergency response procedures.
• Train — Train all employees and supervisors on heat illness prevention.
• Provide Water — Offer fresh, pure, suitably cool drinking water free of charge, enough for each worker to drink at least one quart per hour, and encourage workers to do so.
• Encourage Rest — Allow workers to take a cool-down rest in the shade for at least five minutes when they feel the need to protect themselves from overheating. Workers should not wait until they feel sick to cool down.
• Provide Shade — Supply proper shade when temperatures exceed 80 degrees. Workers have the right to request and be provided shade at any time.
• Observe — Closely observe all employees during a heat wave and any employee newly assigned to a high heat area. Lighter work, frequent breaks or shorter hours help employees who have not been working in high temperatures adapt.

Why acclimatization matters in a late-season heat wave

The observation requirement carries particular weight during sudden heat events. Employees who have not been working in high temperatures — a common situation when a heat wave arrives in October — need time to adapt. Cal/OSHA recommends lighter workloads, more frequent breaks or shorter shifts as adjustment tools.

That acclimatization risk also extends to supervisors, who must receive the same training as employees and are responsible for spotting early warning signs before they become emergencies.

What happens next?

The advisories expire Friday morning, but Cal/OSHA's enforcement posture is not weather-dependent: the written plan, training and water requirements apply year-round whenever temperature thresholds are met. Employers can find detailed requirements at www.99calor.org and on Cal/OSHA's Heat Illness Prevention website, and with California heat events stretching later into the fall, HR teams with operations in the state should expect heat compliance to remain a seasonal planning item well beyond summer.