Heat Illness Prevention: What Employers and Workers Must Know

Heat kills workers. According to OSHA, dozens of workers die from heat exposure each year, and hundreds more suffer heat stroke or other serious illness. Most of those cases are preventable.

The three-word summary of heat illness prevention is water, rest, and shade. But that shorthand doesn’t capture what it actually takes to protect a crew working in high heat, and it doesn’t reflect what OSHA expects employers to have in place.

Types of Heat Illness: Know What You’re Dealing With

Heat illness isn’t one thing. It’s a spectrum, and where a worker falls on that spectrum determines how fast you need to act.

Heat cramps are muscle spasms caused by fluid and salt loss through sweating. They’re painful but not life-threatening on their own. The fix is moving the worker to a cool area and having them drink water or a sports drink with electrolytes. Heat cramps are often the first sign that someone isn’t acclimatized or isn’t drinking enough.

Heat exhaustion is more serious. Symptoms include heavy sweating, cool and pale skin, weakness, nausea, headache, and a fast but weak pulse. A worker with heat exhaustion needs to stop working immediately, move to a cool environment, loosen clothing, and drink cool fluids. If symptoms don’t improve within 15 minutes, treat it as an emergency.

Heat stroke is a life-threatening emergency. The body’s temperature regulation system has failed. Symptoms include a body temperature above 103 degrees Fahrenheit, hot and red skin (sometimes dry if sweating has stopped), a rapid strong pulse, confusion, slurred speech, and possible unconsciousness. Call 911 immediately and begin aggressive cooling. Don’t give fluids to someone who is confused or unconscious. Every minute of delay increases the risk of organ damage or death.

Supervisors and workers need to know the difference between heat exhaustion and heat stroke. The two conditions can look similar in early stages, and mistaking heat stroke for exhaustion costs lives.

Risk Factors That Matter

Not every worker faces the same heat risk, even on the same job site. OSHA and NIOSH identify several factors that increase vulnerability:

Direct sun exposure raises the apparent temperature above the ambient air temperature, sometimes by 10 to 15 degrees. Workers in full sun face higher risk than those in partial shade.

High humidity slows sweat evaporation, which is the body’s primary cooling mechanism. A 95-degree day with 60% humidity is more dangerous than a 100-degree day with 20% humidity. Heat index calculations account for this.

Physical exertion speeds up heat generation inside the body. A worker doing light tasks in the shade is in a very different situation than one doing heavy work in direct sun.

Lack of acclimatization is the most commonly overlooked factor and the most dangerous. A worker’s first day on a hot job is their highest-risk day. The body needs time to adapt, producing more sweat, increasing plasma volume, and sweating at lower core temperatures. Without that adaptation, the risk of heat stroke is significantly elevated.

Certain medications increase heat illness risk: diuretics, antihistamines, beta-blockers, and some antidepressants can impair the body’s heat response. Workers won’t always disclose what they’re taking, so acclimatization and hydration protocols matter for everyone, not just visibly at-risk workers.

Acclimatization: The Step Most Employers Skip

Acclimatization is the process of gradually exposing workers to heat so the body adapts. According to OSHA and NIOSH, most heat-related fatalities occur in the first few days on the job. That’s not a coincidence.

The recommended schedule for new workers is to start at 20% of the full workload on day one and increase by 20% each subsequent day, reaching full exposure by day five. For experienced workers returning after time away (illness, vacation, or any absence longer than a week), OSHA recommends a shorter adaptation period of 7 to 14 days, starting at 50% exposure and building up.

Your heat illness prevention plan should include a written acclimatization protocol. It should specify how new employees are identified, who monitors their condition during the first two weeks, and what reduced-duty assignments look like for your specific work tasks. Verbal policies don’t hold up in an inspection. Written ones do.

During acclimatization, supervisors should check on new workers more frequently than they check on experienced crew. A new hire who seems to be “pushing through” discomfort is a heat stroke risk, not a model employee.

OSHA’s National Emphasis Program on Heat

In 2021, OSHA launched a National Emphasis Program (NEP) targeting heat illness in outdoor and indoor work environments. The NEP means OSHA proactively inspects worksites when the heat index reaches 80 degrees Fahrenheit or higher, regardless of whether a complaint or incident triggered the visit.

Under the NEP, OSHA inspectors arriving during high heat periods will look for a written heat illness prevention plan, availability of cool drinking water (at least one quart per worker per hour is the standard recommendation), accessible shade or cool rest areas, and evidence of an acclimatization program for new and returning workers.

Federal OSHA doesn’t yet have a specific heat standard. Enforcement relies on Section 5(a)(1) of the OSH Act, the General Duty Clause, which requires employers to provide a workplace free from recognized serious hazards. Courts and OSHRC have consistently upheld General Duty Clause citations for heat-related fatalities where employers lacked basic preventive measures. See the OSHA fines and penalties guide for what those citations cost.

A dedicated federal heat standard is in the rulemaking process as of 2026. When it passes, it will likely codify the water-rest-shade requirements along with specific temperature thresholds and written plan requirements. Getting compliant now means you won’t face a scramble when the rule takes effect.

Cal/OSHA’s Heat Standard: The Strictest in the Country

California has had a mandatory heat illness prevention standard since 2005, and it’s the most protective occupational heat standard in the United States. Any employer with outdoor workers in California must comply, regardless of industry or company size.

The Cal/OSHA standard at Title 8, Section 3395 requires cool drinking water at a ratio of one quart per worker per hour, shade sufficient for all workers during rest periods (or shade provided within two minutes when requested), a written heat illness prevention plan in English and any language understood by a majority of workers, and supervisor and worker training before work begins in hot conditions.

The most significant threshold in the Cal/OSHA standard is 95 degrees Fahrenheit. When the heat index reaches 95, employers must implement a high-heat procedure. This includes mandatory cool-down rest periods of at least 10 minutes every two hours, direct observation of workers for heat illness symptoms, pre-shift meetings to review heat illness procedures, and contact with workers every 20 minutes if they’re working alone.

Cal/OSHA’s indoor heat illness standard, which took effect in 2024, extends these protections to indoor workplaces where temperatures reach 82 degrees. This covers warehouses, distribution centers, restaurant kitchens, and manufacturing facilities that have historically been outside the scope of outdoor heat rules.

Other states with their own OSHA plans may have similar state-level heat rules. Washington, Minnesota, and Oregon have adopted heat illness standards as well. If you operate in multiple states, check each state’s OSHA plan requirements separately.

Heat Index and Action Levels

The heat index combines temperature and relative humidity to estimate how hot it actually feels to the human body. OSHA uses a four-tier action level system based on heat index:

At 80 to 90 degrees, the risk level is lower but real. Workers should have access to water, and new employees should be in acclimatization mode. Supervisors should be alert to symptoms.

At 91 to 103 degrees, the risk is high. In addition to water and shade, employers should implement additional rest breaks and increase supervisor monitoring. Workers should work in pairs when possible.

At 103 to 115 degrees, the risk is very high. More frequent rest breaks, active cooling measures like misting fans or cooling towels, and close supervision are needed. Alert workers to symptoms and remove anyone showing early signs immediately.

Above 115 degrees, conditions are extreme. Reschedule strenuous work to cooler parts of the day or consider suspending outdoor work entirely until conditions improve.

OSHA’s heat app and website provide daily heat index readings by location, which supervisors can use for daily planning.

What a Compliant Heat Illness Prevention Plan Looks Like

A written plan doesn’t need to be long. It needs to be specific. A general statement that “we follow OSHA guidelines” is not a plan.

Your plan should identify who is responsible for heat illness prevention, how water will be provided and replenished, where shade or cooling areas are located and how workers access them, what the acclimatization schedule is for new and returning workers, what symptoms supervisors and employees are trained to recognize, and what the emergency response procedure is if a worker shows heat stroke signs.

The plan should be accessible to all workers and supervisors, not filed in a binder in the main office. For multilingual crews, it needs to be translated.

Training is a required component, not an optional one. Employer safety training requirements cover what documentation to keep. For heat training specifically, document who received training, when, and what topics were covered. Sign-in sheets work. Digital training records work. A supervisor’s recollection does not.

One direct recommendation: if your site runs summer work in a high-heat region, designate a heat illness prevention officer for each crew. That person’s job during high heat days is to monitor workers, track water consumption, enforce rest breaks, and call for help if symptoms appear. One person with clear authority and a clear checklist prevents more heat illness cases than any amount of policy language. Pair that with the right training from an OSHA 30 certification and your supervisors will know what to watch for.

Building a safety culture where workers feel comfortable flagging early symptoms without fear of losing work time is what actually reduces heat deaths. Workers who push through because they’re afraid to speak up are your highest risk.