Heat Safety for Outdoor Workers — OSHA Guidelines and Best Practices

Employers must keep workers safe from heat, but the legal route is narrower than many people assume. OSHA's heat pages state that there is no federal OSHA heat standard; instead OSHA relies on the General Duty Clause, which requires employers to provide a workplace free from recognized hazards that are causing or likely to cause death or serious harm. Five states, which OSHA lists as California, Colorado, Minnesota, Oregon and Washington, have their own heat rules. Whatever the legal setting, OSHA's practical guidance rests on three things: water, rest and shade, plus time for new workers to adjust to the heat.

Water, rest and shade

• Water: OSHA says workers should be encouraged to drink water frequently while working in the heat, not just when they are thirsty. Its page does not set a fixed number of ounces. For jobs lasting longer than two hours, it says to provide electrolyte-containing drinks such as sports drinks.
• Rest: OSHA says workers should take hourly breaks whenever heat stress exceeds the limits in its own table of recommended limits, that breaks should lengthen and become more frequent as heat stress rises, and that in hot conditions skipping breaks is not safe.
• Shade: workers need a cool place to recover. For outdoor jobs OSHA names a shady area, an air-conditioned vehicle, a nearby building or tent, or an area with fans and misting devices. Indoor workers need a cool area away from heat sources such as ovens and furnaces.

Acclimatization: the rule of 20 percent

People who have not recently spent time in warm or hot conditions need time to build tolerance. OSHA's guidance for new workers is the rule of 20 percent: work no more than 20 percent of a normal day on day one, then add 20 percent each day until a normal schedule is reached. OSHA stresses reducing the duration of the work, not its intensity. It says the rule protects most physically fit workers but that others may need up to 14 days. The schedule below is simple arithmetic from that rule.

The same protections apply to returning workers. OSHA's guidance covers people back after an absence of a week or more, workers in the spring as temperatures climb after winter, and anyone facing a sudden heat wave, and it advises keeping the extra protections in place for at least a week.

Worked example: turning a forecast into a plan

OSHA's overview refers to wet bulb globe temperature (WBGT) measurement and does not give heat index category thresholds, so the heat index is best treated as a quick screening number. The National Weather Service says heat index values assume shade and that direct sunshine can add up to 15°F. The table applies both ideas to four sample forecasts using the heat index calculator's formula, rounded to one decimal place, with the NWS cutoffs of 80, 90, 103 and 125.

A forecast that looks like Extreme Caution in the shade can sit in the Danger category for a crew working in an open field. That is the reason to plan from the sun-adjusted number, to confirm with WBGT where an employer measures it, and to schedule the heaviest tasks for the cooler part of the day.

Recognizing heat illness and responding

OSHA lists fatigue, irritability, thirst, nausea or vomiting, dizziness or lightheadedness, heavy sweating, and an elevated body temperature or fast heart rate as heat exhaustion signs. For heat stroke it lists confusion, slurred speech, unconsciousness, seizures, heavy sweating or hot, dry skin, very high body temperature and a rapid heart rate. OSHA's instruction when heat stroke symptoms appear is to call 911 immediately and cool the worker with ice or cold water until help arrives. It also says never to leave a worker with heat illness alone, and when in doubt, call 911.

Common mistakes

• Waiting for thirst. OSHA says to drink frequently, not just when thirsty.
• Skipping breaks to finish a job. OSHA's guidance is blunt that this is not safe in hot conditions.
• Treating a returning worker like an acclimatized one after a week away.
• Using the shaded heat index for crews in direct sun.
• Assuming one rule covers everyone. The five states with their own heat rules apply them to different settings, from outdoor work to indoor workplaces and agriculture.