Both the outdoor and indoor heat illness prevention standards require employers to develop and implement a Heat Illness Prevention Plan that meets specific requirements set forth in the respective regulations. To help ensure your plan and procedures comply with the law, it’s important to take note of key definitions included in these regulations. Both the outdoor and indoor heat standards define key terms, some of which are the same in both standards and others that are distinct to the individual standards.

The heat illness standards provide the following definitions common to both standards:

• “Acclimatization” means the body’s temporary adaptation to work in the heat that occurs gradually when a person is exposed to it. Acclimatization peaks in most people within four to 14 days of regular work for at least two hours per day in the heat.
• “Heat illness” means a serious medical condition that results from the body’s inability to cope with a particular heat load. Heat illness includes heat cramps, heat exhaustion, heat syncope and heat stroke.
• “Environmental risk factors for heat illness” means working conditions that create the possibility that heat illness could occur. Environmental risk factors for heat illness include air temperature, relative humidity, radiant heat from the sun and other sources, conductive heat sources, air movement, workload severity and duration, protective clothing and PPE that employees wear.
• “Personal risk factors for heat illness” means factors that affect the body’s water retention or other physiological responses to heat. Personal risk factors for heat illness include a person’s age, degree of acclimatization, health, water consumption, alcohol consumption, caffeine consumption and prescription medication use.
• “Temperature” means the dry bulb temperature in degrees Fahrenheit obtainable by using a thermometer to measure the outdoor temperature in an unshaded area. Though the temperature measurement must be taken in an area with full sunlight, the thermometer bulb or sensor should be shielded while taking the measurement (e.g., with the hand or some other object from direct contact by sunlight).

The definition unique to the outdoor heat illness prevention standard is:

• “Shade” means blockage of direct sunlight. One indicator that blockage is sufficient is when objects do not cast a shadow in the area of blocked sunlight. Shade is not adequate when heat in the area of shade defeats shade’s purpose, which is to allow the body to cool. For example, a car sitting in the sun does not provide acceptable shade to a person inside it, unless the car’s air-conditioning is running. You can provide shade by any natural or artificial means that does not expose employees to unsafe or unhealthy conditions or deter or discourage access to use.

Definitions unique to the indoor heat illness prevention standard include:

• “Administrative control” means a method to limit exposure to a hazard by adjusting work procedures, practices or schedules. Examples of administrative controls that may be effective at minimizing the risk of heat illness in a particular work area include, but are not limited to:
• Acclimatizing employees;
• Rotating employees;
• Scheduling work earlier or later in the day;
• Using work/rest schedules;
• Reducing work intensity or speed;
• Reducing work hours;
• Changing required work
• Clothing that restricts heat removal" means full-body clothing covering the arms, legs, and torso that is any of the following:
• Waterproof;
• Designed to protect the wearer from a chemical, biological, physical, radiological or fire hazard; or
• Designed to protect the wearer or the work process from contamination.
• “Clothing that restricts heat removal” does not include clothing demonstrated by the employer to be all of the following:
• Constructed only of knit or woven fibers, or otherwise an air and water vapor permeable material;
• Worn in lieu of the employee’s street clothing; and
• Worn without a full-body thermal, vapor or moisture barrier.
• “Cool-down area” means an indoor or outdoor area that’s blocked from direct sunlight, shielded from other high-radiant-heat sources to the extent feasible, and is either open to the air or provided with ventilation or cooling. One indicator that sunlight blockage is sufficient is when objects don’t cast a shadow in the area. A cool-down area does not include a location where:
• Environmental risk factors defeat the purpose of allowing the body to cool; or
• Employees are exposed to unsafe or unhealthy conditions; or
• Employees are deterred or discouraged from accessing or using the cool-down area.
• “Engineering control” means a method of control or a device that either removes or reduces hazardous conditions or creates a barrier between the employee and the hazard. Examples of engineering controls that may be effective at minimizing the risk of heat illness in a particular work area include, but are not limited to:
• Isolation of hot processes;
• Isolation of employees from sources of heat;
• Air conditioning;
• Cooling fans;
• Cooling mist fans;
• Evaporative coolers (also called swamp coolers);
• Natural ventilation where the outdoor temperature or heat index is lower than the indoor temperature or heat index;
• Local exhaust ventilation;
• Shielding from a radiant heat source; and
• Insulation of hot surfaces.
• “Globe temperature” means the temperature measured by a globe thermometer, which consists of a thermometer sensor in the center of a six-inch diameter hollow copper sphere painted on the outside with a matte black finish, or equivalent. The globe thermometer may not be shielded from direct exposure to radiant heat while the globe temperature is being measured.
• “Heat index” means a measure of heat stress developed by the National Weather Service (NWS) for outdoor environments that takes into account the dry bulb temperature and relative humidity. For purposes of this section, heat index refers to conditions in indoor work areas. Radiant heat is not included in the heat index. See Appendix A to section 3396 for the required NWS heat index chart (2019).
• “Heat wave” means any day in which the predicted high outdoor temperature for the day will be at least 80 F and at least 10 F greater than the average high daily outdoor temperature for the preceding five days, for the purpose of this section only.
• “High-radiant-heat area” means a work area where the globe temperature is at least 5 F greater than the temperature.
• “High-radiant-heat source” means any object, surface or other source of radiant heat that, if not shielded, would raise the globe temperature of the cool-down area 5 F or greater than the dry bulb temperature of the cool-down area.
• “Indoor” refers to a space that’s under a ceiling or overhead covering that restricts airflow and is enclosed along its entire perimeter by walls, doors, windows, dividers or other physical barriers that restrict airflow, whether open or closed. All work areas that are not indoor are considered outdoor and covered by section 3395.
• An exception to the definition of “indoor” is that it does not refer to a shaded area that is used to shade workers covered under the outdoor heat illness standard.
• Personal heat-protective equipment" means equipment worn to protect the user against heat illness. Examples of personal heat-protective equipment that may be effective at minimizing the risk of heat illness in a particular work area include, but are not limited to:
• Water-cooled garments;
• Air-cooled garments;
• Cooling vests;
• Wetted over-garments;
• Heat-reflective clothing; and
• Supplied-air personal cooling systems.
• “Preventative cool-down rest” means a rest taken in a cool-down area to prevent overheating.
• “Radiant heat” means heat transmitted by electromagnetic waves, but not transmitted by conduction or convection. Sources of radiant heat include the sun, hot objects, hot liquids, hot surfaces and fire.
• “Relative humidity” means the amount of moisture in the air relative to the amount that would be present if the air was saturated.
• “Shielding” means a physical barrier between radiant heat sources and employees that reduces the transmission of radiant heat.