Wet Bulb Globe Temperature more directly measures temperature as it relates to a person’s risk of heat illness by factoring the effects of air temperature, relative humidity, wind velocity and radiant heat.  Wet Bulb Globe Temperature, WBGT, is the weighted of average of Wet Bulb, Black Globe and Air Temperature in which WBGT = 0.7Tw + 0.2Tg + 0.1Td.

Employers with workers who are at risk of heat illness are required by OSHA to establish a complete heat illness prevention program.  Pursuant to such a program, an employer must have an adequate means by which they can measure and monitor heat stress factors.
OSHA has published guidance documents, such as the 42-page “Using the Heat Index: A Guide for Employers;” however, the specific regulation under which heat stress is governed is OSHA 29 U.S.C. § 654, 5(a)1.

“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.”

3,442 people died from heat-related illness in the US from 1999 to 2003, and from 1992 to 2006, 423 deaths were US workers.  In 2014 alone, 2,630 workers suffered from heat illness and 18 died [US Department of Labor].

Between 2012 and 2013, the heat illnesses of 20 people, 13 of whom died, resulted in federal enforcement citations. 13 of the 20 employers were found to have not incorporated an approach to identifying heat illness risk (e.g., heat index) into their heat illness prevention program [CDC].

Heat illness is caused by uncompensible heat stress.  This occurs when the rate of heat generated exceeds the rate at which it is dissipated.  “Combustion” of oxygen and adenosine triphosphate is the primary means by which the human body converts chemical energy into kinetic energy.  As much as 80%, however, is converted into heat.  In a similar manner, when oxygen and gasoline combust in an engine, the majority of energy is also converted into heat.

When performing office work, the human body generates the power of two 60 Watt light-bulbs (which impatience when replacing one reminds us that they can get rather hot).  However, while performing heavy construction, the body may generate as much as 500 Watts.  If the heat stress is uncompensible, then core body temperatures rises, and heat illness develops.

Exertional heat stroke is a deadly category of heat illness in which body temperature exceeds 104 °F.  The person may first experience headache, dry skin, increased respiration, increased heart rate, nausea, confusion and dizziness.

Among many other factors, environmental conditions such as temperature, sunlight, wind and humidity dictate how well the body can dissipate heat.   Wet Bulb Global Temperature more specifically measures temperature as it relates to these environmental conditions.

Air temperature, typically denoted as Td, TA or simply T, is an incomplete measure of environmental heat.  It measures the kinetic energy associated with the movement of the molecules of air.  This type of energy transfer occurs through conduction.   Air temperature measurements fail to capture heat transfer through vaporization, radiation and convection.

Wet Bulb temperature, denoted Tw, is a measure of temperature affecting the powerful evaporative cooling process.  Perspiration, and to a minimal extent respiration, is the way in which the human body dissipates heat through evaporative cooling.  It occurs due to the intermolecular force of water, known as the Van der Waals force.  The hydrogen atoms of water molecules are loosely attracted to the oxygen atoms of adjacent water molecules.  When evaporation occurs, the energy associated with this attraction must be overcome in order to separate the molecules.  This force, similar to pulling apart two magnets, is known as the enthalpy of vaporization.

Water has a very high enthalpy of vaporization of 15 kilocalories per ounce.  Theoretically, for every ounce of perspiration that evaporates, it carries with it 15 kilocalories of heat.   For conceptual reference, a loss of 15 kilocalories is enough to cool a 61 pounds of water by 1° Fahrenheit.

Humidity disables the evaporative process.  There is a maximum amount of water vapor that can occupy a given volume of air or vacuum.  This capacity is based upon the equilibrium vapor pressure of water.  The relative humidity is the percent value of partial vapor pressure compared to the equilibrium pressure at a given temperature.  In other words, relative humidity is the humidity compared to the humidity capacity.

Effectively, as the relative humidity increases, the evaporative rate decreases and the efficiency of evaporative cooling from sweat decreases.  Evaporative cooling cannot occur in relative humidity near 100%.

The official highest temperature on Earth was recorded in Death Valley, California in 1913.  The air temperature was 134° F.  Hypothetically, if the relative humidity were to have been a dry 13% on that day, then the Wet Bulb temperature would have been 85° F.

Compare this to the equivalent Wet Bulb temperature of 85° F on July 13, 1995 during the deadly heatwave in Chicago, that killed 739 people.  The air temperature was only 99° F.

Wet Bulb temperature is the temperature that factors for the reduction of evaporative cooling due to relative humidity.  A WBGT device also incorporates instrumentation to capture the heat acquired through radiant heat and partially captures heat loss through wind, known as Globe, Black Globe or Mean Radiant Temperature, usually denoted as Tg.

As the air temperature and Wet Bulb temperature measure heat energy transfer through conduction and vaporization, the Black Globe accounts for heat transfer through electromagnetic radiation and convection.

The human body radiates infrared radiation and reabsorbs radiation from the surroundings.  In typical surroundings, a net loss of about 100 Watts of body heat occurs via IR radiation.  However, in the presence of sunlight, hot asphalt or a furnace, the additional sources of electromagnetic radiation result in a significant net gain.

Black asphalt absorbs more than 90% of light energy from the sun, and it reemits infrared radiation.  According to the Stefan–Boltzmann Law, a 145° F parking lot emits 650 W/m2. Theoretically, 650 W/m2 is enough power to heat 100 pounds of water by 14° F in 1 hour (over 2 m2).

In direct sunlight and at sea level, a person may receive around twice this amount of radiation as infrared, visible and UV light (53, 42 and 4 percent, respectively).  The sunlight irradiance diminishes according to the angle due to geographical location, season and time of day.  The Black Globe measures this radiative effect.

Wind, however, counteracts the effects of heat accumulated through radiation via convection.  Since wind accelerates the heat exchange between air and the Black Globe, it serves to cool the globe as it would serve to cool the body.  However, on the body, wind additionally serves to accelerate the evaporative process.  Therefore, Black Globe temperature only partially captures the convective cooling effects of wind.  How to choose an adequate WBGT meter?  Vet the instrument and vet the manufacturer.

For a true WBGT meter, it is important to verify that the meter is equipped with a Black Globe parameter in addition to the Wet Bulb feature.  When reviewing specifications, remember that it is not just a matter of the humidity and temperature range; it is a matter of a range of humidity across the domain of temperature.  In other words, if the maximum Tw range is 120° F and the maximum Td range is 140° F, this could mean that in 5% RH conditions, the Wet Bulb temperature range is actually only 79° F and not 120° F.  Clarify this specification with the manufacturer.

A meter with a data-logging or wireless data transmission function is beneficial, as the data is useful with regulatory and accrediting bodies and is essential to validation and verification of your heat illness prevention program.

Confirm that the instrument comes with Certificate of Calibration that is NIST traceable according to ISO/IEC 17025 testing criteria.  Pursuantly, ensure that the instrument comes with a warranty and secure return policy.

Determine if the manufacturer has an established quality program.  In addition to accreditations and years in business, indications of a robust quality program include the ability of a manufacturer to answer scientific and regulatory questions.   A manufacturer should be prepared to promptly answer questions regarding function, quality, specifications, validation and regulation.  An additional measure of an established quality department is the capacity to perform calibrations, method development and proof of concept testing for their customers.