DEFINITIONS

Acclimatisation

It has been well documented that when people work in a hot environment for the first time, their work performance is reduced, their heart rate and core temperature increases, and they are more prone to heat disorders, such as heat syncope and heat exhaustion and the far more serious condition of heat stroke. Acclimatisation advantages a person by:

The duration of time it takes to bring about these adaptive changes varies depending on whether the person is active in the heat and the duration of heat exposure. It is generally thought that 4-5 days is sufficient to acquire a good percentage of the advantageous adaptation however, to fully acclimatise takes at least 14 days exposure.

A new worker may require work hardening and/or acclimatisation prior to commencing a physically demanding job in a hostile climate; recognition of this must be incorporated into heat management protocols.

This information regarding Acclimatisation was obtained from the Point Health Web site.

Air Cooling Power

Is the cooling power of the air on a sweating person, measured in watts per square meter of body surface area. Clothing has a major impact on the ability of the body to cool itself via sweating as the following Air Cooling Power calculations shows:

Environmental conditions: 28º C wet bulb air temperature, air speed 1 m/s:

ACP for unclothed body         313 W/m2

ACP for lightly clothed body    168 W/m2

ACP for heavily clothed body   127 W/m2

Key issues for clothing include: fabric vapour permeability and conductivity, clothing design (ventilation, e.g. “bagginess”), amount of clothing and type of personal protective equipment. For example. Wearing leather boots is better than impermeable rubber boots.

Dew Point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapour to condense into water. The condensed water is called dew. The dew point is a saturation temperature.

The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.

Dry-Bulb Temperature

The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.

Mean Radiant Temperature (MRT)

Is defined as the uniform temperature of a surrounding surface giving off blackbody radiation (emissivity e = 1) which results in the same radiation energy gain on a human body as the prevailing radiation fluxes which are usually very varied under open space conditions.

MRT is the most important parameter governing human energy balance, especially on hot sunny days. MRT also has the strongest influence on thermo physiological comfort indexes such as PET (Physiological Equivalent Temperature) or PMV (Predicted Mean Vote) which are derived from heat exchange models.

Another definition of MRT is the MRT of non-uniform environment (e.g., walls, overhead, deck and objects of different emissivity at different temperatures) is defined as the temperature of a uniform black enclosure in which a solid body or an occupant would exchange the same amount of radiant heat as in the given non-uniform environment. It is estimated from dry-bulb and globe temperatures and air movement and is useful in determining radiative heat transfer (net gain or loss) relative to humans.

Relative Humidity

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapour in the mixture to the saturated vapour pressure of water at a prescribed temperature. Relative humidity is expressed as a percentage and is defined in the following manner.

RH Formula Where:

RH is the relative humidity of the mixture being considered;

P(H2O) is the partial pressure of water vapour in the mixture; and

P*(H2O) is the saturated vapour pressure of water at the temperature of the mixture.

Thermal Work Limit

Thermal Work Limit is a heat stress index designed primarily for self-paced workers. It represents the limiting sustainable metabolic rate that a well hydrated, acclimatised worker can maintain in a specific thermal environment within safe limits of core body temperature and sweat rate.

Wet Bulb Temperature (WB)

The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapour, usually air and water vapour.

For air that is less than saturated (100 percent relative humidity), the wet-bulb temperature is lower than the dry-bulb temperature; and the dew point temperature is less than the wet-bulb temperature.

Cooling of the human body through perspiration is inhibited as the wet-bulb temperature (and relative humidity) of the surrounding air increases.

Wet Bulb Globe Temperature (WBGT)

The wet bulb globe temperature (WBGT) is a calculated temperature that was developed in the late 1950’s for the US Marine Corps Recruit Depot on Parris Island South Carolina, to minimize heat stress during training. The index is calculated by measuring the natural wet bulb, globe temperature and dry bulb temperature. The calculated WBGT values are compared to reference values corresponding to different metabolic workloads to establish if the environment is excessive given the task being performed. Should the environment be excessive a work/rest cycling can be implemented for work to continue. The value is a time weighted average (TWA) based on the different tasks a particular person may perform over the shift. Some of the benefits of the WBGT are; it is relatively easy to measure, the instrumentation is simple and not excessively expensive. WBGT values are now used in the ISO 7243 standard and by NIOSH to set work limits.