Implementing and managing a heat stress program can be a frustrating and demanding task for any safety and health professional. The dynamics of the heat balance process taking place in the body coupled with the dynamics of the workplace and the variations of jobs a worker faces throughout the shift, make the job complicated and challenging. One approach is to have a monitoring program that employees can perform themselves.

Lets face it, health and safety pros cannot constantly look over everyone's shoulder to ensure compliance with regulatory guidelines and company policies. The most effective programs put employees in charge. Careful program development, training and management support make this approach work.

Heat stress can cause serious medical injury, impair judgment, increase job stress and lead to low morale. The heat stress program outlined below provides employees with guidelines and recommendations to prevent adverse effects of high heat conditions. Each person should be aware of the adverse effects of high heat conditions, how the medications they may be taking exacerbate the effects, and the importance of proper dietary and physical conditioning.

The program, however, cannot take into account all factors contributing to heat stress and should be used only as a guideline.

Heat and our bodies

Our bodies strive to maintain a steady state temperature range of 98 degrees to 102 degrees F (37 degrees to 39 degrees C). Heat gains to our system from environmental and internal sources are balanced by losses to the environment. Heat exchange is the composite of four mechanisms in which the body can gain, lose or both. During the course of the day, the body dissipates approximately as much heat as it produces and absorbs to maintain its steady state temperature.

Evaporative Cooling = Metabolic Heat + Convective Heat + Radiant Heat EC = MH + CH + RH

Metabolic heat (MH) Metabolism is a sequence of chemical reactions that provides energy to working muscles. Some of the energy is used to contract muscles, the remainder is released as a by-product -- heat. The harder your muscles work, the more internal heat is generated. Generally, this is the most significant contributor to heat stress. The body dissipates this heat by circulation. As blood goes through the muscles, it absorbs heat. This warmed blood circulates to the skin where the heat leaves the blood through the skin's surface. The skin serves as the principle site for heat exchange with the environment.

Convective Heat (CH) Convection represents the rate of heat transfer between the air and the skin surface. Three factors influence the rate of transfer:

1. Temperature difference between the air and the skin -- this factor provides the gradient for the direction of heat exchange. The larger the gradient the greater the heat transfer. If the air temperature is higher than the skin temperature, the heat will transfer into the body. When the air temperature is cooler than the skin temperature, the heat will transfer to the environment.

2. Air velocity -- To a certain extent, the higher the air velocity, the greater the rate of heat transfer.

3. Heat transfer coefficient (which includes the insulation factor of clothing). The thicker or more impermeable the clothing layer, the lower the value of heat transfer to the environment.

Radiant Heat (RH) Radiant heat transfer or thermal radiation between the skin surface and the environment is a subtle heat exchange because the heat is from invisible infrared energy. Radiant heat exchange occurs between two solid bodies. Two factors affect the rate of radiant heat transfer:

1. Temperature difference between objects -- This determines the thermal gradient and direction of heat transfer.

2. Clothing insulation -- As in convection, the more skin is covered or the thicker or less permeable the clothing material, the less heat transfer. Also, if clothing has a reflective surface, the infrared radiation is reflected away and helps cooling.

Evaporative Cooling (EC) Evaporative cooling, which occurs due to sweat evaporation, is the primary mechanism for cooling the body. As sweat is secreted onto the skin, it evaporates into the air. The required rate of sweat evaporation is adjusted to balance the heat absorbed from the sum of metabolic heat, convective heat, and radiant heat. Evaporative cooling is controlled by physiological and environmental factors:

1. Physiological -- There is a limit to the amount of sweat a person can produce and the rate of cooling that the sweat can provide. This limit is about one liter per hour, up to a total of six liters per day.

2. Environmental -- clothing insulation, air velocity, and humidity all affect evaporation. If the humidity is high, the air cannot absorb moisture provided by sweat, if the humidity is low, then air can absorb more water vapor.

Setting up a self-directed heat stress management program

If you want to set up a self-directed heat stress program, consider the factors mentioned above.

1. Clothing reduces the effectiveness of sweating as a means of cooling the body. While each specific clothing ensemble has its own unique insulating and vapor permeation characteristics as shown in Table A, we can break down the combinations into five generic ensembles. (The number and combination of clothing ensemble will depend on your workplace.)

2. Metabolism is a major contributor to heat stress. Work performed by the muscles as various tasks are performed will be a significant consideration in your heat control program.