Sweat rate

The value used in ISO 7933, required sweat rate, SW is based on the heat balance equation (6.1). Assuming the heat storage is equal to 0, the necessary evaporation from the skin, > to ensure a heat balance is calculated as foil... 

These parameters are used to evaluate how stressful a given hot working environment is. Depending on the physiological limitations for factors such as sweat rate, total sweat loss, heat storage, and skin wettedness, which are listed in Table 6.8, it is possible to evaluate whether a given environment is acceptable for continuous work. The method also allows calculation of an acceptable working time. Detailed equations for the calculations can be found in the standard (ISO 7933). The relation between the operative temperature and for different... 

A computer program is provided for ease of calculation and efficient use of the standard. This rational method of assessing hot environments allows identification of the relative importance of different components of the thermal environment, and hence can be used in environmental design. The WBGT index is an empirical index, and it cannot be used to analyze the influence of the individual parameters. The required sweat rate (SW. ) has this capability, but lack of data may make it difficult to estimate the benefits of protective clothing. 

TABLE 6.9 Required Sweat Rate Index SW q W/m, and Wettedness (w q) as a Function of Clothing, Temperature, Air Speed, and Humidity at an Activity Level M Equal to 70 W/m ... 

P4SR Predicted 4-hour sweat rate. A scale used to predict the evaporation rate from a body under hot conditions. 

A normal diet contains a sufficient amount of both sodium and potassium. The body has specific mechanisms for regulating the Na ion concentration, which is important in the control of blood pressure (Chapter 22). However, there is less potassium in the normal diet and it may not always be sufficient (e.g. if the sweating rate is high). Fruit and fruit juices are good natural sources of this electrolyte. 

Vellar, O.D. (1968) Studies on sweat losses of nutrients. I. Iron content of whole body sweat and its association with other sweat constituents, serum iron levels, hematological indices, body surface area, and sweat rate. Scand. J. Clin. Lab. Invest. 21, 157-167,... 

Inuoe, Y. Schibasaki, M. Hirata, K. Araki, T. Relationship between skin blood flow and sweating rate, and age related regional differences. Eur. J. Appl. Physiol. 1998, 79, 17-23. 

A sweat rate of one liter per hour, as occurs with continuous moderate exercise, would lead to the loss of about 40 mmol of Na per hour. A 5-hour running marathon would result in the loss of 200 mmol of sodium. This loss represents depletion of about 12% of the sodium in the ECF. This loss does not, however, result in a drop in plasma sodium concentrations. Plasma sodium is maintained during prolonged exercise by the loss of plasma water, resulting in a drop in plasma volume. Experiments involving human subjects exercising on a stationary bicycle for 3 hours in a warm humid room suggested that about 10% of the water lost in sweat comes from plasma, 38% from interstitial fluid, and about 52% from intracellular fluids (Table 10.9). 

Specimen Weight and Collection Time To have a valid sweat testing result, determination of and adherence to a minimum sweat weight or volume are critical. The requirement for a minimum amount is physiological, to ensure an appropriate sweat rate and sweat electrolyte concentration. It is independent of the instrumentation used to measure sweat electrolytes. Unfortunately, this is poorly understood, leading to false-positive and false-negative sweat test results, which have a significant implication for patient care. Sweat electrolyte concentration is related to sweat rate. 

At low sweat rates, sweat electrolyte concentration decreases and the opportunity for sample evaporation is increased. To ensure a valid result, the average sweat rate should exceed Ig/mVmin. The minimum acceptable volume or weight depends on the size of the electrode and stimulation area, the type and size of collecting devices, and the length of time the sweat is collected. If a laboratory deviates from standard parameters, the minimum acceptable sweat volume or weight will change. The stimulation and/or collection requirement applies to each site independently, so insufficient samples must not be pooled for analysis. 

Unreliable methodology, technical errors, and errors in interpretation can all lead to erroneous sweat test results. Methods that do not quantitate sweat collected or do not have an established minimum sample volume or weight are subject to false-negative results because an adequate sweat rate cannot be ensured. 

It should be noted that the moisture contained in the clothing need not be only that which is collected by absorption. It is also possible in cold damp or extreme cold environments that sweat which is evaporated at the skin will recondense when it reaches colder layers of clothing. Alternatively the sweat rate may be so high that some of it will not evaporate from the skin. In nude man this drips off, but in clothed man it is blotted up by clothing to evaporate after sweating ceases. 

If the mean surface temperature is independent of metabolic rate, a heat exchange mechanism other than convection or radiation must be responsible for removing excess heat generated when a subject exercises. One s attention turns immediately to sweating, a phenomenon which has been studied in many laboratories. For subjects in equilibrium with the environment, the sweat rate is directly proportional to the total heat production rate. Perhaps this is expected because regulating central temperature by controlling the blood flow rate to skin, and, thus, the skin temperature is relatively ineffective. As the skin temperature rises, the difference between the central temperature and the skin temperature... 

The mechanism by which sweating is regulated has not been established with certainty. Many attempts have been made to correlate the sweat rate with the central and mean skin temperatures. Recently, Nadel, Bullard, and Stolwijk (6) proposed the following relation for steady-state conditions. 

Another unresolved question concerns the necessity for including nonthermal contributions in the equations which define sweat rate. This... 

Figure 10.4 (a) Sweat rate sensor developed at the University of Pisa during the BIOTEX project, (b) Humidity sensor integrated into textile pockets of the sensing device at different distances from the skin. 

Salvo, P., Di Francesco, F., Costanzo, D., Ferrari, C., TriveUa, M.G., De Rossi, D., 2010. A wearable sensor for measuring sweat rate. IEEE Sensors Journal 10 (10), 1557—1558. 

During exercise in the heat, sweat loss is the main mechanism to lose heat. Core temperature increases more when hypohydrated as compared to euhydrated subjects. Therefore, an international standard, ISO 7933, was based on the ratio between the amount of sweat that should be evaporated to stay in thermal equilibrium and the amount of sweat that can be evaporated maximally. This required sweat rate index (Ereq) standard is replaced with the Predictive Heat Strain Index developed by Malchaire (2006). This index is mainly used for the industry. Sometimes thermal strain is combined with thermal stress indicators in order to make an individual recommendation for performance limits (Epstein and Moran, 2006). 

Repeated exercise leads to functional adaptations in humans. The aerobic capacity, quantified using the maximum oxygen uptake, may increase to values over 61/min (Figure 7.2). This means that more than 2000 W of heat can be produced for a short period of time (for calculation of the heat balance, see Daanen et al., 2006 Astrand et al., 2003). This amount of produced heat has to be lost quickly if not, the core temperature will increase with 5 °C in 10 min. The mechanism to enhance sweating is strongly dependent on body core temperature when a certain threshold is surpassed, the sweat rate shows a sharp increase. 

These indicators are temperature, cardiovascular sh ain, and sweat rate. 

Human sweating is minimal (about 350ml/24h) under a subject-specific sweat threshold, but increases to values up to several liters per hour during exercise. The maximum sweat rate recorded is 3.7 1/h from Alberto Salazar (Armstrong et al., 1986). An important heat strain indicator is the sweat efficiency, defined as the ratio of evaporated versus produced sweat. In protective clothing, the sweat efficiency is low and heat strain is imminent. In windy, hot dry environments, sweat efficiency can become close to 100% and humans can cool efficiently. Subjects with sweat abnormalities such as hypohidrosis or anhidrosis are not able to produce sufficient sweat to cool and may have additional health risks in the heat. The sweat efficiency is the basis of the ISO standard 7933 (Malchaire, 2006). 

Sweating is the body s principal method for removing excess heat. Sweat consists of water and electrolytes (salts). An individual at rest and not under stress, sweats about one liter per day. The sweating rate for an individual under stress of heavy woric or high temperatures is about four liters in four hours. The body must replace water and electrolytes to prevent heat stress or sickness. This is why many people who try to walk out of stranded situations in a desert with only a few liters of water fail and die. They do not realize how profusely they will sweat and how dehydrated they will become. In the pages that follow, we will take a closer look at temperature hazards the process technician may be exposed to in the processing industry. 

Smith, C. J., 2009. Regional sweat rates in humans, PhD Thesis, Loughborough University, Loughborough, UK. 

Exposure to extreme environments continuously may require medical monitoring. One aspect of such monitoring is continuously measuring the deep body temperature. There must be someone in place who can get a worker out of the extreme condition if the deep body temperature approaches or reaches 38 °C. In addition, medical monitoring may involve continuous measurement of heart and respiratory rates, and sweat rate. Visual observation may be part of medical monitoring. A worker in an extreme environment should have rescue and recovery equipment. There must be an emergency plan that includes a rescue from danger. 

Predicted four hour sweat rate (P4SR) can be obtained from charts and takes into account work rate and clothing. 

P4SR Predicted four hour sweat rate... 

There are two other indices called the Fangers PMV (Predicted Mean Comfort Vote) and the P4SR (predicted 4 hour sweat rate). 

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