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Understanding the Heat Burden While Wearing Personal Protective Clothing
Abstract
Environmental factors (e.g., humidity, wind, temperature, radiant heat, clothing, and workload i.e., metabolic rate) are considered when determining if a heat risk is present indoors or outdoors. There is little doubt that heat stress affects many workers adversely and the additional heat load from protective clothing adds to the risk. The ACGIH Threshold Limit Value TLVs and the National Institute for Occupational Safety and Health (NIOSH) Criteria Document Recommended Exposure Levels (RELs) for heat stress are two guiding documents to evaluate heat stress. Adjustment factors have been evaluated to reflect the change in heat stress imposed by different clothing ensembles. While some Clothing Adjustment Factors (CAFs) were calculated with limited experimental data and some professional judgment, heat balance methods in the laboratory have yielded better estimates for a wider selection of clothing ensembles. These experiments provide the starting point to evaluate work clothing and personal protective clothing in heat balance evaluations based on sweat and heart rate, blood pressure and skin temperature. Proposed CAFs in the ACGIH TLVs and NIOSH REL provide a framework to rethink a corrected Wet Bulb Globe Thermometer (WBGT) measurement based on the work activity, environment, human physiology, frequency and duration of the exposure.
Introduction
Anyone who has worked in industry indoors while wearing personal protective clothing knows about the additional heat load during the summertime or working outdoors in a hot indoor environment. The National Institute for Occupational Safety and Health (NIOSH) and other researchers looked at the increased risk of heat stress while wearing different types of personal protective equipment (PPE). Wearing PPE can often increase the risk for heat-related illnesses by increasing blood pressure and core body temperature (i.e., internal temperature) more quickly than wearing other types of loose fitting PPE in the same work environment. Some information is discussed in the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values documentation for Heat Stress and Strain but does not address the additional metabolic heat load from wearing PPE and the need to correct environmental measurements.
Wearing PPE reduces the body’s normal way of getting rid of heat by sweating and evaporation. Moisture from sweating may not be wicked away and the radiant heat may not be released by convection. It may trap excess heat and moisture inside, making the worker’s body even hotter. Furthermore, it can increase the physical effort to perform work tasks and job duties while carrying the extra weight of the PPE and lead to heat stress faster (e.g., working muscle increases body heat production). While this information may seem intuitive on the surface, the issue is not fully realized by stakeholders developing standards and providing guidance to protect workers against heat stress. Nor are the controls and adjustments made when evaluating the work environment to reduce the additional heat load by clothing ensembles.
Responding to hazmat incident or a fire with PPE in the summer are some of the worst heat transfer scenarios due to the inability to use evaporative cooling, which is the primary means heat being released from the body. Wearing PPE in extreme heat like structural firefighters can become dangerous and results in degradation of the effectiveness of individual performance and, if left unchecked, may lead quickly to heat stress and physiological strain, including heat stroke and death. There are many more indoor and outdoor work environments where workers need to wear their PPE to prevent other indoor concerns like foundries and smelters. Wearing PPE in conjunction with heat loads can be detrimental to workers depending on exposure, especially if they are unacclimated to hot work environments or compromised by illness or disease or taking prescription medication.
Assessing the Risk
Heat index is often referred to as humiture, which is a measure of discomfort due to the combined measurement of temperature and humidity, rather than the actual air temperature. For example, an air temperature of 83°F with a relative humidity of 70% would result in an estimated 88°F perceived heat index. The difference in perceived and actual temperature is based on a combination of air temperature, relative humidity, and wind speed. The risk also depends on the type of work task that includes travel distances and work speed for walking, lifting, hiking and climbing, and activities associated shoveling, digging, transporting, pushing, pulling, carrying and other labor intensive work tasks that increase the metabolic rate. Continuous work or work done with little rest, acclimation, and hydration also increases the relative risk of heat stress. The effect of body weight on metabolic rate can be determined by multiplying a ratio of a person’s actual body weight to a standard 70 kg (154 lbs.) person.
The perception of heat is subjective, and it can be affected by various factors such as menopause, pregnancy, and the effects of drugs or withdrawal, as well as differences in hydration, body shape, and metabolism. A higher relative humidity affects normal body cooling by reducing the rate of evaporation of sweat. The human body cools itself through perspiration, where heat is removed from the body as a result of convention of the heat from the skin and the evaporation of sweat. A lower rate of evaporation subsequently lowers the rate at which the body cools, increasing the perception of heat and discomfort. The perception of heat stress is what the heat index seeks to measure, and while it can technically be used indoors, it is most often used in reference to the outside temperature with a solar load from infrared radiation. Heat index is a screening tool that uses temperature and relative humidity to calculate an adjusted temperature, representing how the conditions feel more accurately than just the ambient temperature alone.
The Wet Bulb Globe Temperature Index (WBGT) and exposure guidelines from the ACGIH TLVs and NIOSH RELs are used to protect workers. Small businesses that do not have access to instruments or standards can measure the heat index by using the OSHA-NIOSH Heat Safety Tool App or the NOAA heat index chart. These applications can help better understand the health risk but these tools fail to consider wind speed and solar load from the sun outdoors and corrections using different fabrics and clothing ensembles. Clothing Adjustment Factors (CAFs) are needed to determine the corrected-WBGT which can vary throughout any given workday.
Heat wave warnings from local weather forecasts can help to prevent heat-related illnesses. The benefit for occupational health is limited due to the fact that factors like heat sources, different workplace microclimates, reflected infrared radiation sources, endogenous heat production, PPE, etc. are not taken into account for the actual health effect on workers. Heat wave warnings are based on large geographical areas so they are not suitable for workers in every workplace.1
Additional Heat Load Wearing PPE
Clothing insulation and evaporative resistance hinders heat transfer through convection, radiation and evaporation from the body to the ambient environment, leading to increased heat strain. In the presence of sunshine (solar heat load), clothing should be lightly colored and loosely fitted to the body to better reflect rather than absorb solar infrared radiation and facilitate convective and evaporative heat exchange.
With powerful radiant heat sources, flames and hot liquid or molten metal, protective clothing with several layers (e.g. reflective layer and outer shell against flames) are required to protect workers in order to avoid burn injuries. When a heat exposure is unavoidable and the work must be done, personal and microclimate cooling measures may be the only way to alleviate heat strain, improve thermal comfort, protect health and maintain productivity. Moreover, wearable and movable personal microclimate cooling (fans), in contrast to air conditioning for an entire room or building, can increase energy efficiency and help to cope with heat waves in areas where air conditioning is not commonly used, such as in the Nordic or developing countries.
In an effort to understand the effects of protective clothing and heat stress, heat balance equations provide a framework to evaluate the heat stress in the workplace based on laboratory evaluation of clothing ensembles. The rationale for assigning CAFs to correct the WBGT measurements has been studied along with how data might be further developed for a broader range of fabrics and sweat rate. While limited in scope, CAFs should be used to evaluate the real heat stress for workers in hot environments. The data doesn’t cover all types of PPE worn in various industries or while performing various types of work tasks or activities. If CAFs are available, they should be used to recalculate the corrected WBGT when being used throughout the workday.
Research and Standards to Evaluate PPE
In 1999, Thomas Bernard studied the emerging health concerns of heat stress while wearing PPE. CAFs reflect the change in heat stress imposed by different clothing ensembles. While the first proposed values started with limited experimental data and professional judgment, heat balance methods in the laboratory have yielded better estimates of CAFs for selected clothing ensembles. The experiments provided a reference point to account for other nonporous clothing in heat balance evaluation based on sweat and heart rate, blood pressure and metabolic heat load. CAFs for firefighter turnout gear and encapsulating Level A suits worn by hazardous material emergency responders can be significant.
The International Standards Organization (ISO) 9920:2007 specifies methods to estimate the thermal characteristics (resistance to dry heat loss and evaporative heat loss) in steady-state conditions for a clothing ensemble based on values for known garments and textiles. The standard examines the influence of body movement and air penetration on the thermal insulation and water vapors resistance. It does not deal with other effects of clothing, such as adsorption of water, buffering or tactile comfort or take into account the influence of thermal characteristics, consider special protective clothing (water-cooled or ventilated suits). The standard does not consider the impact of work load or intensity while using other types of PPE.
Besides ISO 9920:2007, there are several test methods available to measure or estimate clothing items or their combination for insulation. These include American Standard Test Method (ASTM) F12917, ISO 158318), ISO 99209). ISO 99209) databases for clothing ensembles to calculate effect of wind and motion. However, it does not account for many effects using multiple layers of clothing, clothing fit and other factors. Some of these effects may counterbalance or negatively accentuate each other. There is no consideration for different clothing styles or clothing commonly associated with workwear.
It would be labor intensive to test all manufactured PPE ensembles but a summary table by the manufacture would be very useful. The information could be utilized to evaluate of human thermal environments and apply standards for ISO 793314 Predicted Heat Strain (PHS), ISO 773015 Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfaction (PPD) to evaluate stress, comfort and/or thermal climate. Some standards evaluate work exposure by combining environmental and clothing parameters and human activity levels on thermo-physiological basis to predict thermal stress and estimate exposure. Mobile apps help to make risk decisions to plan and cope with unfavorable climate conditions. Even the recent version of ISO 724320, the WBGT includes a limited table of clothing ensembles and CAFs based on the exposure measurement.
NIOSH found that different types of PPE were linked to increases in heart rate, blood pressure and core body temperature during physical exertion. The first test ensemble included a face shield and a fluid-resistant surgical gown for healthcare workers. Other forms of PPE included goggles, coveralls and highly fluid-resistant suits, with and without a hood, and a surgical mask or a NIOSH approved N95 respirator. Participants wore each ensemble over their standard medical scrubs.
Compared to participants wearing the first ensemble, workers wearing the other ensembles had significantly higher heart rates, blood pressure and core body temperatures after exercising on a treadmill. Those participants wearing the second and third ensembles reported feeling hotter and more tired. These findings underscore the need to train workers in proper application of CAFs to prevent heat stress. Besides training, it is important to balance work in hot conditions with adequate rest using heat-prevention strategies such as outdoor shade, air conditioning, cooling vest and portable fans.
The ACGIH TLV for Heat Stress and Strain uses a table of selected PPE in combination with the WBGT values to determine the additional heat load. CAFs are added to the WBGT indices to formulate a corrected WBGT determination [Coles, 1997]. CAFs are not intended to be used for totally encapsulating suits or Level A PPE. Workers wearing Level A PPE should be monitored for their sweat or heart rate and blood pressure [Paull and Rosenthal, 1987] or their skin or core body temperature as proposed by Pandolf and Goldman (1978). CAFs should not be used for multiple layers of PPE. In the case where a construction worker is wearing a lightly colored shirt underneath coveralls, the use of coveralls would not be considered an additional layer of clothing. There are no CAFs for wearing long sleeve shirts and pants or coveralls. Double layer woven clothing would add 3 °C to the WBGT Index (°C) as opposed to 0.5 °C CAF for polypropylene coveralls and 1 °C for polyolefin coveralls. ACGIH notes 11 °C CAF for workers with limited-use vapor barrier clothing.
Other researchers looked at the ACGIH TLV for heat stress and the upper limit of the WBGT for ordinary work clothes and CAFs for other clothing ensembles. A study aimed to determine the CAF for four clothing ensembles (cotton coveralls, Tyvek™ 1424 coveralls, NexGen™ coveralls, and Tychem® QC coveralls) against a baseline of cotton work clothes and to determine what effect relative humidity. When skin and core body temperature began to increase, this was defined as an inflection point. In this study, environmental temperature was recorded 5 minutes before the inflection point to calculate the WBGT for each ensemble. A three-way analysis of variance with different clothing ensembles, humidity interactions and a multiple comparison test were compared. Only the vapor-barrier ensemble (Tychem® QC) demonstrated a significant interaction with rising humidity levels. CAFs were proposed for cotton coveralls (0 °C), Tyvek™ 1424 coveralls (+1 °C), NexGen™ Coveralls (+2 °C), and Tychem® QC Coveralls (+10 °C) to more accurately determine the actual heat load and corrected WBGT.
Ashley and Bernard also looked at Tychem® QC, polyethylene-coated Tyvek™ with and without hoods and flame-retardant (FR) fabrics. The difference suggested a CAF of 1 °C for hoods. There were no significant differences of CAFs among other FR ensembles and cotton work cloths. Cortés-Vizcaíno and Thomas Bernard explored effects on heat stress from FR clothing used by aluminum smelters (Zirpo wool shirt and FR8 denim pants) with typical cotton work clothing. During each simulation, heart rate, blood pressure, skin and core body temperature were continuously monitored and recorded every 5 minutes. After achieving a physiological steady state, temperature and humidity were increased to maintain a relative humidity of 50%. The climatic conditions at the inflection point were used to assign a critical WBGT. A three-way analysis of variance examined the effects on WBGT, clothing, work level, subjects, and interaction between clothing and work activity. There were no significant findings. The authors concluded that there was no difference in heat stress between either clothing ensembles under similar environmental and work conditions and metabolic rate.
The U.S. Navy Physiological Heat Exposure Limit (PHEL) curves and rational models for heat stress have not been examined for different clothing ensembles other than a work uniform nor do the PHEL curves consider time as a risk factor for exposures above the WBGT. Moreover, the PHEL charts were not evaluated for clothing ensembles used for firefighting and chemical, biological, radiological, nuclear and environmental CBRNE) warfare. Short-term exposure limits were studied only for exposure to heat stress levels above environmental exposure guidelines.
Twelve participants were studied by the U.S. Navy at a moderate to heavy metabolic rate (380 W) while wearing three different clothing ensembles. CAFs for work clothes (0 °C), NexGen microporous coveralls (2.5 °C), and vapor-barrier coveralls (6.5 °C) and CAFs of 7.0, 8.0, 9.5, 11.5 and 15.0 °C) for CBRNE ensembles. Combinations of metabolic rate, clothing, and environment were selected so participants would reach a physiological limit in less than 120 min. WBGT-based CAFs were used to analyze the different clothing ensembles. No significant differences were reported, suggesting that some CAFs can be used in WBGT-based time limited exposures under 120 minutes.
In the construction industry, work clothing can protect against exposure to the sun can limit or prevent sweat evaporation and convective cooling [Davis and Bishop 2013]. In general, the thicker or less permeable the clothing, the more it hampers heat exchange [NIOSH 2016]. Cool, dry air moving freely over exposed skin effectively removes or dissipates heat from the body. Some types of clothing materials can interfere with cooling, even in cooler ambient conditions [ACGIH 2017].
Work clothing and PPE like impermeable coveralls or suits, can reduce sweat evaporation, and clothing and PPE can create microenvironments that trap heat and moisture from sweating close to the skin. When evaporation of heat from sweating is impeded, it can increase heart rate, blood pressure, skin and core body temperature. Adjustments to exposure limits and work-rest schedules are recommended based on the type of clothing and/or PPE worn based on the physical demands of the work activity performed. Employers must address the additional metabolic heat and the need to take appropriate steps to reduce the risk during daily work routine at their worksites. NIOSH has encouraged construction workers to wear clothing made of breathable fabrics like cotton and avoid wearing non-breathable synthetic fabrics [NIOSH 2010].
PPE to Protect Against Heat Exposures
Wearable PPE that protects against heat exposures are called auxiliary cooling systems or personal cooling systems (e.g., water-cooled or air-cooled garments, ice vests, and wetted overgarments) and can range in simplicity, cost, and maintenance. Ice vests are cheap, but their temperature cannot be controlled and they often do not stay cool long enough to be practical. If the air cooling system is too cold, this will result in reduced heat transfer from the body to the environment. Water-cooled garments require the worker be tethered to a system that circulates the cool water, which limits the person’s range of operation. Many wearable personal cooling systems are too heavy or too cumbersome to be practical in a work environment.
Portable air fans that clip to a worker’s belt or placed around the neck area are being marketed to improve thermal comfort by convection and evaporation. Forced air is blown by the fan under the work clothes or around the neck to reduce skin temperature and help evaporate moisture from sweating. There is no scientific evidence to support the efficacy but the concept is intriguing. More scientific research should be done to evaluate this new technology to lower the core body temperature.
Newer technologies allow workers to simply wet cooling fabric with water, wring it out, wave it or snap it in the air a few times, and the fabric cools down below elevated skin temperature. These fabrics stay cool for several hours and can be reactivated again and again during the workday. Other options like skull caps for under hard hats, bandanas, neck gaiters, and wet towels also offer temporary relief. Clothing selection is critical for some large job sites and efforts should be made to evaluate various fabrics and clothing ensembles before purchase.