Protection and comfort. When selecting protective garments, these are the two key garment characteristics purchasers consider to determine the best suit for the job. The majority of apparel purchasers are most concerned with protection. Because if the garment doesn't protect the wearer from hazards in the work environment, it won't matter how comfortable the garment is...it will be dangerous.
With all protection characteristics being equal, however, the garment's comfort becomes a key factor in gaining wearer compliance and in guarding against heat stress injuries. Because, again, a garment that is modified to be more comfortable, or a garment that is not worn because it is uncomfortable, creates a dangerous situation.
There are numerous methods for measuring the protection a garment offers, but the issue of comfort has been more difficult to measure quantitatively. Some researchers are now using a new method for measuring garment comfort. This method measures the micro-environment inside the protective suit, assessing the humidity and temperature of the thin layer of air that closely surrounds the body. The micro-environment is a more useful indicator of how a user perceives comfort when wearing a suit than other analytical methods traditionally used to study the material's breathability.
Traditional Garment Comfort Measurements
Material comfort characteristics are typically measured via two methods. "Air Permeability "(ASTM D737) measures the rate and volume of air flow through a fabric. Under controlled conditions, a suction fan draws air through a known area of fabric. The results are reported in cubic feet per minute (the volume of air that passes through the fabric over the duration of the test). "Moisture Vapor Transmission Rate," MVTR, (ASTM E96-80) determines the rate of water vapor (i.e., evaporated perspiration) movement through a fabric sample. A container of liquid is covered with a sample of fabric. The container is put into an oven for 24 hours at a specified temperature and then measured to see how much liquid has evaporated. The MVTR is measured in grams per square meter per a 24-hour period (g/m2/24h). Figure 1 shows typical air permeability and MVTR measurements for two common apparel fabrics.
While these material tests provide useful data for garment purchasers, they are conducted on fabric swatches in controlled laboratory settings, which is quite different from mobile workers in work situations wearing a complete, functioning garment.
However, some tests involving human subjects do exist to help bridge the gap between material testing measurements and human measurements. Traditionally, testing garment performance properties on human subjects requires measuring core temperature (the definitive measure of the body's response to heat stress) and heart rate (which indicates the amount of overall body strain). These measurements indicate how long workers can remain safe and productive in their protective garment, but don't necessarily indicate how the wearers feel while they work.
Figure 2 provides an example of core temperature measurements taken from human test subjects. It is important to note, when looking at the lower mean core temperature of Garment A compared with Garment B, that the entire range of safe core temperature is only 2.0o C (37-39o C). If the trend seen with Garment A continued over a full work day, the advantages could be substantial in terms of reduced heat stress, resulting in reduced heat strain for the worker.
Heart rate is also monitored, since lower heart rates indicate reduced cardiovascular strain, which means less overall body strain, for workers and a greater reserve capacity for handling emergencies and complex tasks. Figure 3 shows an example of actual average heart rates when wearing two different types of protective garments. Again, if the trend with Garment A continues throughout a full work day, one would expect to see the worker functioning optimally over longer periods of time.
Better Indicator of Comfort
The micro-environment (conditions inside the suit) can be substantially different from the ambient or work environment. In fact, conditions inside the suit can be 10o C WBGT above the ambient work environment if the wearer is working hard and producing high metabolic heat. Essentially, the micro-environment inside the suit is the only environment the body experiences, regardless of the environmental surroundings. It is therefore a more accurate indicator of comfort than other environmental measurements.
The study of the micro-environment focuses on two things: the water vapor pressure between the suit and the skin (micro-humidity), and the mean skin temperature. Under semi-permeable or impermeable suits, macro-environmental (outside the suit) air movement makes only a small impact on the micro-environmental conditions affecting heat stress. Humidity in the macro-environment (outside the suit) also does not contribute a great deal to heat stress.
The body's primary cooling mechanism, sweat evaporation, is controlled by micro-humidity and temperature. Lower micro-humidity means that more sweat can be evaporated from the skin and workers will remain cooler longer, increasing productivity and safety. And, since the micro-humidity represents a continuous condition, even reducing the humidity slightly can induce greater evaporative cooling, and thus, greater comfort. A more permeable suit material (as indicated by high air permeability and MVTR measurements) will support more sweat evaporation, providing a mechanism to reduce micro-environmental humidity and increase wearer comfort and safety. Figure 4 provides an example of how human test subjects in two commonly available protective suits were measured for micro-humidity. The early advantage (slower rise) in Garment A helps keep overall heat stress (core temperatures) lower throughout the duration of the exercise.
Mean skin temperature is a secondary indicator of heat strain, but is even more important as a comfort factor, since the body "feels" temperature through the skin-to-air temperature contrast. A lower mean skin temperature for Garment A suggests a greater heat loss at the skin surface, attributable to the lower micro-humidity and concurrent evaporative cooling described above. Workers with lower skin temperatures feel more comfortable.
The micro-environment is dynamic, yet reaches a near plateau by 30-40 minutes. It is important to note that the heat stress that workers experience does change over time when wearing a protective garment. Therefore, it is important for apparel purchasers to review the comfort-related performance of garments at various points during the work shift. Testing on human subjects should show performance throughout a minimum 1.5-hour work bout.
Other tips to help purchasers make a fair comparison among different garments include:
- Full garment testing should supplement material bench testing to obtain a true evaluation of comfort. Tests of the micro-environment are a very good way to characterize comfort in addition to traditional tests.
- Full garment testing should be conducted in realistic work conditions (hot temperatures, high humidity as indicated by high WBGT measurements) comparable to field conditions. Since heat stress is not a major issue in cool environments, testing should occur in warm to hot conditions.
- Physiological monitoring must be conducted to measure the heat strain experienced, regardless of self-reported comfort.
- Fit is also a key comfort factor. Look for designs that fit the great variety of body shapes seen in the workforce.
In conclusion, when choosing safety apparel based on comfort and safety, micro-environmental test results can help you bridge the gap between material characteristics and human comfort characteristics. A garment with superior micro-environmental characteristics will minimize the potential for heat stress and result in higher productivity.
Kimberly Dennis is a research scientist with Kimberly-Clark Safety Division, Alpharetta, Ga. Dr. Phillip Bishop is a professor and director of the Human Performance Laboratory at the University of Alabama.