Winter is certainly here, when the annual battle to keep warm during the coldest months of the year can hit its (frozen) peaks. If you're lucky enough to work in a climate-controlled environment or in warmer temperatures, cold weather may seem like a foreign nuisance. But for thousands of us, managing the cold is a harsh reality.
When considering what to wear in the fight against icy temperatures, wind, and humidity, other workplace hazards must be accounted for. What keeps you warm also must keep you safe. In many work environments, exposure to a litany of hazards remains, whether it is cold outside or not. Yet protection from the bitter cold may not be ideal protection from direct flame, extreme heat, arc flash, molten metal, or other thermal hazards.
We are all aware that dressing in layers is the most effective method for keeping warm. But it is less common knowledge that layering has the same insulating effects for keeping heat trapped against your body when it is cold as it does in keeping high heat out during an extreme thermal event. Greater insulation is provided by air trapped between thinner or lighter layers when worn together than a single, heavier layer might provide. Lighter layers also allow for better mobility and flexibility.
The primary objective of outerwear in cold weather conditions is to provide warmth. However, in many sub-zero temperature industrial workplaces, there is also the potential for exposure to electric arc or a flash fire event. Whether it be a jacket, an insulated coverall, or bibs, the outermost layer of a PPE ensemble designed to defend against these threats must be flame resistant (FR). Where there is a possibility of an arc flash exposure, it also must be arc rated. If the outer layer is not FR and the outerwear is exposed to an electric arc or flash fire, that layer will ignite and burn.
In fact, in extreme conditions involving arc flash, flame, molten metal, and other thermal hazards, primary protection layers—even if they are FR—will eventually break down, allowing the hazard to penetrate to the inner apparel or to the body. In these dangerous situations, layering with a non-flammable base layer can provide additional, life-saving seconds to escape without serious injuries.
A Solid Foundation
A comfortable and highly protective base layer forms a solid foundation on top of which other secondary and primary protection layers can be added as barriers against specific dangers (such as aluminized coats for reflecting radiant heat, treated materials for shedding hot liquids, triple-layer arc hoods for protecting against arc flash, etc.). The layered approach can be adapted to any situation across multiple applications and industries, be it hot or cold.
When you are all layered up, however, you still need to be able to function and be comfortable. Layered PPE ensembles must allow for good maneuverability, fit, and styling relative to the job function and worker’s activity level. Apparel that is too heavy or thick restricts movement and can inhibit moisture management. Comfort is determined by evaluating the combination of the air permeability, breathability, weight, thickness, and warmth of a garment or combination of layers. Of course, comfort is subjective, but the industry has devised metrics to help standardize measures of comfort. When trying to determine comfort and effectiveness in cold weather, we can look to a garment's Clo Value and its Temperature Rating.
Clo Value is a measure of the thermal insulation of apparel and is defined as "the amount of insulation that allows a person at rest to maintain thermal equilibrium." Basically, a Clo Value indicates how warm a garment will keep you, and it is determined by measuring the dry heat transfer, evaporative heat transfer, and moisture management capabilities of a garment. It is scored on a scale, with a minimum Clo Value of zero. A Clo Value of zero is equivalent to a person not wearing any clothing. A Clo Value of 1 indicates an outfit that is comfortable in a normally ventilated room at 70 degrees F with less than 50 percent humidity in a sitting/resting position. The higher the Clo Value, the more thermal insulation a garment provides or the warmer a person will be.
The colder the environment, the higher the Clo Value that is needed to maintain warmth. But don’t be deceived by clothing weights: Lighter-weight apparel can have higher Clo ratings than heavier apparel.
While Clo Value can be used to compare the insulation properties of two different garments, this rating will not indicate the coldest outdoor temperature you can work in and still stay warm.
Temperature Rating specifically states the lowest temperature at which you can wear a garment and remain warm. The Temperature Rating can change, however, depending on the wearer's activity level. It is outlined in ASTM F2732, Standard Practice for Determining the Temperature Ratings for Cold Weather Protective Clothing.
The Temperature Rating takes into account the Metabolic Equivalent of Task, or MET. MET is an expression of the energy cost of a physical activity, which relates to the amount of internal body heat generated by the wearer’s activity level. ANSI defines 1 MET as "the amount of heat produced by a person at rest." A person who is walking slowly generally produces about 2 MET of heat, whereas a person performing at a moderate activity level, such as walking at a fast pace, can produce about 4 MET. As a person’s activity level increases, the amount of metabolic heat produced by the body increases, which helps keep you warm and means less insulation is needed to provide thermal comfort. The ASTM F2732 standard requires Temperature Rating to be stated at 2 MET and 4MET. Simply put, the harder you work, the less cold weather protection you will need, no matter the temperature outside. But the colder it is, the more comfort that can be gained by wearing protective clothing with higher Clo Values and optimal Temperature Ratings to keep warm. Inevitably, a PPE ensemble will likely need to be adjusted according to the activity or MET level of your task. This is what makes a layered solution, which includes a lightweight but warm non-flammable baselayer and FR outer layer, ideal.
Just as in the outdoors, outer layers and shells can be easily swapped out to accommodate changing weather conditions and activity levels, but the base layer almost always remains the same and is there to protect against all of the elements, whether hot or cold. Regardless of the temperature outside, you should always wear PPE that will keep you safe from the worst-case scenario of your work environment.