Heat Balance
Your body works constantly to keep your microclimate balanced. The amount of heat your body produces must match the amount it loses, or else it reacts: get too hot, you sweat; get too cold, you shiver. You can help your body maintain its temperature balance by adjusting your gear (removing or adding layers, opening and closing garments or vents) whenever possible. There are three main processes by which heat is transferred through clothing
Evaporation
Being dry equals being warm — it’s that simple. When you become overheated, you begin to sweat. As your sweat evaporates, your skin cools down. However, if your clothes are not breathable, the moisture vapor from your sweat cannot escape. This causes the sweat to stay on your skin, which makes your feel damp and uncomfortable.
Conduction
Sit on the cold ground in the winter, and you immediately feel the effects of conduction — the process of a cold object transferring heat away from a warmer one. Conduction happens very fast; in fact, water transfers heat up to 23 times faster than air. Cold moisture can be from rain or snow on the outside or sweat on the inside, which explains why it is just as important for a fabric to be breathable as it is to be waterproof.
Convection
Better known as windchill, convection occurs when air moves through your gear. As cold wind blows through your clothing, gloves, or boots, it robs your body of the warmth in your microclimate. This is why fabrics that are windproof help you feel warm on windy days, even though you may not have a lining in your jacket.
You can’t always control the environment you operate in, but you can reduce its effect on your performance. Your environment may change from day to day — hot and sunny one day, raining the next, and snowing the next. In addition, you need protection from the real hazards you may encounter on the job — hazards such as fire, fuel, chemicals, or blood and body fluids. Your best defense is a clothing system made up of specific components that combine durable protection and comfort.
Activities
Keeping your body’s temperature in balance while active is extremely important. Your personal level of comfort can be affected by your exertion level, physical condition, and perception.
Exertion Level
As you move around, your body produces heat. Excess heat makes you sweat, which creates moisture that your gear absorbs. Wet gear causes you to lose heat 23 times faster than dry gear.
Physical Condition
If you are physically fit, you will experience less discomfort and heat stress. However, if you are not in good condition, you will become uncomfortable more quickly at a similar activity level. Your physical response can affect your mental state and cause distress.
Perception
Two people in the same place and identically dressed can have very different perceptions of the temperature — as much as 10°F apart.
Gear
What you wear from head to toe influences how you perform. To stay comfortable and dry, your gear must protect you from water, wind, and other external liquids. At the same time, it must also allow your perspiration vapor to escape, so you don’t feel the effects of heat stress. Otherwise, your focus may shift because significant heat stress can lead to disorientation and impaired judgment.
Your garments, boots, and gloves must fit well and complement the rest of your equipment. For example, your jacket should be abrasion-resistant to withstand strap friction from a heavy pack, your gloves should be dexterous to allow you to operate your equipment easily, and your boots should be light weight so they don’t compromise your endurance.
Clothing
Wearing layers is often the best way to balance heat and remain protected. Typical clothing systems (excluding the unique layering required for firefighters’ turnout gear) consist of three basic layers
Base Layer
Your base layer — worn directly next to your skin — draws moisture away from your body to prevent the conductive heat loss that can cause you to feel clammy and cold. Good wicking fabrics are nonabsorbent synthetic fibers that insulate while drawing liquid away from the skin. Most natural fibers like cotton and silk do not make good base layers because they retain moisture. Wicking and breathability have different purposes. Wicking fabrics physically move liquid away from your skin, while breathable fabrics allow moisture vapor to pass through and escape from your clothing system. Both work to keep you dry, but in different ways
Midlayer
Worn over the base layer, your midlayer traps air and prevents heat from escaping your microclimate. This layer should fit snugly, or air will be lost each time you move. Good options for the midlayer include synthetic and woolen fabrics because they don’t soak up much water; at the same time, they retain much of their insulating ability even when wet. Firefighters’ turnout gear has a thermal barrier that acts as an insulating layer to keep heat out.
Outer Layer
The outer layer protects you and all the other layers of your clothing system. In addition to efficiently blocking the wind, water, and sun, the outer layer can provide protection against contaminants, such as blood, body fluids, and chemicals. With some fabrics, this layer may also provide protection against heat and flames.
Footwear
Your feet are typically the first part of your body to encounter the harsh elements of your environment — water, snow, mud, or chemicals. Each foot perspires roughly one-quarter of a cup of moisture per day while at rest. If you are active or in hot weather, each foot can sweat up to one full cup. So where does all of this moisture vapor go? It stays inside your boots if your footwear isn’t breathable. This combination of sweat and exposure to the environment can cause blisters and other foot ailments. To ensure comfort and safety on the job, you need to consider your socks and shoes as an integrated system.
Socks
If you’re active and on your feet all day, you need a sock that enhances your comfort and is absorbent and fast-drying so your feet stay dry. For improved comfort, wear lightweight polyester socks in warm weather. In winter conditions, wool keeps you warm because it is a better insulator, wet or dry. Cotton is not recommended, because it traps moisture in its fibers and takes a long time to dry.
Boots
Your footwear supports all your weight, plus the weight of your gear. A comfortable, secure fit is key. Boots are available in a variety of styles for different types of uses. To keep feet dry, choose boots that are breathable — letting perspiration out while repelling water and other hazardous fluids. You should select footwear engineered to handle the various conditions and hazards you might face.
Head Protection
Your head plays an important role in protecting you from cold and bad weather. Because your head can release more heat than your body is capable of producing, good headwear is essential. Good headwear should also protect against outside moisture like rain and snow, and let perspiration escape so that your head stays dry.
Gloves
Your hands can sweat a lot, and they endure reduced blood circulation in cold weather. You need gloves that lock in warmth, allow sweat to escape, and ensure that no moisture penetrates from the outside. At the same time, gloves shouldn’t compromise your ability to do your job. If you have to remove a glove to use a tool or shoot a weapon, you probably aren’t wearing the right gloves.
In addition to dexterity, gloves are expected to provide the right degree of thermal stability, abrasion resistance, and protection from liquid penetration. Additional layers of padding in the palm and fingers should be a consideration.