1.1 Heat in, heat out: the five pathways

1 Where the heat comes from

Two sources add heat to a worker, and only one of them is the weather.

Metabolic heat is made inside. Muscles are inefficient engines: most of the energy they burn comes out as heat, not motion. That is why workload is a heat input in its own right and why OSHA's guidance keeps returning to it — reduce the physical demands, use relief workers, spread the work over more people, and you have lowered the temperature inside the worker without touching the thermometer.

Environmental heat comes in from outside: hot air, and radiant energy from the sun or from hot surfaces. OSHA's Technical Manual lists the workplace radiant sources plainly — direct sunlight, and machinery, furnaces and hot structures near the workplace. That is why a black membrane roof at noon and a shaded slab at the same air temperature are two different jobs, and why the heat index, which knows nothing about radiant heat, cannot tell them apart.

2 The four ways heat leaves — and how each one fails

Heat leaves a body by four pathways. Understanding which one is blocked tells you which control will actually work.

Evaporation is the big one in hot work. Sweat carries heat away as it turns to vapor — but only if it can evaporate. In humid air it cannot, so it runs off the skin and soaks the shirt, doing almost nothing. Sweat dripping off a hard hat brim is not cooling anybody; it is water leaving the body with the job undone. This is why humidity matters so much, and it is why the natural wet-bulb sensor carries 70 percent of the weight in the WBGT equations you will meet in Part 2.

Convection moves heat into passing air. It works while the air is cooler than skin, roughly below 95 °F. Above that the wind is bringing heat in, which is the reason OSHA's guidance cautions that fans improve heat exchange between the skin and the air unless the air temperature is higher than skin temperature, and why the proposed heat rule would make an employer evaluate fan use above an ambient 102 °F.

Radiation works both ways. A worker radiates heat to cooler surroundings and absorbs it from hotter ones. Under a July sun or in front of an open furnace door, the net flow is inward and no amount of breeze fixes it. Shade and shielding are the only controls that touch this pathway, which is why OSHA's proposed rule asks specifically for shade that blocks direct sunlight — and states that shade from equipment does not count.

Conduction is contact: a hand on hot steel, a knee on a sun-baked deck, boots on asphalt. It is small in most jobs and dominant in a few.

3 What the body does about it

The body has two moves against heat, and both cost something.

First it sends blood to the skin, where heat can escape. That blood is no longer available to the muscles and the brain, so the heart beats faster to cover both jobs. Second it sweats, which cools by evaporation but drains fluid and salt. Keep it up long enough without replacing the fluid, and blood volume falls; the heart has to work harder still to move less blood between a working muscle and a hot skin.

That chain explains the symptoms in the order they appear. Fatigue and irritability first, then headache, nausea, dizziness and heavy sweating — the heat exhaustion picture in the OSHA-NIOSH InfoSheet, along with elevated body temperature and decreased urine output. If the system fails outright, temperature regulation goes with it: confusion, loss of consciousness, seizures, very high body temperature, and skin that may be hot and dry or profusely sweating. That is heat stroke, and the InfoSheet calls it a medical emergency that may rapidly result in death.

4 Clothing: the control that works both ways

Clothing sits directly across the evaporation pathway. Anything that traps vapor against the skin — a rain suit, chemical coveralls, a welding jacket, a full-body Tyvek layer — turns the body into a closed system: the sweat is made, it cannot evaporate, and the heat stays in. This is why the OSHA-NIOSH InfoSheet lists heavy personal protective clothing and equipment among the risk factors, and why OSHA's proposed rule would require an employer whose employees wear vapor-impermeable clothing to evaluate the heat stress that clothing creates and write the resulting policies into the plan.

The same guidance points the other way for cooling: reflective clothing such as safety vests, worn as loosely as possible, can minimize heat illness; water-dampened cotton whole-body suits are described as an inexpensive and effective personal cooling technique; and cooling vests with pockets holding cold packs are called comfortable and effective. If your employer issues cooling PPE, the proposed rule would require its cooling properties to be maintained at all times during use — a cooling vest with warm packs in it is just a vest.

5 The same four pathways, running backwards

Cold is not a different subject. It is the same four pathways with the arrows reversed, and the same equation solved for the other failure.

1. 1
   Convection: wind strips the thin layer of warm air held against the skin. That is what the wind chill number describes — heat loss from exposed skin, nothing else.
2. 2
   Evaporation: sweat under insulated clothing wets the layers, and wet insulation stops insulating. In cold work, sweating hard is a hazard, not a sign of effort.
3. 3
   Conduction: OSHA's Cold Stress Guide notes that wet feet lose heat 25 times faster than dry feet. Contact with cold metal and wet surfaces does the same thing to bare hands.
4. 4
   Radiation: a body radiates heat to a cold sky and cold surfaces all day, which is why radiant heaters in outdoor stations are on OSHA's list of engineering controls for cold.

The body's defense is the mirror image too. OSHA's Cold Stress Guide describes it: in a cold environment most of the body's energy goes to keeping the core warm, and over time blood flow shifts from the extremities and outer skin to the chest and abdomen. Hands, feet, ears and nose are sacrificed to protect the core — which is exactly why frostbite takes fingers and toes while the person is still walking and talking.

6 Why this matters before any number does

Crews argue about thresholds: what temperature is too hot, what wind chill is too cold. The physics says the threshold question is the wrong one. Two workers standing side by side in identical weather are in different environments if one is in the sun and one is in shade, if one is in a rain suit and one is in a t-shirt, if one is swinging a sledge and one is holding a clipboard, if one has been doing this all summer and one started Monday.

That is why OSHA reports heat stroke deaths on days when the maximum heat index was only 86 °F, and why its Cold Stress Guide puts hypothermia above 40 °F for someone chilled by rain, sweat or immersion. The weather is one input in a balance with four other terms. Learn the balance and the numbers in the rest of this course become tools instead of permissions.