Heat Doesn’t Have to Be Hot

The conditions that produce exertional heat illness and significant UV exposure do not require a hot day. They require exertion, gear, and time outside. May has all three, and the thermometer reads nothing alarming.

That last part is the problem. Temperature is not a reliable indicator of what the body is managing under load and in gear. A 68-degree overcast morning on a range is not a neutral environment. It is a comfortable one, which is a different thing.

Two separate threats follow from this: UV exposure, which has no relationship to air temperature, and exertional heat illness, which is driven by core temperature rather than ambient conditions. Both are underestimated in spring for the same reason. The environment does not feel dangerous. That is exactly why it is.

UV Has Nothing to Do with Temperature

Temperature and UV radiation are independent variables. The UV index measures the intensity of ultraviolet radiation reaching the surface at a given location and time. Air temperature does not factor into that calculation. A 62-degree spring day with a UV index of 8 carries the same skin damage potential as a 95-degree summer afternoon at the same index. The thermometer is not a useful indicator of UV exposure.

Cloud cover does not solve this. Clouds block 20 to 40 percent of UV radiation, which means a solid overcast sky still passes the majority through. Personnel who would not think twice about extended outdoor exposure on a cool, gray day are absorbing meaningful UV throughout. The absence of heat or bright sun removes the sensory cues people use to judge risk, so they stay out longer and cover less skin.

Reflective surfaces add to the load. Pavement, water, vehicles, and light-colored concrete all reflect UV back upward, increasing effective exposure beyond what the index alone would indicate. Personnel working in parking lots, on ranges, near waterways, or in open urban environments are managing reflected UV on top of direct.

The UV index scale runs from 0 to 11+. According to the EPA, a reading of 6 or 7 is classified as high, and unprotected fair-skinned individuals can begin to burn in approximately 15 to 25 minutes. Index readings of 8 to 10 compress that window to roughly 15 minutes or less. Those readings are routine across much of the U.S. in May, including on days that feel nothing like a beach afternoon. Checking the index before extended outdoor operations takes seconds. Using air temperature as a substitute does not work.

Exertional Heat Illness Is Driven by the Body, Not the Thermometer

Exertional heat stroke is a medical emergency, and one of the more underrecognized heat stroke risks in moderate temperatures. It is distinct from the classic form most people have encountered in general health contexts, which typically affects the elderly or young children during sustained heat wave exposure. Exertional heat stroke occurs in otherwise healthy individuals under physical load, which makes it an occupational hazard specific to military personnel, law enforcement, and firefighters operating in gear.

The military uses Wet Bulb Globe Temperature, or WBGT, as its standard measure of environmental heat stress. Unlike the standard heat index, WBGT accounts for humidity, solar load, and wind, producing a more complete assessment of the actual thermal environment. What it does not account for is gear. Current Defense Health Agency guidance applies a 5-degree Fahrenheit adjustment to the WBGT when personnel are wearing body armor or a rucksack in humid conditions, and 10 degrees for full chemical protective equipment. A WBGT reading that looks manageable can shift into a higher risk category once gear load is applied.

Exertional heat stroke (EHS) is clinically defined as a core body temperature above 104 degrees Fahrenheit combined with central nervous system dysfunction. In the field, core temperature is almost never known. The behavioral signs are what matter: altered mental status, confusion, disorientation, slurred speech, combativeness. Someone who was functional 20 minutes ago and is now stumbling or not responding coherently is a heat stroke casualty until proven otherwise. Do not wait for a temperature reading to begin treatment.

Earlier on the continuum, heat exhaustion presents with heavy sweating, nausea, headache, weakness, and dizziness, without the mental status changes. Core temperature is elevated but typically below 104 degrees. Prompt intervention resolves most cases: move the person to shade, remove excess clothing and gear, encourage controlled oral hydration within established limits. The window to intervene before exhaustion becomes stroke is real and it closes.

One hydration risk worth naming separately: overhydration. The military guideline caps fluid intake at one quart per hour under normal conditions and no more than one and a half quarts per hour during intense activity. Exceeding those limits dilutes sodium and can cause hyponatremia, which presents with some of the same symptoms as heat exhaustion. Treating one aggressively while the person actually has the other is a real mismanagement scenario.

Cool First, Transport Second

The Joint Trauma System Clinical Practice Guidelines establish the treatment sequence for suspected EHS: cool first, transport second. Duration of hyperthermia is the primary determinant of organ damage and mortality, which means cooling begins at the point of collapse and continues throughout evacuation. In field settings, the expedient method is ice sheeting, bed linens or similar material soaked in ice water applied to the body. It is not a substitute for definitive care. It is what keeps the casualty viable until definitive care is available.

The Month That Catches People Off Guard

Heat casualties in the military are reported every month of the year, including in conditions that would not register as dangerous by any intuitive measure. The common thread is not temperature. It is exertion, gear, and the absence of the environmental cues that normally prompt people to act differently.

May is that month. The air is comfortable. Nothing looks like a heat day. The UV index is high by midmorning, and nobody checks it. Personnel are back in full operational tempo after winter, and the acclimatization that reduces EHS risk takes one to two weeks to develop. None of it has happened yet.

The countermeasures are known. What is harder is applying them on a morning that feels like nothing is wrong, when the WBGT looks manageable and the sky is overcast and the temperature is reasonable. That gap between what the environment feels like and what it is actually doing to the body is where most May casualties come from.