In July 2025 Virginia wrote student-athlete heat protection into state law. Every school division now sets practice by wet bulb globe temperature, and above a hard cutoff nobody goes outside. Table 1 gives the limits. They read as a safety schedule, and they are also a temperature curve for enzyme activity, converted into hours and equipment.

Air temperature is the wrong number

Wet bulb globe temperature is not what a thermometer in the shade reads. It is 70 percent a wet-bulb reading, 20 percent the temperature inside a black globe in the sun, and only 10 percent the air. The wet bulb measures whether sweat can evaporate, and sweat that does not evaporate carries away no heat. On a humid day the air can read 88 °F while the body has almost no way to shed what it is making, which is exactly the situation plain air temperature hides.

Two halves of one curve

An enzyme is a protein — a polymer of amino acids folded into a shape — and it works as a catalyst, lowering the activation energy a reaction needs so that reactants become products fast enough to keep you alive. Your enzymes are built for about 37 °C. Below that, warming helps: across most of the biological range a rise of 10 °C roughly doubles or triples reaction rate, because molecules collide more often and with more energy. That is the rising half of the curve, and it is why the first part of a hot practice feels fine.

Past about 40 °C the curve does not flatten out. It collapses. Heat shakes apart the hydrogen bonds and ionic attractions holding the protein in its folded shape, while the covalent bonds of the chain itself hold fast — the sequence survives, the fold does not. Since the active site is a shape, losing the fold loses the site, and the substrate no longer fits. The enzyme is denatured, and past a point it will not refold.

Exertional heat stroke is diagnosed above 40.5 °C, or 105 °F, core temperature. That figure is not an arbitrary line on a chart. It is roughly where the enzymes running an athlete's brain and muscles begin coming apart.

Cool first, transport second

Nearly every emergency is transport first. Heat stroke is the exception, and athletic trainers are taught the reverse: cold-water immersion, on site, immediately, aiming to bring core temperature under 38.9 °C within 30 minutes of collapse.

The biology explains the order. Damage tracks how long the body stays above 40.5 °C, not how quickly someone reaches a hospital, so twenty minutes in an ambulance buys twenty more minutes of denaturation. Across more than 2,000 heat stroke patients, no deaths were recorded where cooling began inside that 30-minute window. None of this makes you a first responder. It makes the case for getting an athletic trainer or coach the moment a teammate is confused, staggering or unresponsive in the heat, instead of waiting to see whether they walk it off.