Heat load is the total amount of thermal energy your body (or a building, or any system) must deal with from both internal and external sources. When heat coming in exceeds heat going out, the system’s temperature rises. For the human body, that means your core temperature climbs; for a building, the air conditioning works harder. The concept applies across physiology, workplace safety, and building design, but the underlying principle is always the same: heat gained minus heat lost equals heat load.
Where Heat Load Comes From
Heat load has two broad sources: internal and external. Internal heat is generated by metabolism, machinery, lighting, electronics, or any process that converts energy into work (and waste heat as a byproduct). External heat comes from the surrounding environment, primarily air temperature, solar radiation, humidity, and radiant heat from nearby surfaces.
In the human body, internal heat production scales dramatically with activity. At rest, your metabolism generates roughly 80 to 100 watts of heat, comparable to a standard light bulb. During moderate cycling exercise at normal atmospheric pressure, metabolic heat output can reach around 550 watts. That’s a fivefold increase your body must somehow shed to stay at a safe temperature.
In buildings, internal heat gains come from occupants, lighting, and appliances. Each person adds about 230 BTU per hour of direct (sensible) heat plus another 190 BTU per hour of moisture-related (latent) heat. All lighting energy converts to heat. Virtually 100% of the energy consumed by indoor appliances ends up as heat that the cooling system must remove.
How Your Body Sheds Heat
Your body has four channels for dumping thermal energy: radiation, convection, evaporation, and conduction. At rest in a comfortable room (around 21°C or 70°F), radiation dominates, accounting for about 60% of heat loss from a nude body. Convection, the movement of air across your skin, handles roughly 18%. Evaporation through the skin covers about 15%, and direct conduction to surfaces you’re touching accounts for a small 3%.
These proportions shift as conditions change. In hot environments, radiation and convection become less effective because the temperature gap between your skin and the air shrinks. Your body then relies more heavily on sweating. If humidity is high, even evaporation slows down because the surrounding air is already saturated with moisture. This is why humid heat feels so much worse: your primary cooling mechanism is partially disabled, and heat load accumulates faster.
Measuring Environmental Heat Load
Two common tools estimate how much heat stress an environment imposes on people: the Heat Index and the Wet Bulb Globe Temperature (WBGT). They measure different things and are designed for different situations.
The Heat Index is the one most people encounter in weather forecasts. It combines air temperature and relative humidity to produce a “feels like” number. What many people don’t realize is that the Heat Index assumes you’re in the shade. It doesn’t factor in direct sunlight, wind speed, or cloud cover.
WBGT is the more comprehensive measurement, used by the military, OSHA, and sports organizations to guide decisions about outdoor work and exercise. It incorporates temperature measured in direct sunlight, humidity, wind speed, sun angle, and cloud cover. The calculation weights humidity most heavily (70% of the score comes from the wet-bulb temperature), followed by radiant heat from the sun (20%) and actual air temperature (10%). For anyone working or exercising outdoors in full sun, WBGT gives a far more accurate picture of heat stress than the Heat Index alone.
What Happens When Heat Load Overwhelms the Body
During exercise or exposure to high temperatures, a core body temperature between 38°C and 40°C (100.4°F to 104°F) is normal and generally well tolerated, especially in people who are physically fit or acclimatized to heat. Problems begin when core temperature pushes above 40°C (104°F).
Heat-related illness exists on a spectrum. Heat exhaustion occurs when the cardiovascular system can no longer keep up with the competing demands of cooling the skin and supplying blood to muscles and organs. Symptoms include heavy sweating, weakness, nausea, and dizziness. Heat injury is a step beyond, involving mild confusion, disorientation, and early organ damage. Heat stroke is the most severe form: a life-threatening emergency marked by delirium, seizures, or loss of consciousness, with core temperatures typically exceeding 40°C. At the cellular level, proteins in human cells begin to break down in the range of 40°C to 45°C, causing cell injury and death.
Workplace and Regulatory Thresholds
Occupational safety agencies set guidelines to prevent workers from accumulating dangerous heat loads. NIOSH has published a recommended standard for occupational heat stress exposure that includes guidance on work-rest cycles, hydration, and acclimatization schedules. California’s Heat Illness Prevention Standard is one of the most specific state-level regulations, requiring employers to provide training, water, shade, and an emergency plan whenever temperatures reach 80°F (27°C).
These standards typically use WBGT readings, adjusted for clothing and workload intensity, to determine when work should be slowed, paused, or moved indoors. The heavier the physical labor, the lower the WBGT threshold at which restrictions kick in.
Practical Ways to Reduce Heat Load
Reducing heat load means either lowering heat gained or increasing heat lost. For people working or exercising in hot conditions, several evidence-based strategies help.
Pre-cooling, which means lowering your body temperature before exposure, is one of the most effective approaches. It essentially creates a larger thermal “buffer” so your core temperature takes longer to reach dangerous levels. A meta-analysis found that pre-cooling improves exercise performance in ambient temperatures above 30°C (86°F) by an average of 5.7%. Cold water immersion, cold water or ice slurry ingestion, cooling vests (kept at 10 to 20°C), and ice packs all work. Combining multiple techniques produces the largest effect: whole-body cooling paired with cold fluid intake outperforms any single method.
Beyond pre-cooling, the basics matter most: staying hydrated replaces the fluid lost through sweat and keeps evaporative cooling functional. Lightweight, light-colored, loose-fitting clothing allows air circulation and reflects solar radiation. Scheduling heavy exertion for cooler parts of the day directly lowers environmental heat gain. Gradual heat acclimatization over 7 to 14 days improves your body’s sweating efficiency and cardiovascular response.
Heat Load in Buildings
In building science, heat load determines how much cooling (or heating) capacity a structure needs. Engineers calculate heat gains from sunlight hitting windows and walls, outdoor air temperature, and internal sources like occupants, lighting, and equipment. A building with large south-facing windows, poor insulation, and a high occupant density will have a substantially higher cooling load than a well-shaded, tightly insulated building with the same floor plan.
Building simulation tools account for all of these factors. They track how every watt of energy consumed indoors, from a refrigerator to an overhead light, becomes heat that the HVAC system must remove. This is why energy-efficient appliances and LED lighting don’t just reduce electricity bills; they also reduce cooling costs by generating less waste heat.
Why Heat Load Is Increasing
Global surface temperatures are rising steadily, which directly increases external heat load for both people and buildings. NOAA ranked 2025 the third-warmest year on record, with global surface temperatures 1.34°C (2.41°F) above the pre-industrial average. The ten warmest years ever recorded have all occurred since 2015. Higher baseline temperatures mean the body starts closer to its thermal limits on any given day, and cooling systems in buildings run longer and harder. For outdoor workers, athletes, and vulnerable populations, the margin between a manageable day and a dangerous one continues to narrow.