Assessment of Heat Stress in Iranian Industries and Temporal Trends over the Last Two Decades: A Systematic Review and Meta-Analysis

Heat is a silent, invisible force that can overwhelm the human body long before a person feels the first wave of exhaustion. When people work in hot environments, their bodies generate internal heat from movement and metabolism, while the surrounding air, sun, and machinery pump in more warmth. The body tries to cool itself by sweating and sending blood to the skin, but if the heat load becomes too great, this cooling system fails. The result is a cascade of physical distress, ranging from cramps and fainting to life-threatening heat stroke. To measure this danger, scientists and safety officials rely on a specific tool called the Wet Bulb Globe Temperature. Unlike a standard thermometer that only measures air temperature, this tool combines three different readings: the temperature of the air, the heat radiating from surfaces like the sun or a furnace, and the cooling power of the air's humidity. By blending these factors, it gives a single number that reflects how hot a worker actually feels and how hard their body must work to stay safe.

For decades, researchers have known that Iran, with its vast deserts and humid southern coasts, presents a particularly harsh landscape for outdoor and industrial labor. Yet, until now, there was no single, comprehensive picture of just how severe the problem has become across the country's many different industries. A team of researchers set out to fill this gap by gathering every available study on heat stress in Iran from the last twenty years. They did not conduct new measurements themselves; instead, they acted as archivists and statisticians, hunting down dozens of scattered reports from factories, mines, farms, and bakeries. Their goal was to combine these fragmented pieces of data into a clear, national overview, looking for patterns in how hot it has gotten, where the danger is highest, and whether the situation is improving or worsening over time.

The researchers sifted through nearly six hundred records, eventually selecting forty-three high-quality studies that met strict criteria. These studies covered a wide spectrum of work, from the scorching steel mills and foundries of the industrial north to the date palm groves of the south and the underground tunnels of mines. They separated the data into two distinct categories: work done under the open sky and work done inside buildings or covered structures. When they combined the results, a stark reality emerged. In open-air environments, the average heat stress level was 25.71 degrees Celsius. Inside covered buildings, the average was even higher, reaching 28.11 degrees Celsius. These numbers are not merely uncomfortable; they frequently exceed the safety limits set by international health organizations for moderate to heavy physical labor, indicating that a significant portion of the Iranian workforce is regularly exposed to dangerous thermal conditions.

The danger is not distributed evenly. The analysis revealed that the type of industry a person works in is the most powerful predictor of how hot their environment will be. In open fields, workers harvesting dates faced the highest average temperatures at 33.20 degrees Celsius, while those working in underground mines were the coolest at 12.30 degrees Celsius. Inside buildings, the steel industry stood out as the most extreme, with one study recording a staggering average of 40.20 degrees Celsius. This extreme heat in steel mills and foundries is driven by the massive furnaces and molten metal that radiate intense energy, often trapping heat inside the structure. The study also found that the location matters significantly for indoor work, with different provinces showing vastly different levels of heat stress, whereas the specific province mattered less for outdoor workers, who face the broad, uniform heat of the climate regardless of their exact location.

Perhaps the most concerning finding concerns the passage of time. The researchers looked at whether heat stress has changed over the last two decades, from 2004 to 2023. For workers outside, the data shows a clear and worrying upward trend. The average heat stress level in open environments has been rising steadily, increasing by nearly 0.7 degrees Celsius for every year that passed. This suggests that the combination of global climate change and perhaps changing industrial practices is making outdoor work increasingly perilous. In contrast, the heat levels inside covered buildings have remained relatively stable over the same period. This stability likely comes from the fact that indoor environments are somewhat shielded from the direct effects of the changing climate, though they remain dangerously hot due to the nature of the work itself.

The study also highlighted a critical limitation in how we currently measure heat. While the Wet Bulb Globe Temperature is the global standard, the researchers noted that it is not a perfect tool for every situation. In Iran's hot and dry regions, the index might underestimate the true strain on a worker's body because it does not fully account for the intense radiant heat from the sun or machinery. Conversely, in the humid coastal areas, it might overestimate the risk. The authors argue that relying on this single number can lead to flawed safety decisions. They suggest that a more complete picture requires combining environmental measurements with direct checks on the workers' bodies, such as monitoring their heart rates or body temperatures. This dual approach would ensure that safety rules are based on how the human body is actually reacting, rather than just on the weather outside.

Ultimately, this comprehensive review paints a picture of a workforce facing a growing thermal challenge. The heat is not just a temporary discomfort; it is a persistent, escalating hazard that threatens the health and productivity of millions of workers. The findings call for a shift in how industries manage heat, moving away from a one-size-fits-all approach toward strategies that are tailored to specific regions and types of work. As the climate continues to warm, the need for better ventilation, smarter work schedules, and more accurate monitoring becomes not just a matter of comfort, but of survival. The data confirms that without significant changes in how heat is managed, the gap between the environment and the human body's ability to cope will only continue to widen.