Is Heat Index Enough? One Lesser-Known Measure May Be Better at Judging Outdoor Heat Danger

With summer in full swing and heat advisories becoming more frequent, many people turn to the heat index to gauge whether it’s safe to work or exercise outside. But experts say that familiar number might not be telling the full story.

According to the Centers for Disease Control and Prevention (CDC), heat is the leading weather-related cause of death in the U.S., often silently affecting outdoor workers, athletes, and vulnerable populations before symptoms appear. The tools we use to assess that danger—especially the heat index—could be overlooking key risks.

What the Heat Index Misses

The heat index blends temperature and humidity to give a “feels like” number. It’s the most widely known and publicly reported metric for gauging heat stress. But as meteorologists and occupational health experts point out, it only tells part of the story.

“The heat index doesn’t account for wind, sun angle, or cloud cover—things that dramatically affect how your body responds to heat,” says local weather expert Caleb Saylor.

Imagine standing outside on a cloudless, windless day, with the sun beating down at a steep afternoon angle. Even at a moderate air temperature—say 65°F—you might feel far warmer because of solar radiation and the lack of airflow. The heat index would not register that elevated strain.

A Better Measure: Wet-Bulb Globe Temperature

Enter the wet-bulb globe temperature (WBGT)—a more comprehensive, science-based measure used by military units, sports organizations, and some workplaces. Unlike the heat index, WBGT includes:

Air temperature
Humidity
Wind speed
Sun angle
Cloud cover/radiant heat

This makes it far more effective at estimating how hard your body is working to cool down, especially during exertion. The U.S. military uses WBGT thresholds to determine when soldiers must hydrate, rest, or stop training altogether.

Why This Matters for Workers and Athletes

For construction workers, delivery drivers, landscapers, and athletes, heat risk is not just about sweating—it’s about core body temperature rising faster than the body can cool down. With heat index alone, dangerous conditions can go unrecognized, especially in full sun or low-wind environments.

Many occupational safety experts and meteorologists now urge employers and local governments to consider using WBGT as a standard, especially in regulated industries and organized sports.

So, Is the Heat Index Useless?

Not at all. The heat index remains a good general guideline for the public, especially when issued by the National Weather Service during heat advisories. But for task-specific risk assessment, especially in high-stakes environments, it simply doesn’t go far enough.