New KSI Study Finds Apps Can't Match the Accuracy of On-Site WBGT Monitoring

July 7, 2025 Jackie Fitzpatrick Comments

New research from the Korey Stringer Institute shows that relying on smartphone apps like Zelus may underestimate heat risk.

When it comes to heat stress prevention, accurate data is everything. Athletic trainers, safety officers, and outdoor professionals depend on Wet Bulb Globe Temperature (WBGT) to make informed decisions that protect lives in extreme heat. But a new peer-reviewed study from the Korey Stringer Institute (KSI) and leading climate researchers raises concerns about the reliability of smartphone apps for WBGT estimation—especially in high-heat environments.

In the 2025 GeoHealth study, "Evaluating Heat Risk: Comparing On-Site WBGT Measurements Versus Smartphone Application Estimates" by Grundstein et al., over 1,000 data points were collected from a range of climates and surface types across the United States. Researchers compared WBGT readings from the Kestrel 5400 Heat Stress Tracker to those generated by the Zelus smartphone app, which is commonly used in athletic settings.

The Key Takeaway: Apps Underestimate Heat Risk

On average, the phone app underestimated WBGT by about 1°C—and in hotter environments, discrepancies of 2–3°C were observed. That may not sound like much, but when it comes to preventing heat illness, every degree matters. These lower readings led the app to recommend fewer activity modifications than the on-site measurements, which could pose serious risks to athletes, workers, and military personnel operating in high-heat conditions.

"The phone app tended to underestimate the need for activity modification," said Rebecca Stearns from KSI. "This indicates potential safety concerns for athletes and other active populations relying solely on WBGT estimates from a phone."

The study's authors emphasize the need for further validation of phone-based WBGT estimates before they are adopted as a replacement for on-site instruments.

Why On-site WBGT Is Still the Gold Standard

The Kestrel 5400 Heat Stress Tracker is designed specifically for on-site WBGT monitoring. Unlike estimations based on air temperature and humidity alone, the 5400 captures all critical parameters—globe temperature, humidity, wind speed, and radiant heat—providing an accurate representation of heat stress risk in real time. It also features pre-programmed zones for athletics, military, and occupational use, as well as alerts, data logging, and exporting, so you are equipped with a comprehensive heat safety solution.

This level of accuracy is why athletic programs, emergency services, and industrial safety teams worldwide trust Kestrel Instruments.

When Safety Is On the Line, Accuracy Matters

While phone apps may offer convenience, this new study reinforces a hard truth: only on-site WBGT measurements provide the reliability needed for life-saving decisions. With rising global temperatures, new heat safety legislation, and increasing awareness of heat-related illnesses, the demand for trustworthy heat stress tools is more important than ever.

Read the Full Study

Evaluating Heat Risk: Comparing On-site WBGT Measurements Versus Smartphone Application Estimates

Stay Safe. Stay Informed. Choose Kestrel.

Whether you're on the field, at a job site, or in a training facility, Kestrel Heat Stress Trackers offer the data you need to make smart, safe decisions in the heat.

Explore our Heat Stress Solutions ➡️ www.kestrelinstruments.com/heat-stress

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Jackie Fitzpatrick