Methodology
HeatSafe's calculations, thresholds, and data sources, with citations. Written for journalists, researchers, and anyone fact-checking the science.
What HeatSafe calculates
HeatSafe calculates the wet-bulb temperature at a user-selected location and maps it onto a four-tier risk index. The wet-bulb temperature is the lowest temperature human skin can reach through sweat evaporation in the current air — the physiological boundary of the body's primary cooling mechanism.
The reading is presented as the Heat Safety Index with four zones: Manageable, Caution, Danger, Critical. A fifth state, "Heat is not a concern", appears in genuinely cool conditions where the summer-calibrated zone language would read out-of-context.
Wet-bulb formula
HeatSafe does not use a physical wet-bulb thermometer. Wet-bulb temperature is calculated from two inputs available via weather APIs — dry-bulb temperature (ordinary air temperature) and relative humidity — using:
Stull, R. (2011). Wet-bulb temperature from relative humidity and air temperature. Journal of Applied Meteorology and Climatology, 50(11), 2267–2269. doi:10.1175/JAMC-D-11-0143.1
Stull's empirical formula is accurate to within approximately 0.3°C under normal atmospheric conditions. It is the standard approach in applied meteorology when a direct wet-bulb measurement is unavailable.
Why this formula: computationally simple (runs instantly in JavaScript, no external API dependency for the calculation), widely validated, and well-documented. Iterative psychrometric methods are marginally more accurate but unnecessary for a public-safety tool — the 0.3°C error margin is far smaller than the real-world variability across individuals and conditions.
Physiological thresholds
Two papers anchor HeatSafe's thresholds — one theoretical, one experimental.
The theoretical limit — Sherwood & Huber (2010)
Sherwood, S. C., & Huber, M. (2010). An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences, 107(21), 9552–9555. doi:10.1073/pnas.0913352107
This paper established the widely-cited 35°C wet-bulb figure. Human skin sits at approximately 35°C. If ambient wet-bulb exceeds 35°C, heat cannot flow from skin to air — thermodynamically impossible — and the body cannot cool itself regardless of hydration, fitness, or acclimatisation.
The 35°C figure is a theoretical calculation. It was not tested on humans.
The experimental limit — Vecellio et al. (2022)
Vecellio, D. J., Wolf, S. T., Cottle, R. M., & Kenney, W. L. (2022). Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects. Journal of Applied Physiology, 132(2), 340–345. doi:10.1152/japplphysiol.00738.2021
This study, part of the Penn State HEAT Project, ran climate-chamber experiments on young, healthy, resting, hydrated adults. Subjects were exposed to progressively higher heat-and-humidity combinations until their core body temperature began rising uncontrollably.
The measured uncompensable heat-stress limit was approximately 30.6°C wet-bulb — significantly below the theoretical 35°C figure.
Subsequent work from the same lab has shown the threshold is roughly 2–3°C lower for adults over 65, and lower still for people with cardiovascular conditions, outdoor labourers, and anyone not resting in ideal conditions.
Implication for safety guidance: the widely-cited 35°C figure is a theoretical ceiling, not a practical safety threshold. For public health, the 30–31°C range is more defensible as the point at which danger becomes real for vulnerable people.
HeatSafe's operational thresholds
HeatSafe uses four colour-coded zones, with boundaries set conservatively below the experimental limit:
| Wet-bulb range | Zone | Rationale |
|---|---|---|
| Below 26°C | Manageable | Within the compensable range with normal hydration. |
| 26–28°C | Caution | Vulnerable groups should limit outdoor exposure. |
| 28–32°C | Danger | Approaching and crossing the experimental threshold (Vecellio 2022 ≈ 30.6°C). |
| 32°C and above | Critical | Beyond experimental human tolerance. Theoretical 35°C limit being approached. |
°F display. US users see rounded °F equivalents on the dial: 79°F (26°C), 82°F (28°C), 90°F (32°C). Zone boundaries and the wet-bulb calculation remain anchored to °C; only the visible dial labels change.
Why 32°C, not 35°C, as the Critical boundary
The 35°C figure is a theoretical limit derived from skin-temperature thermodynamics for an idealised resting body. Real-world conditions — sun load, exertion, age, illness, prolonged exposure — lower the practical limit substantially. 32°C is set as a conservative operational ceiling. A life-safety tool should warn earlier, not later. This is a deliberate choice, not a misreading of the research.
Why "Manageable" rather than "Safe" for the lowest zone
"Safe" misleads in high-temperature, low-humidity conditions — for example, Delhi 42°C dry, low humidity. The body's cooling mechanism functions there, but the label "Safe" understates the demand on the body. "Manageable" is honest: cooling works, with normal hydration. There is no fully unconditional state in Indian summer.
Real-world calibration
The Chennai Air Show on 6 October 2024 illustrates why HeatSafe presents the experimental thresholds as baseline assumptions rather than absolute risk levels.
An estimated 1.5 million attendees gathered on Marina Beach. By afternoon, five people had died from heat-related causes and approximately 200 required hospital care. The wet-bulb temperature that day was approximately 28°C — at the upper edge of HeatSafe's Caution zone, near the boundary with Danger. The Vecellio (2022) experimental threshold (~30.6°C) was not crossed.
The discrepancy is not a tool failure. The experimental thresholds assume healthy young adults at rest in shade with adequate hydration. The Air Show conditions broke every one of those baseline assumptions: full sun for hours, prolonged standing, dense crowds, mixed-age population, varying fitness and hydration.
HeatSafe shows the baseline. Real-world conditions add stressors on top. A Caution reading under ideal conditions can carry Critical risk for someone in full sun, someone elderly, or a child playing outdoors. This is why HeatSafe's safety messaging consistently states that vulnerable groups face higher risk at every reading, and that the displayed zone is a baseline, not a personal risk assessment.
Data sources
- Weather data: Open-Meteo — a free, open-source weather API that aggregates data from multiple national weather services, including the India Meteorological Department (IMD) station network. Updated approximately hourly.
- Forward geocoding (place name → coordinates): Open-Meteo Geocoding API.
- Reverse geocoding (coordinates → place name): Nominatim — the open-source geocoder used by OpenStreetMap.
- Pincode lookup (India): postalpincode.in — a free public API mapping Indian PIN codes to districts.
HeatSafe runs entirely client-side. No HeatSafe server stores user data, queries, or location information. See the Privacy page for the complete data-handling statement.
What this page is not
This page is a methodology reference, not a clinical guideline.
- The cited thresholds describe healthy young adults at rest in shade with adequate hydration. They are not adjusted for individual age, fitness, medication, or health condition.
- The displayed zones describe conditions, not personal risk. The same reading carries different actual risk for different people and different exposure durations.
- HeatSafe does not replace official heatwave alerts from IMD or state authorities. Where guidance conflicts, follow the official alert.
See the Disclaimer for the full informational-not-medical statement.
Last updated: May 18, 2026