Table 1.

Examples of common direct and rationale indices of heat stress

		Air temperature	Heat index	Humidex	Wet Bulb Globe Temperature (WBGT)	Universal Thermal Climate Index (UTCI)
Typea	Direct	Simplified rationale	Direct	Direct	Rationale
Measured or derived		Measured	Derived	Derived	Derived	Derived
Equation		NA	See footnoteb	See footnoteb	WBGT = 0.7Tnwb + 0.2Tg + 0.1Ta	Mathematical model
Heat stress quantification	Environmental inputsc	T a	Ta, RH, or Td	Ta, vapor pressure or Td	Ta, Tnwb, Tg, Vw	Ta, Vw, RH, MRT
	Model assumptions/-range of validity	NA	Shaded	Shaded	Sun exposure	Sun exposure
			Ta: 26.67–43.33 °C (80–110 °F)	Ta: 21–43 °C (70–109 °F)	Ta: wami weatherd	Ta: all ranges
			Humidity: RH ≥ 40%	Humidity: vapor pressure > 10 mb	Humidity: Tnwb range 5–40 °C	Humidity: RH or vapor pressure, all ranges
			MET = 180 W·m−2	MET: non-active	MET: Nonspecific; the WBGT limit values set by ACGIH and ISO 7243 (2017) are used for resting (115 W), low (180 W), moderate (300 W), high (415 W), and very high (520 W) work rates. Differing work rates may be used in athletic or military situations based on easy, moderate, or hard work	MET = 135 W m−2
			Vw: wind speed and person movement = 2.57 m s−1 (5 kts)	Vw: “ventilated” from wind speed	Vw: natural ventilation, all ranges	Vw: wind speed of 0.5–30.3 m s−1 and person movement of 1.1 m s−1
			Clothing: trousers, T-shirt	Clothing: unknown	Clothing: “light” (0.60 clo)e	Clothing: varies with Ta
Public data availability		Common from weather station data	Common; derived from weather standard station data	Common; derived from weather standard station data	Uncommon; in situ collection	Less common, derived from standard weather station data plus solar radiation data
Ease of interpretation		Simple	Moderate	Moderate	Moderate	Complex
Sample applications		Some National Weather Service (NWS) forecast offices in arid climates use Ta to inform issuance of heat alerts, such as the HeatRisk tool in Phoenix	Inform issuance of heat alerts via NWS	Environment Canada issuance of heat alerts; CCOHS for outdoor work	Used for activity modification in athletics, the military, and occupational safety	General thermal comfort of population; occupational heat exposure
Reference(s)		Hawkins et al. (2017)	Rothfusz (1990)	Masterton and Richardson (1979)	Yaglou and Minard (1957)	Bröde et al. (2012)
		NOAA (2017)	Hawkins et al. (2017)		ISO 7243 (2017)
					Armstrong et al. (2007)
					NIOSH (2016)
				Government of Canada (2017)	Departments of Army and Air Force (2003)
					ACGIH (2017)
				CCOHS (2017)

^aSee Epstein and Moran (2006)

^bFull equations can be found in Gosling et al. (2014)

^cAir temperature (T_a), relative humidity (RH), dew point temperature (T_d), natural wet bulb temperature (T_nwb), globe temperature (T_g), mean radiant temperature (MRT), wind speed (V_w), and metabolic activity (MET)

^dISO 7243 states that for accurate WBGT, the T_nwb should measure a range of 5–40 °C and the T_g should measure a range of 20–120 °C

^eWBGT can be adjusted for changes to clothing/equipment (see ISO 7243 (2017) and Havenith and Fiala (2015))

ACGIH the American Conference of Governmental Industrial Hygienists, CCOHS Canadian Centre for Occupational Health and Safety