2007, Piran, Slovenia

Measurement methods
HUMIDEX: CAN THIS BIOMETEOROLOGICAL INDEX BE USED
FOR THE ASSESSMENT OF THE INDOOR THERMAL
ENVIRONMENT?
F. R. d’Ambrosio Alfano 1 , B. I. Palella 2 , G. Riccio 2
1 DIMEC - Dipartimento di Ingegneria Meccanica, Università di Salerno. Via Ponte Don
Melillo, 84084 Fisciano (Salerno), 2 DETEC - Dipartimento di Energetica,
Termofluidodinamica Applicata e Condizionamenti Ambientali, Università degli Studi di
Napoli Federico II Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy.
Contact person: : fdambrosio@unisa.it
INTRODUCTION
Thermal sensation depends on the subject-environment heat transfer according to the
following energy balance equation [1]:
M−W−E−E −C −C− R = S(1)
res res
In addition to equation (1), the assessment of the thermal environment also requires
information regarding the subjective sensation of the thermal environment.
The assessment of the thermal environment therefore depends on solving eq. 1, and
calculating the appropriate microclimate index using dedicated software [2]. Due to the
computing complexity, simple temperature-humidity indices have gained much popularity,
especially those that report the thermal condition as a “felt temperature”. Among these indices
is the Humidex (HD), originally a bio-meteorological index. An increasing number of climate
stations now provide an on-line estimate of the summer thermal discomfort. Humidex was
introduced in 1979 by Masterton and Richardson [3] for correlating outdoor thermal
discomfort in mild regions of Canada, based on two meteorological parameters: air
temperature and relative humidity. Its calculation is based on a Thom’s index modification:
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HD= ta + ⋅( pas − 10)
(2)
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In Table 1 HD limit values are reported.
Table 1. Ranges of the Humidex index corresponding to increasing thermal discomfort
conditions [3].
Humidex range Thermal discomfort level
20°C ≤ HD ≤ 29°C Comfort
30°C ≤ HD ≤ 39°C Some discomfort
40°C ≤ HD ≤ 45°C Great discomfort, avoid exertion
Above 45°C Dangerous
Above 54°C Heat stroke imminent
Humidex was initially developed for outdoor weather forecasts. Its scale has been correlated
to the death risk in summer hot waves [4]. Its use has been extended to the assessment of both
moderate and hot thermal environments. Indeed, the Use and Occupancy of Buildings
Directive of the Treasury Board of Canada [5] recommends the use of Humidex for the
thermal comfort assessment in office accommodations, suggesting an unsatisfactory condition
when the Humidex value exceeds 40°C. The HD Index has also been proposed as a simplified
method for heat stress assessment in indoor environments by the Ontario Ministry of Labour,
resulting in the “Humidex Based Heat Response Plan” [6].
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