LettersHong Kong mustn’t use the wrong index to gauge outdoor heat stress
Readers discuss how to better assess heat exposure at work, the accessibility of the Wang Fuk Court fire inquiry documents, and the US-Iran war
The reason for this absurdity is the use of the Hong Kong Heat Index (HKHI) in determining the threshold for issuing the three levels of warning – amber at 30, red at 32 and black at 34.
HKHI was developed through research by the Hong Kong Observatory and Chinese University as an indicator of the risk of emergency hospital admission for the public in Hong Kong. The Wet Bulb Globe Temperature (WBGT), on the other hand, is widely used around the world to assess heat exposure at work, including by international standard ISO 7243.
Both HKHI and WBGT utilise the same three primary measurements: natural wet-bulb temperature (Tnw), globe thermometer temperature (Tg) and dry-bulb or ordinary air temperature (Ta). Following physical principles, natural wet-bulb temperature is always lower than air temperature, except when relative humidity reaches 100 per cent. The globe thermometer, which measures solar radiation, almost always shows a higher reading than air temperature for outdoor environments in the daytime.
HKHI is calculated using the weightings of 0.8, 0.05 and 0.1 for Tnw, Tg and Ta, while the corresponding weightings for WBGT are 0.7, 0.2 and 0.1. Mathematically, an HKHI reading will tend to be lower than a corresponding WBGT reading.