Illuminance and Indoor Heat Stress Index (WBGT) in Tropical Classrooms: Assessment against Legal Standards in Opportunity Expansion Schools in Surat Thani Province, Thailand

Abstract

This survey study aimed to assess area illuminance, lighting uniformity, and indoor heat stress conditions in classrooms of opportunity expansion schools in Surat Thani Province, Thailand. Illuminance levels were compared with the criteria specified in the Ministerial Regulation on the Prescribing of Standards for Safety Management and Administration Relating to Heat, Light, and Noise B.E. 2559 (2016). Lighting uniformity was evaluated using Uo = Emin/Eavg, with Uo ≥0.40 adopted as the reference criterion based on EN 12464-1:2021. The study was conducted in two opportunity expansion schools participating in the Safe School Project. Area illuminance was measured using a lux meter, and indoor heat stress was assessed using a Wet Bulb Globe Temperature (WBGT) meter. Data were analyzed using descriptive statistics.

The results showed that, in Case Study School 1, 22 of 33 rooms (66.7%) met the average illuminance criterion of ≥300 lux, 24 rooms (72.7%) met the minimum illuminance criterion of ≥150 lux, and 30 rooms (90.9%) met the lighting uniformity criterion of Uo ≥0.40. Secondary classrooms, the shared classroom, and the meeting room met all criteria, whereas primary classrooms, laboratories, and the library had substandard average illuminance, minimum illuminance, or lighting uniformity in some areas. In Case Study School 2, 11 of 15 rooms (73.3%) met the average illuminance criterion, 10 rooms (66.7%) met the minimum illuminance criterion, but only 7 rooms (46.7%) met the Uo ≥0.40 criterion. The academic room met all criteria, while primary classrooms, secondary classrooms, the meeting room, library, and teachers’ room showed issues related to uneven lighting distribution and/or substandard illuminance levels. Indoor WBGT values ranged from 25.4–28.0°C in School 1 and 26.2–28.6°C in School 2, which did not exceed the legal standard during the measurement period.

These findings suggest that schools should improve both illuminance levels and lighting uniformity by checking luminaire placement, cleaning or replacing deteriorated lamps, and adding task lighting in areas with low minimum illuminance. Heat management should also be supported through improved ventilation, reduction of heat gain through openings or roofs, and periodic environmental monitoring in accordance with legal requirements.