2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
ESTIMATION OF THE CONTRIBUTION OF SOLAR RADIATION TO
THERMAL STRESS DURING OUTDOOR ACTIVITY
Ju Youn Kwon, Ken Parsons
Human Thermal Environments Laboratory, Department of Human Sciences, Loughborough
University, Loughborough, Leicestershire LE11 3TU
United Kingdom
Contact person: j.y.kwon@lboro.ac.uk
INTRODUCTION
The sun provides heat in the form of electro-magnetic radiation to the surface of the earth’s
atmosphere at a level of around 1370 Wm -2 (Monteith and Unsworth (1990); Parsons
(2003)). This arrives on the surface of the earth at a maximum of around 1000 Wm -2 ,
depending upon weather conditions. People working outdoors will receive this heat as part of
their thermal environment. Some of the heat will be absorbed by the person and it will
influence the heat exchange between the body and the environment. Any comprehensive
method for assessing thermal strain in outdoor conditions must therefore consider the
contribution of solar radiation. It was the aim of this study to estimate the contribution of
solar radiation to people when working outdoors using three methods: human subjects; a
thermal manikin and theoretical analysis. The results not only provide estimates of the
contribution of solar radiation but also of the relative strengths of the three methods used.
METHODS
STUDY ONE: Estimation of the contribution of the sun to thermal stress using a human
subject: A fit male subject (30 y, 1.76 m, 91 kg) conducted a step test for one hour at 20 steps
per min with a step height of 0.2 m, first in a car park outdoors in the sun and later at the same
time of day, in a thermal chamber. The subject was weighed before and after the ‘exposure’
using Mettler 1D1 Multi-range Digital Dynamic Scales. Oral temperature was measured
before and after the exposure using a mercury in glass clinical thermometer. Heart rate was
measured using a Polar Sports Tester. Environmental conditions were recorded around the
subject. Air temperature was measured using shielded thermistors placed at heights equally
spaced from 0.2 m to 1.7 m. Globe temperature was recorded at 1 m and 1.6 m both in the
shade and in the sun (outdoors). Radiation levels were measured using a Skye pyranometer
SP 1110. Air velocity was measured using a hot wire anemometer. Humidity was measured
using a whirling hygrometer. Clothing worn was a white cotton/polyester shirt, beige trousers,
briefs, socks and trainers.
RESULTS
The environmental conditions outdoors and in the thermal chamber are presented in Table 1.
It can be seen that conditions indoors were a good simulation, on average, of the outdoor
conditions but indoors the globe temperature and hence mean radiant temperature were close
to air temperature as there is no contribution from the sun.
Humidity and solar radiation in outdoor condition and indoor humidity were relatively
constant at 54%, 423 Wm -2 , and 52% respectively. The fluctuation in conditions over the one
hour of exposure is shown in Figure 1. Of particular interest is the simulation of air
movement. Indoors this was achieved using fans. In practice it is difficult to simulate the
three-dimensional and dynamic nature of air movement around a subject so it is assumed that
simulation on average provides a good approximation to heat transfer conditions.
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