2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
family of lines with different slopes depending on the relative humidity that pivot
around the data at the lower end. Therefore some caution is needed in using this
relationship to generalize the data available.
The ability to distinguish the total resultant evaporative resistance between ensembles
and to appreciate the confidence interval around the sample mean, the variance (or
standard error) of the distribution of values is necessary. The standard error was
estimated as 0.0044 kPa m 2 W -1 , and the 95% confidence interval was 0.0040 to
0.0050 kPa m 2 W -1 or about ±10%. Based on the upper limit of the standard error, the
resolution on the mean value is less than 0.005 with a sample size of 4, drops to 0.003
with n = 11, and does not get much better than 0.002 with large sample sizes (greater
than 23).
In conclusion, the progressive heat stress protocol is a useful method to determine the
total resultant evaporative resistance, which does not rely on the direct determination
of sweat rate. The results indicate there is little practical difference among single
layer configurations with modesty clothing of work clothes, cloth coveralls and Tyvek
coveralls. There were significant increases in evaporative resistance for a specific
vapour permeable water barrier coverall (NexGen) and for a vapour barrier coverall.
In addition, values for total static evaporative resistance and permeability index for
four ensembles not reported in ISO/FDIS 9920 (2007) were determined from wear
tests. These values can be useful rational models of heat stress.
Under the current test conditions, specifically a progressive heat stress protocol at
50% relative humidity, there appears to be a linear relationship between total resultant
evaporative resistance and WBGT clothing adjustment factors. The relationship may
break down at different relative humidities when the evaporative resistance is high
(above 0.02 kPa m 2 W -1 ); and this requires further investigation. This large study also
allowed for a good estimate of the standard error for the progressive protocol method
of determining total resultant evaporative resistance, which is 0.0044 kPa m 2 W -1 .
ACKNOWLEDGMENTS
This research was supported by a grant from the US National Institute for
Occupational Safety and Health (1R01 OH03983); and its contents are solely the
responsibility of the authors and do not necessarily represent the official views of
NIOSH or the CDC.
140