APPENDIX
2
Discussion of Heat
Stress
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igh values of leisure thermal comfort that exceed the P.O. Fanger scale (that is, greater than “+3”) are considered to lie within the “heat stress” domain. In this section, we look at the definition of heat stress presented in the U.S. Government Occupational and Environmental Health Bulletin* and we apply this definition to July sunshine conditions for Miami, Florida. Miami is an arbitrary example chosen to illustrate the concept of heat stress.
As discussed in Chapter 10, PMVSUN values were estimated at 4.4, 4.8 and 5.3 for presumed values of MRTSUN of 110ºF, 115ºF and 120ºF, respectively. (These extrapolated values of PMVSUN are more suspect as values extend farther and farther away from the Fanger upper-limit value of “+3”.)
Associated values of PMVSHADE and MAXTMP temperature for July (summarized in Table 5-3) are 2.27 and 89ºF.
The heat stress formula referenced by the Occupational and Environmental Health document is:
WBGT = 0.7 x WB + 0.2 x BG + 0.1 x DB (A-1)
Where: WB is the wet-bulb temperature
BG is the 6-inch Vernon Black Globe temperature
DB is the dry-bulb (ambient) temperature
WBGT is the wet-bulb globe temperature index
The WBGT equation can be recast into the following form by inserting our alias variables:
WBGT = 0.7 x WB + 0.2 x MRTSUN + 0.1 x MAXTMP
(A-2)
The July (average-maximum) wet-bulb temperature, available from ISMCS (and NOAA), is 76.8ºF. Upon inserting the wet-bulb and MAXTMP, along with three estimates for MRTSUN temperatures into Equation A-2, we obtain the following:
(1) WBGT = 84.7 = 0.7 x 76.8 + 0.2 x 110 +0.1 x 89
(2) WBGT = 85.7 = 0.7 x 76.8 + 0.2 x 115 +0.1 x 89
(3) WBGT = 86.7 = 0.7 x 76.8 + 0.2 x 120 +0.1 x 89
The WBGT equation evolved from the work of Yaglou and Minard*and was originally based on Marine Corps recruits doing close order drill in the utility or fatigue uniform of the day, which was probably a heavy 100% cotton two-piece uniform with a field cap (about 0.7 clo – see Appendix 1 for definition of “clo”).
The U.S. Government Occupational and Environmental Health Bulletin lists several of the WBGT index general guidelines, which include the following:
WBGT Index = 78.0 – 81.9ºF. Extremely
intense physical exertion may precipitate heat exhaustion or heat stroke;
therefore caution should be taken.
WBGT Index = 82.0 – 84.9ºF. Discretion should be used
in planning heavy exercise for unseasoned personnel. This is sometimes used as
the marginal limit of environmental heat stress.
WBGT Index = 85.0 – 87.9ºF. Strenuous exercise such as
marching at standard cadence should be suspended in unseasoned personnel during
their first three weeks of training. Training activities may be continued
on a reduced scale after the second week of
training. Outdoor classes in the sun should be avoided above this temperature.
WBGT Index = 85.0ºF or greater. Outdoor classes in the sun
should be avoided.
WBGT Index = 88.0ºF. Strenuous exercise should
be curtailed for all recruits and other trainees with less than 12 weeks
training in hot weather.
WBGT Index = 90.0ºF or greater. Physical training and
strenuous exercise should be suspended for all personnel (excluding
essential operational commitments not for training purposes, where risk of heat
casualties may be warranted).
On the basis of temperatures alone, PMVSUN indices of 4.4, 4.8 and 5.3 correspond to WBGT values of about 84.7, 85.7 and 86.7, respectively. PMVSUN indices, being derived directly from PMVSHADE values, imply sedentary activity and light summer clothing (about 0.5 “clo”). On the other hand, WBGT indices relate to non-sedentary activity and military fatigue clothing (about 0.7 clo). Accordingly, direct comparison between PMV indices and WBGT indices cannot be made. Nevertheless, the magnitudes of these WBGT values suggest the relative unpleasantness of the counterpart PMV indices.
In summary, by illustrating this brief application of heat stress to an example city (such as Miami, Florida), we gain some idea of the relationship between extrapolated PMVSUN values (4.4, 4.8 and 5.3) and counterpart WBGT indices (84.7, 85.7 and 86.7). ■
