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Effect of WBGT Index Measurement Location on Heat Stress Category Classification


Medicine & Science in Sports & Exercise: September 2015 - Volume 47 - Issue 9 - p 1958–1964
doi: 10.1249/MSS.0000000000000624
Applied Sciences

The location of the wet bulb globe temperature (WBGT) index measurement may affect heat stress flag category classification.

Purpose This study aimed to compare WBGT measurements at three locations along the Boston Marathon race course and compare WBGT estimates for meteorological stations and 72-h advanced WBGT forecasts.

Methods WBGT was measured hourly from 1000 to 1400 h at approximately 7 km, approximately 18 km, and approximately 30 km on the Boston Marathon race course. Simultaneous WBGT estimates were made for two meteorological stations southeast of the course via a commercial online system, which also provided 72-h advanced forecasts.

Results The measurement difference (mean ± SD) among course locations was 0.2°C ± 1.8°C WBGT (ANOVA, P > 0.05). The difference between course and stations was 1.9°C ± 2.4°C WBGT (t-test, P < 0.05). Station values underestimated (n = 98) or overestimated (n = 13) course values by >3°C WBGT (>0.5 flag category) in 111 of 245 paired comparisons (45%). Higher black globe and lower wet bulb temperatures explained over- and underestimates, respectively. Significant underestimates of WBGT resulted in misclassification of green (labeled white) and black (labeled red) course flag categories (χ 2, P < 0.05). Forecast data significantly underestimated red (labeled amber) and black (labeled red) course flag categories.

Conclusions Differences in WBGT index along 23 km of the Boston Marathon race route can be small enough to warrant single measurements. However, significant misclassification of flag categories occurred using WBGT estimates for meteorological stations; thus, local measurements are preferred. If the relation between station WBGT forecasts and the race sites can be established, the forecast WBGT values could be corrected to give advanced warning of approximate flag conditions. Similar work is proposed for other venues to improve heat stress monitoring.

Supplemental digital content is available in the text.

1U.S. Army Research Institute of Environmental Medicine, Natick, MA; and 2Boston Athletic Association, Boston, MA

Address for correspondence: Samuel N. Cheuvront, Ph.D., Thermal and Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine, Kansas Street, Building 42, Natick, MA 01760–5007; E-mail:

Submitted for publication October 2014.

Accepted for publication January 2015.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (

© 2015 American College of Sports Medicine