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ISDS Annual Conference Proceedings 2012. This is an Open Access article distributed under the terms of the Creative Commons Attribution-
Noncommercial 3.0 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permitting all non-commercial use, distribution, and
reproduction in any medium, provided the original work is properly cited.

ISDS 2012 Conference Abstracts

Surveillance of Heat-related Morbidity: Relation to Heat-
related Excess Mortality
Robert Mathes*1, Kristina B. Metzger2, Kazuhiko Ito1 and Thomas Matte1

1New York City Department of Health and Mental Hygiene, Queens, NY, USA; 2City of Austin/Travis County Health and Human
Services Department, Austin, TX, USA

Objective
To describe the extent to which heat-illness indicators increase

with extreme heat and to evaluate the association among daily
weather, heat-related illness and natural cause mortality.

Introduction
The impact of heat on mortality is well documented [1-3] but

deaths tend to lag extreme heat and mortality data is generally not
available for timely surveillance during heat waves. Recently, sys-
tems for near-real time surveillance of heat illness have been reported
[4] but have not been validated as predictors of heat related mortal-
ity. In this study, we examined the associations among weather, indi-
cators of heat-related ambulance calls and emergency department
visits and excess natural cause mortality in New York City (NYC).

Methods
We analyzed daily weather conditions, emergency medical system

(EMS) calls flagged as heat-related by EMS dispatchers, emergency
department (ED) visits classified as heat-related based on chief com-
plaint text, and natural cause deaths. EMS and ED data were obtained
from data reported daily to the city health department for syndromic
surveillance. We fit generalized linear models to assess the relation-
ships of daily counts of heat related EMS and ED visits to natural
cause deaths after adjustment for weather conditions during the
months of May-September between 1999 and 2008.

Results
We observed an increase in mean total calls to EMS and a decrease

in mean total visits to EDs during 10 observed heat waves (maximum
heat index ! 90° F (Fahrenheit) for four or more consecutive days
with the first three days ! 95° F and at least one day !100°F) in NYC
between 1999 and 2008. Both EMS and EDs experienced an increase
in heat-related incidents during heat waves though the increase in
heat-related EMS calls was much greater. A modest increase in mean
natural cause deaths was also observed. Controlling for temporal
trends, an 11% (95% confidence interval (CI): 5-18) and 7% (95% CI:
4-9) increase in natural cause mortality was associated with an in-
crease from the 50th percentile to 99th percentile of same-day and
one-day lagged heat-related EMS calls and ED visits, respectively.

After controlling for both temporal trends and weather, we observed
a 10% (95% CI: 4-16) increase in natural cause mortality associated
with one-day lagged heat-related EMS calls and a 5% mortality in-
crease with one-day lagged ED visits (95% CI: 2-8).

Conclusions
Heat-related EMS calls and ED visits lagged one day predicted

natural cause mortality in our temporal and weather-adjusted model.
In particular, risk of mortality rapidly increased as the number of heat-
related EMS calls approached high levels (>100 heat-related
calls/day). Heat-related illness can be tracked during heat waves using
EMS and ED data which are indicators of heat associated excess nat-
ural cause mortality during the warm weather season.

Keywords
Surveillance; Heat; Morbidity; Mortality

Acknowledgments

This research was funded by the Environmental Protection Agency, STAR
Grant R833623010, and in conjunction with the Alfred P. Sloan Founda-
tion, grant 2010-12-14. We thank the members of the New York City De-
partment of Health and Mental Hygiene Syndromic Surveillance Unit.

References

1. Curriero, F.C., et al., Temperature and mortality in 11 cities of the
eastern United States. Am J Epidemiol, 2002. 155(1): p. 80-7.

2. Dolney, T.J. and S.C. Sheridan, The relationship between extreme heat
and ambulance response calls for the city of Toronto, Ontario, Canada.
Environ Res, 2006. 101(1): p. 94-103.

3. Knowlton, K., et al., The 2006 California heat wave: impacts on hos-
pitalizations and emergency department visits. Environ Health Per-
spect, 2009. 117(1): p. 61-7.

4. Chau, P.H., K.C. Chan, and J. Woo, Hot weather warning might help
to reduce elderly mortality in Hong Kong. Int J Biometeorol, 2009.
53(5): p. 461-8.

*Robert Mathes
E-mail: rmathes@health.nyc.gov

Online Journal of Public Health Informatics * ISSN 1947-2579 * http://ojphi.org * 5(1):e156, 2013