Heat Stress and Sudden Infant Death Syndrome Incidence: A United States Population Epidemiologic Study
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Heat Stress and Sudden Infant Death Syndrome Incidence:
A United States Population Epidemiologic Study
Joshua R. Scheers-Masters, BA*; Mario Schootman, PhD*‡; and Bradley T. Thach, MD*
ABSTRACT. Objectives. To determine the role of heat nations without extreme temperature variations,
stress in sudden infant death syndrome (SIDS) by exam- such as Great Britain, Tasmania, and New Zea-
ining the SIDS rates during periods of extreme environ- land.2–5 Such research has consistently associated in-
mental temperatures in a period when most infants were creased SIDS incidence with colder outdoor temper-
placed prone for sleep. atures. It has been suggested that some parents may
Design. A retrospective study of SIDS rates and mor-
tality rates attributable to excessive environmental heat
respond to such cold waves by overwrapping infants
in relationship to climatologic temperature was per- or excessively increasing room temperatures.3 In
formed. Data were collected for each of 454 counties in 4 contrast, a cross-cultural study found comparatively
states (Arkansas, Georgia, Kansas, and Missouri) from weak relationships between climatic temperature
May 1 to September 30, 1980, and were then summed to and SIDS incidence in communities in the United
yield the mortality rates for each 5°F (2.8°C) temperature States.6 Therefore, it is possible that seasonal over-
range. wrapping of infants may not be a major cause of
Results. 2 analyses revealed significant relation- SIDS in the United States.
ships for heat-related mortality rates and both the maxi- Sections of the United States, particularly the Mid-
mal daily temperature and mean daily temperature but west, are subject to periodic heat waves, which can
demonstrated no such relationships for SIDS rates. No
association between SIDS rates and heat-related mortal-
lead to severe epidemics of deaths attributable to
ity rates was found with Spearman’s ranked correlation, exposure to excessive heat.7 Such conditions present
for either the maximal daily temperature or the mean an opportunity to examine the effect of increased
daily temperature. environmental temperature on SIDS incidence. We
Conclusions. On the basis of our findings of no sig- reasoned that, if thermal stress increases the risk of
nificant association between SIDS and either measure of SIDS, then an increase in the number of SIDS deaths
temperature during periods of high heat stress-related should be observed when there is an increase in the
death rates, it seems unlikely that the heat stress associ- number of heat-related deaths in the general popu-
ated with the combination of prone sleep positions and lation.
elevated environmental temperatures has a significant
role in the development of SIDS. Pediatrics 2004;
113:e586 –e592. URL: http://www.pediatrics.org/cgi/ METHODS
content/full/113/6/e586; SIDS, thermal stress, bedding, Population
overwrapping, sleep position. This study concentrated on the heat waves of the summer of
1980. In that year, excessive heat from all sources was listed as the
underlying cause of death for a total of 1700 deaths across the
ABBREVIATIONS. SIDS, sudden infant death syndrome; Tmax, United States,8 which was notably greater than the median of 274
maximal daily temperature; Tavg, average daily temperature. deaths per year from 1979 to 1998 and was the largest number of
heat-related deaths during that period.9 To avoid a false-negative
finding attributable to seasonal variations in the SIDS rate, the
T
he thermal stress theory for sudden infant period was restricted to May 1 through September 30, 1980. All
death syndrome (SIDS) states that many SIDS deaths attributable to SIDS and exposure to excessive heat result-
deaths occur as a result of increased demands ing from weather conditions in each of the counties of the 4 states
with the greatest numbers of heat-related deaths, namely, Arkan-
on thermoregulatory mechanisms attributable to a sas, Georgia, Kansas, and Missouri, during this period were con-
prone sleep position, increased ambient temperature, sidered.
and/or a decreased ability to eliminate body heat as
a result of excessive clothing covering the infant.1 Data Sources
Although the relationship between climatic temper- SIDS mortality data were defined as all records from the Mor-
ature and SIDS incidence has been examined care- tality Detail Files, 1979 –1980,10 that fit the aforementioned criteria
fully, the studies were performed predominantly in and for which an International Classification of Diseases, 9th revision,
code of 798.0 was assigned for the underlying cause of death.
Heat-mortality data were defined as all records for which an
From the *Edward Mallinckrodt Department of Pediatrics and ‡Department International Classification of Diseases, 9th revision, code of 900.0
of Internal Medicine, Washington University School of Medicine, St Louis, (“death due to excessive heat due to weather conditions”) was
Missouri. assigned. Population data for each county were obtained from the
Received for publication Sep 25, 2002; accepted Dec 29, 2003. Census of Population and Housing, 1980 (United States): County Pop-
Reprint requests to (B.T.T.) Washington University School of Medicine, 660 ulation by Age, Sex, Race, and Spanish Origin,11 with the populations
S Euclid, Campus Box 8208, St Louis, MO 63110. E-mail: thach@kids. being defined as the entire population of the county for heat-
wustl.edu related deaths and the population ⬍1 year of age for SIDS deaths.
PEDIATRICS (ISSN 0031 4005). Copyright © 2004 by the American Acad- Data on the maximal daily temperature (Tmax) and minimal
emy of Pediatrics. daily temperature were determined for each county of the 4 states
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e586 PEDIATRICS Vol. 113 No. 6 June 2004 http://www.pediatrics.org/cgi/content/full/113/6/e586studied from the Record of Climatological Observations.12 Tempera- Furthermore, analysis of the relationship between
tures for the counties were recorded from a single weather station SIDS rates and heat-related mortality rates with
within each county. For counties that were missing data, temper-
ature data were calculated as the average of values from 2 weather Spearman’s ranked correlation indicated no associa-
stations in 2 bordering counties. If original data were missing for tion for either Tmax (P ⫽ .2080) or Tavg (P ⫽ .8979).
all bordering counties, then data for the next closest counties were The 3 infant deaths were diagnosed as being at-
used. Average daily temperatures (Tavg) in each location then tributable to environmental heat. The analysis was
were calculated as the average of Tmax and the minimal daily
temperature. All temperature data were rounded to the nearest
repeated after these deaths had been removed from
integer. the heat-related category and entered as SIDS deaths.
Again, 2 tests demonstrated increases in heat-re-
Statistical Analyses lated mortality rates with increasing Tmax values (P
For each 5°F temperature range (for example, 60 – 64°F [15.6 – ⬍ .0001) but indicated no such relationship for SIDS
17.8°C]), the population of each county was multiplied by the rates (P ⫽ .605) (Fig 3). Heat-related mortality rates
number of days the county experienced that temperature range. increased with Tavg values (P ⬍ .0001), but again
These values then were summed for all counties, to yield the total
population at risk for each temperature range. The total number of
there was no association between SIDS rates and Tavg
deaths that occurred in that temperature range then was divided (P ⫽ .945) (Fig 4). In addition, analysis of the recoded
by the total population at risk to yield the mortality rate for each data with Spearman’s ranked correlation indicated
temperature range. no association for either Tmax (P ⫽ .9572) or Tavg (P ⫽
2 tests for trend were performed to determine the association .4175). Finally, we considered the possibility that
between the SIDS rate and the heat-related death rate for Tavg and
Tmax. To determine the association between the SIDS rate and the there could be a threshold for heat stress, increasing
heat-related death rate, Spearman’s rank correlation coefficient SIDS risks at higher temperature ranges. Therefore,
was used. A P value of ⬍.05 was considered statistically signifi- we analyzed the dichotomized Tavg variable. The rate
cant. These analyses were repeated after recoding of any infant for Tavg of ⬍85°F was 3.0 cases per 1 000 000 popu-
deaths attributable to exposure to excessive heat as deaths attrib-
utable to SIDS. In these cases, the population associated with lation, whereas the rate for ⱖ85°F was 3.4 cases per
heat-related deaths was equal to the entire population of the 1 000 000. This difference was not statistically signif-
county minus the population ⬍1 year of age. icant (P ⫽ .568).
Because of the relatively few cases (111) of SIDS recorded
during the study period, we used a simulation to calculate the DISCUSSION
empirical power to detect a linear increase in the SIDS rate with
increasing temperature. With the available SIDS deaths, the power In this study, we failed to find a relationship be-
was calculated to be 0.78 if the SIDS rate at the highest tempera- tween elevated environmental temperatures and the
ture was 2.7 times that at the lowest temperature. The power was incidence of SIDS during a period in which deaths
0.70 if the SIDS rate at the highest temperature was 1.8 times that attributable to excessive environmental temperatures
at the lowest temperature. If the SIDS rate at the highest temper-
ature was 1.5 times that at the lowest temperature, then the power in the general population were increased to epidemic
was calculated to be 0.63 (Table 1). levels. Physiologic and epidemiologic research sug-
gests increased risks attributable to heat stress
RESULTS among prone-sleeping infants, because their ability
Between May 1 and September 30, 1980, in the to eliminate heat may be reduced in this position.13,14
states of Arkansas, Georgia, Kansas, and Missouri, a During the period of our study, ⬃72% to 77% of
total of 111 SIDS deaths and 402 deaths attributable infants slept prone.15 Therefore, US infants as a
to excessive heat resulting from weather conditions group should have been unusually susceptible to
were recorded. 2 tests demonstrated increased heat- heat stress.
related mortality rates with increasing Tmax values (P The power calculation indicated that the number
⬍ .0001) but indicated no such relationship for SIDS of SIDS deaths studied was sufficient to allow detec-
rates (P ⫽ .362) (Fig 1). Also, there was a strong tion of an increase in SIDS rates of 1.8-fold between
association between heat-related mortality rates and the highest and lowest temperatures. Smaller in-
Tavg values (P ⬍ .0001); however, there was no linear creases would have been undetectable. Increases of
association for SIDS rates and Tavg (P ⫽ .713) (Fig 2). this magnitude are relatively small, considering that
the differences between summer and winter SIDS
rates during the 1980s in New Zealand, for example,
TABLE 1. Empirical Power Calculations* were five- to sevenfold.3 Therefore, if the large sea-
Increase in SIDS Power sonal variations in SIDS rates observed in the past
Rate at Highest were primarily attributable to heat stress, as sug-
Versus Lowest Linear Increase Stepwise Increase
With Increasing With Increasing gested previously,1,16 then we would expect to have
Temperature
Temperature Temperature observed changes of this magnitude among heat-
stressed infants. Furthermore, the present study in-
6.0 0.89 0.95
3.5 0.83 0.93 dicated that heat-related death rates in the general
2.7 0.78 0.88 population we studied were much higher at the
1.8 0.70 0.73 highest temperature, compared with the lowest. If
1.5 0.63 0.64 SIDS infants are to be considered at high risk for
1.2 0.56 0.60
death attributable to heat stress, then an increase in
* For example; the Power is 0.78 if the SIDS rate at the highest death rates of a magnitude comparable to that for
temperature is 2.7 times that at the lowest temperature, with a deaths attributable to heat stress in the general pop-
linear increase in the SIDS rate with increasing temperature. The
power is 0.70 if the SIDS rate at the highest temperature is 1.8
ulation would be expected. The statistical power was
times that at the lowest temperature, with a linear increase in the more than adequate to detect such an increase.
SIDS rate with increasing temperature. These findings suggest that the heat stress theory
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http://www.pediatrics.org/cgi/content/full/113/6/e586 e587Fig 1. SIDS rates versus heat-related mortality rates for Tmax. Error bars indicate ⫾SEM. It should be noted that, because of the repeated
exposure of the populations in this study to the various temperature ranges, resulting in an extremely large population at risk, these daily
rates per temperature range have little relationship to annual rates.
Fig 2. SIDS rates versus heat-related mortality rates for Tavg.
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e588 HEAT STRESS AND SIDS INCIDENCEFig 3. SIDS rates versus heat-related mortality rates for Tmax, with infant heat-related deaths recoded as SIDS.
Fig 4. SIDS rates versus heat-related mortality rates for Tavg, with infant heat-related deaths recoded as SIDS.
for SIDS should be reevaluated. There are confound- theory. Materials that are high in thermal insulation
ing factors in interpreting the insulation data, argu- value (as measured in tog units) also contribute to
ably the most important findings supporting this death through other mechanisms such as rebreathing
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http://www.pediatrics.org/cgi/content/full/113/6/e586 e589of expired air causing asphyxiation. Studies have tendency for emesis as well as refusal to accept orally
found that infant bedding items such as quilts, com- administered fluids when overheated, which make
forters, and blankets have a high potential for caus- infants more susceptible to heat stress and related
ing rebreathing.17,18 Fleming et al16 found that in- dehydration.27,31 Therefore, for any given thermal
fants who died as a result of SIDS were wrapped in exposure, infants may be expected to experience
clothing or covered by bedding with a significantly symptoms of heat stress more rapidly than other age
greater tog value than were control infants. The groups.
increased SIDS risk associated with increased body Second, it might be expected that most parents
insulation was highest among prone-sleeping in- would pay particular attention to keeping infants
fants. Because prone sleeping and increased insula- cool in hot weather. Evidence suggests that this is not
tion can interact to reduce an infant’s ability to elim- the case. A study of heat-stressed infants hospital-
inate heat, the investigators attributed the increased ized for treatment found that 68% of the infants’
SIDS risk in the prone position to thermal stress. caretakers were unaware of the need for increased
However, those investigators did not include bulky fluid intake by their infants during hot weather and
bedding items (such as duvets, quilts, or comforters) had seriously underestimated the seriousness of the
covering infants in their multivariate analysis. These infants’ initial symptoms.27 Other studies also indi-
items not only are very substantial components of cated a lack of parental perception regarding infant
the tog estimate but also place infants at increased heat stress. It has been reported that the practice of
risk for rebreathing. In fact, Ponsonby et al19 found leaving sleeping infants in parked cars while parents
that the increase in the risk of SIDS resulting from run errands is a frequent cause of heat-related mor-
quilt or comforter use was evident only for supine- bidity and death.31–34 It is noteworthy that 24% of
sleeping infants, suggesting that heat stress may not mothers admit to this practice, again indicating seri-
be the primary mechanism for the increased risk ous underestimation by caretakers of their infants’
associated with these items. ability to cope with increased environmental temper-
The thermal stress theory of SIDS originated with atures.32
premortem and postmortem observations of ele- Third, it is unlikely that many infants kept indoors
vated body temperature or excessive sweating during the 1980 heat wave received adequate protec-
among infants with SIDS.20,21 The observations of tion as a result of access to home air conditioning or
overwrapping or elevated room temperature on the fans. A government-sponsored study of the avail-
day of death provided additional support for this ability of home air conditioners in the 4 states we
theory.16,20,22,23 It is important to note that the ther- studied, spanning 1980 to 1984, indicated that most
mal stress theory draws a sharp distinction between households (51%) had inadequate access to air con-
SIDS resulting from heat stress and death attribut- ditioners sufficient to reduce the risk of heat stroke.35
able to extreme hyperthermia leading to heat Even more relevant were the findings of a 1980 study
stroke.22,24 Therefore, it has been suggested that ther- that found that ⬎80% of the households in the lower
mal stress, with or without moderate increases in one-third socioeconomic group did not have air con-
body temperature, can result in fatal disruption of ditioning.36 Fans in the home were not found to
vital systems among infants predisposed to SIDS.1,25 decrease the risk of heat stroke.37 Therefore, it can be
It has been proposed that congenital or acquired inferred that the majority of the infants, including
anatomic abnormalities in brainstem centers regu- those at highest risk for SIDS (ie, infants in low-
lating respiratory or cardiovascular stability could income households), did not have access to environ-
represent the underlying defects that render SIDS ments adequate to eliminate heat stress.
infants susceptible to death resulting from heat Fourth, any consideration of the possible relation-
stress.26 ship of heat stress to SIDS in the United States must
Our reasoning in assuming that a substantial num- also take into account the fact that the population at
ber of infants at risk for SIDS experienced significant highest risk for one is also that at highest risk for the
heat stress is as follows. First, several factors indicate other. It has been well established in several studies,
that young infants can be expected to experience including an analysis of the 1980 heat wave in Mis-
more heat stress than older children or adults when souri, that low-income, urban, and African American
environmental temperatures increase. The full spec- populations are at highest risk for heat stroke be-
trum of heat-related illnesses among infants must be cause of the combination of an urban “heat-island”
considered, because more cases of nonlethal environ- effect that increases both indoor and outdoor tem-
mental heat-related illnesses (eg, heat prostration peratures, substandard housing, poor ventilation,
and water-deficiency heat exhaustion) than actual and decreased access to air conditioning.38–41 When
heat stroke occur among infants.27 The actual death infant physiologic factors, the failure of caregivers to
rates for infants, compared with the elderly, during perform appropriate interventions, and populations
heat waves are less likely to reflect the true incidence at greatest risk for both heat stress and SIDS are
of heat stress, because death seems to be less fre- taken into account, it might reasonably be predicted
quent among infants diagnosed with heat stroke that, if heat stress per se is a significant cause of SIDS,
than in the older population.27 The risk for all heat- SIDS incidence should increase significantly during
related illnesses is thought to be increased among summer heat waves, when the number of deaths
infants, compared with older children or adults.28–30 attributable to overheating in the general population
This is attributed to increased metabolic rate, in- increases.
creased surface area/body mass ratio, and increased Heat stress can be defined as a deviation from a
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e590 HEAT STRESS AND SIDS INCIDENCEthermoneutral environment that requires increased Census of Population and Housing, 1980 (United States): County Pop-
use of the body’s mechanisms for cooling. In con- ulation by Age, Sex, Race, and Spanish Origin were originally col-
lected by the US Department of Commerce, Bureau of the Census.
trast, heat stroke, defined as a body temperature of Neither the collectors of the original data nor the Consortium bear
ⱖ105°F (ⱖ40.6°C) and altered mental status,42 leads any responsibility for the analyses or interpretations presented
to cardiac arrhythmias, seizures, or hypovolemic here.
shock, with tachycardia and eventual circulatory col-
lapse.43 Depression of central nervous system ther-
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e592 HEAT STRESS AND SIDS INCIDENCEHeat Stress and Sudden Infant Death Syndrome Incidence: A United States
Population Epidemiologic Study
Joshua R. Scheers-Masters, Mario Schootman and Bradley T. Thach
Pediatrics 2004;113;e586
DOI: 10.1542/peds.113.6.e586
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Downloaded from www.aappublications.org/news by guest on August 31, 2021Heat Stress and Sudden Infant Death Syndrome Incidence: A United States
Population Epidemiologic Study
Joshua R. Scheers-Masters, Mario Schootman and Bradley T. Thach
Pediatrics 2004;113;e586
DOI: 10.1542/peds.113.6.e586
The online version of this article, along with updated information and services, is located
on the World Wide Web at:
http://pediatrics.aappublications.org/content/113/6/e586
Pediatrics is the official journal of the American Academy of Pediatrics. A monthly publication, it has
been published continuously since 1948. Pediatrics is owned, published, and trademarked by the
American Academy of Pediatrics, 345 Park Avenue, Itasca, Illinois, 60143. Copyright © 2004 by the
American Academy of Pediatrics. All rights reserved. Print ISSN: 1073-0397.
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