Climatic Heat and Human Mortality in Iowa

 
By
Pramod Dwivedi
 
 
The University of Iowa
Center for Global and Regional Environmental Research
1998
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

1. Background

2. Introduction

3. Methodology
         Mortality Data
        Temperature Data

4. Discussion

5. Conclusion

Background:

Human illness and mortality are linked in many ways to the environment and climate (WHO, 1995). The human health and its biology may be hampered not only by climate but also by the way in which the surrounding habitat affects the temperature and humidity (Ibid. 1995). These relationships can be direct, such as that between mortality rates and the frequency and severity of extreme temperatures. Or the relationships may be indirect with complex relationships affecting the life cycles and habitats of disease carrying mosquitoes and ticks. Climatic attributes have been established as factors affecting: respiratory diseases (emphysema, asthma, etc.); chronic diseases (strokes, heart diseases, etc.); contagious diseases (influenza, pneumonia, etc.); allergenic disease (asthma, hay fever, etc.); and vector- borne diseases (encephalitis, lyme disease) (Smith & Tirpak, 1990).

The Centers for Disease and Prevention (CDC) reports that approximately 385 people die each year in the United States from excessive heat.  The mortality associated with excessive heat can be divided into four categories of heat stroke, heat exhaustion, heat syncope, and heat creps.  The Illinois Emergency Management Agency reveals that heat contributes to more illness and death than any other weather related cause.  A number of studies have indicated that increased temperature is directly related to the increased deaths in summer months.     
 
 

Introduction:

This research explores the relationship between climatic temperature and mortality suggested by several researchers in the US and Europe.  The summer of 1998 witnessed a dramatic increase in mortality in India (Hindustan Times).  Our hypothesis is based on the findings of previous studies, which have stated that the rise of temperature causes increased mortality (Kalkstein, 1997).

Our geographical area of study is the 99 counties of the State of Iowa.  The analysis of the data suggests a relationship between temperature and death rate. In general, it is clear that increased climatic temperatures do increase the death rates among Iowans (Fig 1 &2).

This study includes all deaths that have occurred in the stated of Iowa between between 1973 and 1994 during the months of July and August.  This means that we did not specify the underlying cause of death reported on the death certificates.
 

Methodology:

Mortality Data

The data associated with death in the state of Iowa were obtained from the State Health Registry of Iowa (SHRI). The relevant records were copied from the death certificates. The information in which we were interested included 1) date of birth, 2) date of death, 3) gender, 4) county of death, and 5) underlying cause of death. There were no inclusion or exclusion criteria determined for the subjects other than their deaths must have occurred during the months of July and August between 1973 and 1994. Although, in our data analysis, we did not use gender and underlying cause of death, we thought this information may be useful in the future.

For the month of July the number of death ranged between 2139 (average temperature 79 F) in 1992 and 2739 (average temperature 89 F) in 1974. The total number of people, who died between 1973 and 1995 in July was 50386. The average monthly mortality in July was 2290.

The daily deaths in July ranged between 44 in 1979 at a temperature of 75 F and 167 in 1976 at 90 F. It appears from the data that number of deaths 90 and above generally corresponded with the temperature 85 F and above. Approximately 74 people died each day in July (Table 1).

The average death rate in August was 2245, and the mortality ranged between 2144 (average temperature 80 F) in 1994 and 2464 (average temperature 83 F) in 1987. A total of 49385 people died between 1973 and 1994 in August. The August daily average mortality was 72. Forty- five (83 F) was least number of people died on a day in August. The maximum death was 107 at 93 F (Table 2).
 
 
Table 1.

Average Daily Maximum Temperature and Death in July
 
Year  Average Max T Average Daily Death Total July Death
1973 85 80 2474
1974 89 88 2739
1975 87 77 2372
1976 88 74 2281
1977 90 76 2358
1978 83 74 2283
1979 84 71 2202
1980 88 74 2286
1981 82 71 2196
1982  84 72 2241
 
1983 88 76 2344
1984 83 72 2242
1985 86 72 2246
1986 86 71 2203
1987 89 72 2230
1988 92 74 2298
1989 87 74 2286
1990 83 71 2207
1991 87 72 2236
1992 79 69 2139
1993 82 74 2282
1994 82 72 2241
 Total                                               87                                                    74                                                    50386
 
 

Table 2

Average Daily Maximum Temperature and Death in August
 
 
Year Average Max T Average Daily Death Total August Death
1973 87 79 2464
1974 81 76 2371
1975 86 73 1170
1976 85 72 2270
1977 79 71 2188
1978 84 75 2330
1979 82 74 2293
1980 85 69 2153
1981 80 71 2216
1982 80 72 2222
1983 91 74 2300
1984 86 70 2170
1985 81 71 2194
1986 78 70 2171
1987 83 76 2341
1988 90 75 2315
1989 83 70 2178
1990 83 69 2145
1991 84 71 2200
1992 78 71 2199
1993 81 74 2297
1994 80 69 2141
Total 85 72 49385
 

Temperature Data

The temperature data were obtained from the weather laboratory of the State of Iowa.  Average temperature in July was approximately 86 F.  Table 3 summarizes the temperature record of July and August. The average maximum temperature in July ranged between 79 F (69 deaths) in 1992 and 92 F (72 deaths) in 1988. The maximum temperature recorded on a single day in July was 107 F (73 deaths). Average maximum temperature in August ranged between 78 F in 1986 and 1992 (70 and 71 deaths respectively) and 91 F in 1983 (74 deaths). The average maximum temperature in 1988 was recorded at 90 F with a average death of 75. 
 

Table 3
 
 
Temperature in degrees F  Maximum Temperature Minimum Temperature
Mean 84.46 63.13
Median 85.00 63.00
Mode 86.00 63.00
Std. Deviation 6.56 62.00
Range 63.00-105 41-81
 

Discussion:

As stated earlier this analysis is based on the mortality and temperature data in the state of Iowa. Although the mortality data have been obtained from each of the ninety-nine counties in the state, temperature data were recorded only at one place. Based on available July and August mortality data and the temperature data corresponding to the months of July and August, it can be concluded that there is a correlation between the temperature and mortality in the state of Iowa (Figure 1 & 2). In general, for both months, the number of death went up as the temperature increased. More people died in July (50,386) than August (49,385)(Table1 &2). Also the average temperature in July was hotter (86 F) than that of August (83 F). From the Table 1 and 2 it appears that as the temperature went above 85 F, there was a slight increase in mortality. Although the average temperature in July and August of 1988 was hotter than any year, the number of death (2307) was only moderately higher than the average (2268). Most people (2739) died in July 1974, which showed an average monthly temperature of 89 F. Our figures suggest that death may rise after 85 F. We, however, cannot confirm the threshold phenomenon beyond which mortality rises rapidly (Kalkstein, 1997). In Iowa even moderate heat can lead to excess death.
 

Figure 1
 


 
 

Figure 2


 
 
 
 

Temperature Mortality Ratio (TMR)

Temperature Mortality Ratio (TMR) show a clear correlation between TMR and Temperature.  Figures 3 and 4 reveal that TMR is directly affected by the increase or decrease of the temperature. In general rise in temperature also results in the rise of TMR.

Figure 5 shows all the data points between 1973 and 1994.  It clearly shows a strong correlation between temperature and TMR.  As the temperature goes up, so does TMR.  At an extreme temperature (above 105F) it begins to level off, which means that the TMR will not change at a very high temperature.
 
 
 

Figure 3

Relationship between Maximum Temperature and TMR in July


 
 
 
Figure 4

Relationship between Maximum Temperature and TMR in August


 

Figure 5

Temperature Mortality Ratio for July and August


 
 

Conclusion:

Based on our observation of the data we can conclude that death rate certainly rises as the climatic temperature increases in Iowa.  Although we have not been able to say with certainity about the threshold factor, we are in agreement with several research that conclude that mortality increases with the increase in the temperature.

References:

Anonymous (1998). Heat related mortality-United States, 1997. Morbidity and Mortality Weekly Report (MMWR) 47:23.

Kalkstein LS and Greene JS (1997). An evaluation of climate/mortality relationships in large U.S. cities and the possible impact of a climate change. Environmental Health Perspectives 105:1.

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