By Nancy Walsh, Staff Writer, MedPage Today
Published: April 10, 2012
Reviewed by Zalman S. Agus, MD; Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania.
Increased variability in summer temperatures is likely to raise mortality rates among older people with chronic diseases, a large analysis of U.S. cities found.
After adjustment for ozone levels, the risk of death among individuals over 65 with diabetes increased by 4% (HR 1.040, 95% CI 1.022 to 1.059) for each 1° C increase in the standard deviation (SD) in summer temperature, according to Antonella Zanobetti, PhD, of the Harvard School of Public Health in Boston, and colleagues.
And for each 1° C increase in warm weather temperature variability, the risk rose by 3.8% (HR 1.038, 95% CI 1.021 to 1.055) for individuals who had experienced a myocardial infarction, Zanobetti's group reported online in the Proceedings of the National Academy of Sciences.
Previous research has shown that heat waves are associated with increased mortality in the short term, which has “important implications for understanding the health effects of climate change, given that climate change is increasing both the variability of temperatures and the frequency, duration, and intensity of heat waves,” they observed.
However, the results of long-term changes in temperature have been uncertain, so Zanobetti and colleagues designed a cohort study that examined changes by year within 135 individual cities, pooling the results.
The 3.7 million study participants all had been hospitalized for COPD, congestive heart failure, diabetes, or myocardial infarction, and discharged alive from 1985 to 2006.
During follow-up, death rates ranged from 41.9% of the MI cohort to 60.9% of the congestive heart failure group.
In the model that adjusted for ozone, the increase in mortality for each 1° C increase in summer temperature standard deviation was 3.7% (HR 1.037, 95% CI 1.019 to 1.055) for those with COPD and 2.8% (HR 1.028, 95% CI 1.013 to 1.042) for those with congestive heart failure.
The researchers also analyzed models that adjusted for heat waves, as well as heat waves plus ozone, with estimates increasing by about 20% compared with an unadjusted model.
They then looked at the pooled results according to region of the country, and found greater increases in mortality in the hottest areas.
For instance, among patients with MI, the hazard ratio per 1° C temperature SD rose from 0.984 (95% CI 0.952 to 1.018) in the coldest areas of the country to 1.095 (95% CI 1.039 to 1.154) in the hottest areas.
Zanobetti and colleagues then analyzed various other individual factors, and found greater mortality increases for individuals older than 75 compared with those in their late 60s or early 70s.
Demographic factors further contributed, with lower survival being seen in areas with higher proportions of black residents and those living in poverty, and greater survival in areas with larger proportions of college-educated individuals.
The amount of green space in a given zip code was an additional factor in survival, the researchers noted. For example, the hazard ratio for death among individuals with a past myocardial infarction decreased to 0.98 (95% CI 0.97 to 1) with a 15% greater area of local surface green as shown in the National Land Use Cover Dataset.
In discussing their findings, the researchers pointed out that older individuals and those with chronic ailments are less able to adjust physically to temperature fluctuation.
They also noted that their analysis adjusted for heat waves, so the mortality increases were not simply reflecting deaths associated with temporary spikes in temperature.
“This suggests that adaptation and intervention strategies solely targeted to heat waves may miss an important opportunity to improve public health,” they stated.
The 1° C increase in their analysis was “plausible” in some areas of the country, and could be assumed to result in an overall 5% rise in all-cause mortality in older individuals with a prior myocardial infarction.
And with 270,000 deaths annually among patients with the four chronic conditions examined in the study, that would result in an additional 14,000 deaths attributable to temperature variability.
Further investigations will be needed to identify interventions that can modify the risks for these susceptible individuals, the researchers concluded.
The study was supported by the U.S. Environmental Protection Agency and the National Institute of Environmental Health Sciences.
The authors reported no conflicts of interest.
Primary source: Proceedings of the National Academy of Sciences
Source reference:
Zanobetti A, et al “Summer temperature variability and long-term survival among elderly people with chronic disease” Proc Natl Acad Sci 2012; DOI: 10.1073/pnas.1113070109.
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