Background: We estimated the effects of temperature and humidity on hospital admissions for heart disease (International Classification of Diseases, 9th revision [ICD-9] codes 390-429) and myocardial infarction (ICD-9 code 410) of persons age 65 and older in 12 U.S. cities with a wide range of climates. To account for possible delayed effects and harvesting, we examined the impact of weather up to 20 days before each admission.
Methods: Poisson regression models were fitted in each city, with regression splines used to control for season and barometric pressure. We also controlled day of the week. We estimated the effect and the lag structure of both temperature and humidity based on a distributed lag model.
Findings: For cities in both hot and cold climates, we found that hospital admissions for all heart disease increased monotonically with average temperature on the same day as and the day before admission. The effect of very high temperatures had a temporal pattern consistent with harvesting: several days after an episode of high temperature, there were fewer admissions. In contrast, a protective effect of cold temperature persisted without rebound. The effects of either hot or cold temperature disappeared within 10 days of exposure. There was no evidence for a humidity effect. Similar but smaller effects of temperature were seen for admissions for myocardial infarction specifically.
Conclusions: The effects of temperature on hospital admissions predominantly occur within a few days after exposure, and much of the effect of hot temperatures is short-term displacement of events.