• Tiina M Mäkinen, Raija Juvonen, Jari Jokelainen, Terttu H Harju, Ari Peitso, Aini Bloigu, Sylvi Silvennoinen-Kassinen, Maija Leinonen, and Juhani Hassi.
• Institute of Health Sciences, University of Oulu, Finland. tiina.makinen@oulu.fi
• Respir Med. 2009 Mar 1; 103 (3): 456-62.

ObjectiveThe association between cold exposure and acute respiratory tract infections (RTIs) has remained unclear. The study examined whether the development of RTIs is potentiated by cold exposure and lowered humidity in a northern population.MethodsA population study where diagnosed RTI episodes, outdoor temperature and humidity among conscripts (n=892) were analysed.ResultsAltogether 643 RTI episodes were diagnosed during the follow-up period. Five hundred and ninety-five episodes were upper (URTI) and 87 lower (LRTI) RTIs. The mean average daily temperature preceding any RTIs was -3.7+/-10.6; for URTI and LRTI they were -4.1+/-10.6 degrees C and -1.1+/-10.0 degrees C, respectively. Temperature was associated with common cold (p=0.017), pharyngitis (p=0.011) and LRTI (p=0.048). Absolute humidity was associated with URTI (p<0.001). A 1 degrees C decrease in temperature increased the estimated risk for URTI by 4.3% (p<0.0001), for common cold by 2.1% (p=0.004), for pharyngitis by 2.8% (p=0.019) and for LRTI by 2.1% (p=0.039). A decrease of 1g/m(-3) in absolute humidity increased the estimated risk for URTI by 10.0% (p<0.001) and for pharyngitis by 10.8% (p=0.023). The average outdoor temperature decreased during the preceding three days of the onset of any RTIs, URTI, LRTI or common cold. The temperature for the preceding 14 days also showed a linear decrease for any RTI, URTI or common cold. Absolute humidity decreased linearly during the preceding three days before the onset of common cold, and during the preceding 14 days for all RTIs, common cold and LRTI.ConclusionsCold temperature and low humidity were associated with increased occurrence of RTIs, and a decrease in temperature and humidity preceded the onset of the infections.

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