title: Cold temperature and low humidity are associated with increased occurrence of respiratory tract infections.
abstract: OBJECTIVE
  The 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.
  
  
  METHODS
  A population study where diagnosed RTI episodes, outdoor temperature and humidity among conscripts (n=892) were analysed.
  
  
  RESULTS
  Altogether 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.
  
  
  CONCLUSIONS
  Cold temperature and low humidity were associated with increased occurrence of RTIs, and a decrease in temperature and humidity preceded the onset of the infections.
authors.affiliation: Institute of Health Sciences, University of Oulu, Finland. tiina.makinen@oulu.fi
authors.forename: Tiina M
authors.initials: TM
authors.lastname: Mäkinen
authors.forename: Raija
authors.initials: R
authors.lastname: Juvonen
authors.forename: Jari
authors.initials: J
authors.lastname: Jokelainen
authors.forename: Terttu H
authors.initials: TH
authors.lastname: Harju
authors.forename: Ari
authors.initials: A
authors.lastname: Peitso
authors.forename: Aini
authors.initials: A
authors.lastname: Bloigu
authors.forename: Sylvi
authors.initials: S
authors.lastname: Silvennoinen-Kassinen
authors.forename: Maija
authors.initials: M
authors.lastname: Leinonen
authors.forename: Juhani
authors.initials: J
authors.lastname: Hassi
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001249:Asthma / Q000382:microbiology; D003080:Cold Temperature; D003139:Common Cold / Q000453:epidemiology; D005387:Finland / Q000453:epidemiology; D005500:Follow-Up Studies; D006801:Humans; D006813:Humidity; D015994:Incidence; D008297:Male; D008889:Military Personnel; D016017:Odds Ratio; D010612:Pharyngitis / Q000453:epidemiology; D012141:Respiratory Tract Infections / Q000453:epidemiology*; D012306:Risk; D012907:Smoking; D055815:Young Adult