Colder, drier conditions are associated with more respiratory infections.
Drops in outdoor temperature and absolute humidity are associated with an increased occurrence of respiratory tract infections. Researchers followed 892 Finnish military conscripts and related 643 clinically diagnosed respiratory-infection episodes to local temperature and humidity measurements.
Each 1°C decrease in temperature was associated with an estimated:
- 4.3 percent higher risk of an upper respiratory tract infection.
- 2.1 percent higher risk of a common cold.
- 2.8 percent higher risk of pharyngitis.
- 2.1 percent higher risk of a lower respiratory tract infection.
Each one-gram-per-cubic-meter decrease in absolute humidity was associated with a 10 percent higher estimated risk of an upper respiratory tract infection. Temperature and humidity also tended to decline during the days preceding illness onset.
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The findings do not mean that cold temperature creates a viral infection. Cold and dry conditions could influence viral stability, airway defenses, indoor crowding, or transmission behavior. The study was observational, involved young men in a crowded and physically demanding military environment, and did not establish direct causation.
Prolonged exposure to cold weather also differs from a brief, voluntary cold shower or plunge. These results should therefore not be interpreted as contradicting research on intermittent cold exposure; they show that temperature dose, duration, and context matter.