Humidity is a consistent climatic factor contributing to SARS‐CoV‐2 transmission (August, 2020, Sydney / Shanghai)
Transboundary and Emerging Diseases
Spread of SARS-CoV-2
Respiratory droplets and aerosols
Fomites
Faecal–oral
Effects of humidity
Cold air holds less moisture
Warm air holds more moisture
Muggy summers and crisp winters
Exhaled droplets become smaller in dry air when they have less moisture to become enveloped in
Smaller infectious aerosols can stay suspended in the air for longer
In humid air the aerosols are larger and heavier, so they fall out of the air
Indoor heating dries things further through evaporation
Indoor aircon also reduces humidity
Climatic and behavioural factors could influence the evolution of the current COVID‐19 pandemic
Method
Southern hemisphere summer and autumn period
Locally transmitted infections
Relationship between climatic factors and cases in New South Wales
During both the exponential and declining phases in 2020
Results
Increased relative humidity was associated with decreased cases in both epidemic phases
A consistent negative relationship was found between relative humidity and cases
Higher humidity, less disease
Lower humidity, more disease
Similar negative relationship with SARS – 1 and MERS coronavirus
A decrease in relative humidity of 1% was associated with an increase in cases of 7–8%
No relationship with between cases and temperature, rainfall or wind speed
(virus does survive for longer at cool temperatures)
Consistent with work carried out in the Chinese winter
Study confirms low humidity as a driver of SARS‐CoV‐2 transmission
Combined with poor ventilation
Reasons low humidity causes more spread
Respiratory aerosols remaining airborne for longer in low humidity
Survivability of the virus increased in lower humidity
Survivability of the virus decreased in higher humidity
Applications
Could increasing relative humidity contribute to a reduction in SARS-CoV-2 infections when infectious individuals mix with susceptible individuals?
Indoor when cold with heating
Indoors when hot with air-con
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