Humidity is a consistent climatic factor contributing to SARS‐CoV‐2 transmission (August, 2020, Sydney / Shanghai)

Transboundary and Emerging Diseases

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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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